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Electrical Machines Objective Questions & Answers


Electrical Machines Objective Questions & Answers

Q.1 The two winding’s of a transformer are

(A) conductively linked. (B) inductively linked. (C) not linked at all. (D) electrically linked.

Q.2 A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will

(A) come to stop.
(B) continue to run at synchronous speed.
(C) continue to run at a speed slightly more than the synchronous speed.
(D) continue to run at a speed slightly less than the synchronous speed.

Q.3 The d.c. series motor should always be started with load because

(A) at no load, it will rotate at dangerously high speed.
(B) it will fail to start.
(C) it will not develop high starting torque.
(D) all are true.

Q.4 The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about

(A) 50 Hz. (B) 20 Hz.(C) 2 Hz. (D) Zero.

Q.5 In a stepper motor the angular displacement

(A) can be precisely controlled.
(B) it cannot be readily interfaced with micro computer based controller.
(C) the angular displacement cannot be precisely controlled.
(D) it cannot be used for positioning of work tables and tools in NC machines.

Q.6 The power factor of a squirrel cage induction motor is

(A) low at light load only.
(B) low at heavy load only.
(C) low at light and heavy load both.
(D) low at rated load only.

Q.7 The generation voltage is usually

(A) between 11 KV and 33 KV.
(B) between 132 KV and 400 KV.
(C) between 400 KV and 700 KV.
(D) None of the above.

Q.8 When a synchronous motor is running at synchronous speed, the damper winding produces

(A) damping torque.
(B) eddy current torque.
(C) torque aiding the developed torque.
(D) no torque.

Q.9 If a transformer primary is energised from a square wave voltage source, its output voltage will be

(A) A square wave. (B) A sine wave. (C) A triangular wave. (D) A pulse wave.

Q.10 In a d.c. series motor the electromagnetic torque developed is proportional to

(A) Ia . (B) Ia2. (C) 1/Ia (D)I1/Ia2

Q.11 The emf induced in the primary of a transformer

(A) is in phase with the flux.
(B) lags behind the flux by 90 degree.
(C) leads the flux by 90 degree.
(D) is in phase opposition to that of flux.

Q.12 The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a

(A) dc machine.
(B) 3 phase induction machine.
(C) synchronous machine.
(D) single phase induction machine.

Q.13 The current from the stator of an alternator is taken out to the external load circuit through

(A) slip rings. (B) commutator segments.(C) solid connections. (D) carbon brushes.

Q.14 A motor which can conveniently be operated at lagging as well as leading power factors is the

(A) squirrel cage induction motor.  (B) wound rotor induction motor (C) synchronous motor.  (D) DC shunt motor.

Q.15 A hysteresis motor

(A) is not a self-starting motor. (B) is a constant speed motor.(C) needs dc excitation. (D) can not be run in reverse speed.

Q.16 The most suitable servomotor for low power applications is

(A) a dc series motor.(B) a dc shunt motor. (C) an ac two-phase induction motor.(D) an ac series motor.

Q.17 The size of a conductor used in power cables depends on the

(A) operating voltage. (B) power factor. (C) current to be carried. (D) type of insulation used.

Q.18 Out of the following methods of heating the one which is independent of supply frequency is

(A) electric arc heating (B) induction heating (C) electric resistance heating (D) dielectric heating

Q.19 A ceiling fan uses

(A) split-phase motor.
(B) capacitor start and capacitor run motor.
(C) universal motor.
(D) capacitor start motor.

Q.20 A stepper motor is

(A) a dc motor. (B) a single-phase ac motor. (C) a multi-phase motor. (D) a two phase motor.

Q.21 The drive motor used in a mixer-grinder is a

(A) dc motor. (B) induction motor. (C) synchronous motor. (D) universal motor.

Q.22 In a capacitor start single-phase induction motor, the capacitor is connected

(A) in series with main winding.
(B) in series with auxiliary winding.
(C) in series with both the windings.
(D) in parallel with auxiliary winding.

Q.23 A synchro has

(A) a 3-phase winding on rotor and a single-phase winding on stator.
(B) a 3-phase winding on stator and a commutator winding on rotor.
(C) a 3-phase winding on stator and a single-phase winding on rotor.
(D) a single-phase winding on stator and a commutator winding on rotor.

Q24 As the voltage of transmission increases, the volume of conductor

(A) increases. (B) does not change. (C) decreases. (D) increases proportionately.

Q. 25 A commutator in a d.c. machine

(A) Reduces power loss in armature.
(B) Reduces power loss in field circuit.
(C) Converts the induced a.c armature voltage into direct voltage.
(D) Is not necessary.


Electrical Machines

Q.1 The two windings of a transformer is

Ans : B

Q.2 A salient pole synchronous motor is running at no load. Its field current is switched off. The motor will

Ans: B

Q.3 The d.c. series motor should always be started with load because

Ans: A

Q.4 The frequency of the rotor current in a 3 phase 50 Hz, 4 pole induction motor at full load speed is about

Ans: C

Q.5 In a stepper motor the angular displacement

Ans: A

Q.6 The power factor of a squirrel cage induction motor is

Ans: A

Q.7 The generation voltage is usually

Ans: A

Q.8 When a synchronous motor is running at synchronous speed, the damper winding produces

Ans: D

Q.9 If a transformer primary is energised from a square wave voltage source, its output voltage will be

Ans: A

Q.10 In a d.c. series motor the electromagnetic torque developed is proportional to


Q.11 The emf induced in the primary of a transformer

Ans: C

Q.12 The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for a

Ans: all options are correct

Q.13The current from the stator of an alternator is taken out to the external load circuit through

Ans: C

Q.14 A motor which can conveniently be operated at lagging as well as leading power factors is the

Ans: C

Q.15 A hysteresis motor

Ans: B

Q.16 The most suitable servomotor for low power applications is

Ans: B

Q.17 The size of a conductor used in power cables depends on the

Ans: C

Q.18 Out of the following methods of heating the one which is independent of supply frequency is

Ans: C

Q.19 A ceiling fan uses

Ans: D, Explanation : To give starting torque and to maintain speed.

Q.20 A stepper motor is

Ans: D

Q.21 The drive motor used in a mixer-grinder is a

Ans: D

Q.22 In a capacitor start single-phase induction motor, the capacitor is connected

Ans: B, Explanation : To make single phase motor self start. We split the phases at 90 degree. Hence, motor behaves like a two phase motor.

Q.23 A synchro has

Ans: C, Explanation : The basic synchro unit called a synchro transmitter. It’s construction similar to that of a Three phase alternator.

Q24 As the voltage of transmission increases, the volume of conductor

Ans: C, Explanation : Decreases due to skin effect.

Q. 25 A commutator in a d.c. machine

Ans: C , Explanation : As name suggests, it commutes ac into dc.

Electrical Measurements and Instrumentation Objective Questions & Answers


Electrical Measurements and Instrumentation Objective Questions & Answers

[1] A 10MHz CRO has

(a) 5MHz sweep
(b) 10MHz vertical oscillator
(c) 10MHz horizontal oscillator
(d) 10MHz supply frequency

[2] Which of the following instruments can be used to measure AC current only?

(a) Permanent Magnet Type ammeter
(b) Induction type ammeter
(c) Moving iron voltmeter
(d) Moving iron ammeter

choose the right Answer:

  1. D only
  2. B only
  3. A, B, D
  4. B and D only

[3] An oscilloscope indicates

(a) Peak to peak value of voltage
(b) DC value of voltage
(c) RMS value
(d) Average value

[4] In a ballistic galvanometer, the deflecting torque is proportional to

(a) the current through coil
(b) square of current through coil
(c) square-root of current through coil
(d) sine of measured

[5] The error of an instrument is normally given as a percentage of

(a) measured value
(b) full-scale value
(c) mean value
(d) rms value

[6] If the instrument is to have a wide range, the instrument should have

(a) Linear scale
(b) Square-law scale
(c) Exponential scale
(d) Logarithmic scale

[7] The resistance can be measured most accurately by

(a) Voltmeter-ammeter method
(b) bridge method
(c) multimeter
(d) Megger

[8] The repeat accuracy of an instrument can be judged from its

(a) static error
(b) linearity error
(c) dynamic error
(d) standard deviation of error

[9] Which of the following meters has a linear scale?

(a) Thermocouple meter
(b) Moving iron meter
(c) Hot wore meter
(d) Moving coil meter

[10] No eddy current and hysteresis losses occur in

(a) Electrostatic instruments
(b) PMMC instruments
(c) Moving iron instruments
(d) Electrodynamo meter instruments

[11] Two voltmeters have the same range 0-400V. The internal impedance are 30,000 Ohms and 20,000 Ohms. If they are connected in series and 600V be applied across them, the readings are

(a) 360V and 240V
(b) 300V each
(c) 400V and 200V
(d) one of the meters out of the range and other 100V

[12] The full-scale deflection current of an ammeter is 1 mA and its internal resistance is 100Ohm. If this meter is to have full deflection at 5A, what is the value of the shunt resistance to be used?

(a) 49.99 Ohms
(b) 1/49.99 ohms
(c) 1 Ohm
(d) 2 Ohms

[13] The full-scale deflection current of an ammeter is 1 mA and its internal resistance is 100Ohm. This is to have full deflection when 100V is measured. What is the value of series resistor to be used?

(a) 99.99 K ohms
(b) 100 K ohms
(c) 99.99 ohms
(d) 100 ohms

[14] Why is a MISC meter not recommended for DC measurement?

(a) The meter is calibrated for AC and it’s error for DC would be high
(b) The meter does not respond to DC signals
(c) The error is high due to hysteresis effect
(d) The error is high due to eddy current effect

[15] The EMF of Weston standard cell is measured using

(a) Moving- iron meter
(b) Moving-coil meter
(c) Digital Volt meter
(d) Potentiometer


[1] A 10MHz CRO has

Ans: C

[2] Which of the following instruments can be used to measure AC current only?

Ans: 2

[3] An oscilloscope indicates

Ans: A

[4] In a ballistic galvanometer, the deflecting torque is proportional to


[5] The error of an instrument is normally given as a percentage of

Ans: B

[6] If the instrument is to have a wide range, the instrument should have


[7] The resistance can be measured most accurately by

Ans: B

[8] The repeat accuracy of an instrument can be judged from its

Ans: D

[9] Which of the following meters has a linear scale?

Ans: D

[10] No eddy current and hysteresis losses occur in

Ans: A

[11] Two voltmeters have the same range 0-400V. The internal impedance are 30,000 Ohms and 20,000 Ohms. If they are connected in series and 600V be applied across them, the readings are

Ans: A

[12] The full-scale deflection current of an ammeter is 1 mA and its internal resistance is 100Ohm. If this meter is to have full deflection at 5A, what is the value of the shunt resistance to be used?

Ans: B

[13] The full-scale deflection current of an ammeter is 1 mA and its internal resistance is 100Ohm. This is to have full deflection when 100V is measured. What is the value of series resistor to be used?

Ans: A

[14] Why is a MISC meter not recommended for DC measurement?

Ans:  C

[15] The EMF of Weston standard cell is measured using

Ans: D

More Instrumentation MCQ : CLICK HERE

DP Transmitter Interface Level Measurement Principle, Limitations, Selection, Installation, Design & Calibration


The principle of differential pressure level measurement is based on hydrostatic head.

Hydrostatic pressure measurement is the most common means for liquid and interface level measurements. For most applications, differential transmitters are preferred because the range selection is flexible and widely understood. They are used with open and enclosed vessels. Differential transmitters are usually connected to the side of a vessel or tank with isolation facilities.

Interface liquid–liquid level calculation example

The differential pressure DP = hinterface x g x [ ρ2 – ρ1 ] + ρ1 x g x H                    (Equation [1])

The range is:

  • At hinterface = 0     then    DP = ρ1 x g x H
  • At hinterface = H    then     DP = ρ2 x g x H

Interface liquid–liquid level calculation

Figure – DP measurement

ρ1 :  Liquid1 Density (kg/m3)

ρ2 : Liquid2 Density (kg/m3)

hinterface : Interface Level between the Liquid 1 and Liquid (m)

g: 9.81 (m/s2)

Interface measurement requires its own connection into the upper and the lower phase.

Equation [1] is applicable if there is only one variable. For an interface level measurement it should be hinterface.

Using the same principle and Equation [1], density of a single fluid can be measure if both tapping are permanently and fully immersed.

Following the principle, measurement of several interface layers can be considered by staging each interface level measurement. For an interface measurement between two liquids the limitation is derived from Equation [1].

The combination of density difference and the distance between the upper/lower nozzles should result in a minimum DP range of around 30 mbar.


If both density values ρ2 and ρ1 are similar, the interface level measurement may nearly not be detected by the transmitter. This depends on the DP range, accuracy and distance between the upper and lower nozzles. This occurs typically for an interface measurement between oil and water the case of presence of “heavy” oil (the oil density value is nearly the same as the water density value).

Accuracy depends on the liquid density variation. To compensate a density measurement should be provided.

For vessel under vacuum, DP with remote diaphragm seal is recommended. The transmitter should be installed below the bottom nozzle.

For heavy crude oil dirty, foaming, fouling or clogging services the DP with remote diaphragm seal is recommended with nozzle, flushing ring and heat tracing (e.g. freezing oil) as required.

The mounting of heavy instrument (including all accessories, i.e. DBB/SBB valves, flushing ring, etc.) to the nozzles should be verified with the nozzle local stress verification (static and dynamic/fatigue). Sufficient support should be provided for minimizing the weight transferred to the nozzles.


Differential pressure measurement could be considered for most applications with liquid–gas or liquid–liquid interface level measurement.

Differential pressure transmitters can be used in severely turbulent, dirty, in presence of foam above the liquid or fouling service with diaphragm seals and capillaries.

Differential pressure transmitter with diaphragm seals and capillaries are preferred. This should be provided with a flushing ring mounted between the process flange and the diaphragm seal.

Capillaries should be specified at the correct length, without the need for coiling excess capillary that is surplus to the run. Capillaries should be protected from damage using a basic channel system, allowing sufficient bend radius for the capillaries.

Diaphragm material should be carefully selected according to the type of fluid (e.g. gold plated in presence of hydrogen).

The use of wet legs with intermediate liquids and zero adjustment implies more complex range calculation and higher maintenance needs. Differential pressure transmitter used without diaphragm seals and capillaries should have block and bleed valve manifolds as a minimum. In vapour or cryogenic services, the dry leg should have a self‐purge.

A particular attention should be paid to the protection and heat tracing of dry/wet legs. For capillary tubing, the selection of tubing fluid should consider the ambient temperature (to prevent freezing).

The mounting of heavy instrument (including all accessories, i.e. isolation/drain valves, flushing ring, etc.) on the nozzles should be verified with the nozzle local stress verification (static and dynamic/fatigue). Sufficient support should be provided for minimizing the weight transferred to the nozzles.

High static pressure can create a measurement zero and full scale drift. This can be compensated as required, by measuring and compensating the static pressure.

For low range (e.g. below 300 mm) or similar densities between two liquids (for an interface measurement), a particular attention should be paid to sources of measurement error, such as:

  • temperature/density variation of capillary fluid
  • measurement resolution error due to 2″ or 3″ nozzle and diaphragm
  • zero error due to air/liquid pockets in the hook up/transmitter or fouling of the diaphragm
  • uncertainties of the transmitter when maximum possible calibration range of the cell is much greater than actual calibrated range.
1. Impulse piping

For atmospheric vented vessel the low pressure side is connected to the atmosphere. Wind effect or insect should not affect the measurement (e.g. using a bug filter).

The impulse line is used to interface the instrument with the process connection. There are two methods which could be used to connect the instrument the process:

  • using a wet leg
  • using a dry leg.

DP Impulse Line

Figure – DP Impulse lines

Wet leg

If the ‘reference leg’ is filled with a liquid, a permanent zero offset will be created. This offset should be compensated.

The wetted leg liquid should be selected for avoiding the risk of evaporation and leakage.

A trifoliate label in the field should be affixed to the three‐way manifold block, highlighting “This level duty is on a wet leg system. Equalization of the transmitter block will result in loss of the wet leg.”

Dry leg

Gas compatibility with the dry leg material should be considered. Gas change state or liquid presence in the dry leg should be carefully addressed.

Where use of Differential Pressure dry leg system are deployed on a closed tank, they should be assessed to ensure no excess fluid or condensate can build up in the low pressure (dry) impulse leg.

Dry legs should include an isolable drainage pot at their lowest point (below HP tap) for allowing the condensates to be drained.

2. Remote diaphragm seal

Diaphragm seal capillaries filled with oil requires a dedicated configuration of the range with a zero drift.

In case of tall measurement range (e.g. above 6 m), two separate remote sensors may be used. The measurement principle is based on a remote sensor replacing the capillaries. In this case, a detailed procedure for the calibration (including the zero shift) should be studied.

For density measurement the liquid should always above the upper nozzle.

DP Level vs density measurement

Figure – DP Level vs density measurement

3. Symmetric and asymmetric capillaries

Differential pressure seal system is typically specified with identical capillary lengths and seal configurations on both the high and low pressure process connections. This type of system is traditionally specified because it compensates for temperature induced errors.

The oil volume in the capillary will expand and contract causing fluctuations in the internal pressure of the capillary system. This error will be cancelled out because the same expansion and contraction of oil volume will occur on both the high and low sides of the transmitter due to symmetrical construction. The second source of temperature induced measurement drift occurs when a capillary seal system is installed with a vertical separation between the two seals. The density of the fill‐fluid within the capillary will fluctuate with the change in temperature and cause the amount of head pressure force that is measured by the transmitter to vary.

Equal lengths of capillary cannot compensate this change in density due to low pressure side generally being mounted at a higher elevation than the high pressure side. An asymmetrical design minimizes the fill‐fluid volume on the high side in order to counteract the temperature induced density effects always present on any vertical installation.

4. Electronic DP level system

This measurement principle is based on independent pressure measurements. Rather than using a single DP transmitter with mechanical impulse piping or capillary, electronic DP level system uses two direct mount gage or absolute sensors that are connected with a non‐proprietary electrical wire.

Electronic DP level system replaces the long lengths of oil‐filled capillary and impulse piping with an electrical wire that is immune to temperature induced effect as well as the lengthy capillary. This means that it will be possible to get an accurate measurement over a large range of ambient temperatures without fill‐fluid density or volume changes affecting the reading. High and low pressure measurements are fully synchronized to ensure that the differential pressure measurement is accurate.

If the ratio between the DP pressure and the vessel static pressure is (DP/Static) < 1/10 the impact on the accuracy will be non‐negligible.

The effect of a static pressure on both side of a non‐electronic DP system implies a drift that needs to be compensated. Electronic DP system calculates and compensates the pressure effect directly without specific calibration.

Electronic DP Level system solves many of the problems that are traditionally seen when making a DP measurement on tall vessels or towers. Typical problems are:

  • Mechanical installation constraints : two remote seals + capillaries
  • Ambient Temperature effect on the capillaries (fill fluid dilatation/contraction and density variation) results of inaccuracy : insulation or heating tracing of capillaries
  • Plugging condensation/evaporation of reference column
  • Tall measurement range (e.g. above 6 metre).

Note: One of the two sensors calculates the DP and transmits it back to the host system.


1.Level measurement

DP Transmitter signal variation should be directly proportional to the level variation. HP and LP chamber location (i.e. vessel vs dry/wet legs side) should be studied accordingly. Differential pressure transmitters installed above or below the liquid level range or with dry/wet legs may require a zero shift.

For slurry and/or sludge application extended diaphragm may be used. This would eliminate the dead‐ended cavity typically present in the nozzles installed with standard diaphragm seal and minimize error to measurement. The drawback of this is no isolation valves could be installed due to the extended diaphragm inside the nozzle. Total shutdown and isolation of the tank or vessel may be required to remove the diaphragm for maintenance.

2. Range

Ranges for differential pressure transmitters should be calculated using the minimum following information:

  • exact distance between the vessel nozzles
  • specific gravity of liquid in vessel according to the temperature and pressure ranges
  • specific gravity of upper and lower fluid for transmitters in interface service
  • specific gravity of liquid in the reference leg (if applicable)
  • Instrument elevation in relation to higher and lower tapping points
  • at high operating pressures, zero compensation for gas phase weight/density.
3. Process connection with no diaphragm

Differential pressure transmitters with no remote capillaries seals should have their location as follows:

  • keep the impulse tubing as short as possible
  • for liquid tapping connection, slope the impulse tubing at least 1 in./foot (8 cm/m) upwards from the differential pressure transmitter towards the vessel connection
  • for gas tapping connection, slope the impulse tubing at least 1 in./foot (8 cm/m) downwards from the differential pressure transmitter towards the process connection
  • avoid high points in liquid lines and low points in gas lines
  • make sure both impulse legs are the same temperature
  • use impulse tubing large enough to avoid friction effects and clogging
  • prevent sediment deposits in the impulse tubing
  • select instrument manifolds with front facing process connections to avoid pockets in the hook up.

On duties that are fouling, a purge to keep the system clear should be used. The purges work with a constant pressure delivered using a rotameter or other similar system, typically with an inert gas.

4. Process connection with diaphragm

Differential pressure transmitters with diaphragm seals and capillaries should be considered taking into account the following consideration.

When flange reducer is necessary, due to a smaller process connection (25 mm or 50 mm) compared to the 75 mm diaphragm, it is recommended to use flushing and draining connections.

Pressure Transmitter Process connection with diaphragm

Figure –  Process connection with diaphragm

In case there is a risk of freezing liquid in the chamber of the flange adapter/reducer or a high viscosity heat tracing or heating circuit should be considered. Heating medium (steam/oil) should not exceed the fluid boiling point.

Process temperature and ambient temperature should be considered to avoid the fluid boiling or affecting the measurement reaction time (in case of higher fluid viscosity). In a vacuum application this may cause the fluid to reach the boiling point and consequently to blow up the diaphragm and destroy it.

DP Steam heating facilities

Figure – DP Steam heating facilities

Diaphragm material should be carefully selected according to the fluid properties (e.g. gold plated in presence of hydrogen or subject to hydrogen permeation).

Flange connection should be selected according to the piping/vessel code.

DP SM or RTJ diaphragm flanges

Figure – DP SM or RTJ diaphragm flanges

The inner volume of capillary fluid can affect the measurement accuracy and response time. Capillary with internal 1 mm diameter will minimize the effect of temperature variation but will increase the response time. Capillary with internal 2 mm diameter will decrease the response time but will more affected by the fluid dilatation.

Seal fluids compatibility with the line process fluids should be reviewed to confirm it is suitable and prevent contamination of the process stream (e.g. oxygen service).

In the presence of wax, slurries, clogs flushing rings should be considered. Flushing rings should be fitted with vent and drain facilities. Diaphragm seals should include isolation features to enable maintenance.

DP Transmitter Flushing

Figure – DP flushing/draining


Capillary filled seal is sensitive to ambient temperature variation. Protection such as insulation shield, protective cover or installation facing the North should be considered.

Diaphragm seals should have facility to maintain, remove, vent and drain (e.g. isolation features).

The flange fitting should be installed in a vertical orientation. Diaphragm and capillary should be installed in a vertical position.

DP Transmitter capillary protection

Figure – DP capillary protection

The differential pressure transmitter should be mounted below the lowest level to be measured.

The capillary position should avoid any risks of vaporization.

DP Transmitter Capillary arrangement

Figure – DP Capillary arrangement

Capillaries should have a minimum radius of curvature of 150 mm, any vibration or friction should be avoided.

Diaphragm seals should be properly handled in the field to avoid damage to diaphragm seal and capillary tubing and potential loss of sensing fill fluid.

Calibration and configuration

Calibration should be performed at Product Manufacturer premises and verified prior to the commissioning activities. Calibration certificate should be provided.

Calibration may be performed in situ using a field pressure calibrator or using a calibrator bench (e.g. for diaphragm seal or specific low pressure).

Getting started, zero adjustment, scale with seals

The oil column height should be taken into account to a zero offset:

  • When the tank is empty, the sensor measure the LP weight of the oil column, this value causes a zero offset in the negative, which can be adjusted at the span calibration or during the commissioning by a zeroing with empty vessel.
  • Delta scale (or span) by calculation, will normally be adjusted relative to the distance between flanges and to the density of the liquid, then add to the calculated zero value. Or it can be done with full vessel.
  • Zero offset can be therefore even more important than the scale itself (especially with the fluorinated oil). We should choose a sensor whose extent of adjustment allows the zero offset.
  • For density or multi‐layer measurements, it will be necessary to make the zero by adjustment with 100% of the lightest liquid, full scale will be calculated with 100% of the heaviest liquid.
Getting started, zero adjustment, scale with impulse line dry leg on low pressure side (LP)

LP side (upper connection), column vented to vessel atmosphere:

  • When the tank is empty, the sensor measure zero on both side (HP and LP), then DP (differential pressure) is also zero = 4 mA.
  • When the tank is full, measurement is height multiply by density, calibration is done for 100%, In case it is possible to know an intermediate level, calibration could be done for another percentage.
  • Most important thing: “This required that liquid should not arrive into the upper connection, or the dry leg will be filled and this will make a offset of the zero”.
  • For density or multi‐layer measurements, height should be constant, then using a dry leg is clearly not possible, measurement should be done in a vessel which works with an overflow.

Source : International Association of Oil & Gas Producers

Acknowledgements : IOGP Instrumentation and Automaton Standards Subcommittee (IASSC), BG Group, BP, Endress + Hauser, Emerson, Honeywell, Krohne, Petrobras, PETRONAS Carigali Sdn Bhd, Repsol, Siemens, Statoil, Total, Vega, Yokogawa.

Level Instruments Design Rules


The following points are considered during level sensors selection or in time of plant design.

They are

  1. Pressure Vessel Connection
  2. Vessel bottom connection
  3. Range selection
  4. Material
  5. Environment
  6. Standpipes vs. sensor cages
  7. Level sketches
  8. Data
  9. Emulsion
  10. Calibration
  11. Stilling wells
  12. Centring disk
  13. Control and safety
  14. Heat tracing
  15. Maintenance access
  16. Service capabilities

1. Pressure Vessel Connection

Level instrument tapping on vessel outlet piping are not recommended. API RP 551 (reaffirmed in 2007, section 3.2.4) provides guidance with respect to a dynamic flow connection.

Level measurement instruments should be isolatable for maintenance, dismantling/removal and calibration without affecting production, except when stopping the production for such activities is deemed acceptable.

Level instruments may be internally or externally mounted. They should be provided with individual isolation facilities allowing for sensor removal, and cage/chamber cleaning (see Figure 1).

As far as possible, the measurement in the sensor cage/chamber should be representative of the actual level in the vessel. To display a representative level, this might require additional tapping/connection on the vessel.

Isolation valves ID (e.g. used for DP or Radar) should be the same as the nozzle ID.

Level instrument flange should be designed in accordance with the piping/vessel code and material. Flange facing should be free of any coating/insulation and suitable for receiving the piping/vessel gaskets.

Note: Irrespective of the location of instrument nozzles, the weld edge distances requirement from ‘Piping’ Code such as ASME VIII and/or PD 5500 Vessels should have adequate access/distance for constructability and inspection.

Level Sensor Pressure vessel mounting

Figure 1 Pressure vessel mounting principle

2. Vessel Bottom Connection

Connections to bottom vessel heads should be avoided since exact positioning is difficult, dead legs are created and often the vessel skirt has to be penetrated.

3. Range Selection

The normal operating/alarm/trip settings should be defined by a combination of process/vessel/instrument operational limits.

Measurement ranges used for process control system (LT for BPCS and LG) and safety instrumented systems (LT for SIS) should generally have the same range and process tapping elevations to allow for continuous monitoring of any discrepancy between various measurements. However, if for accuracy or sensitivity reasons this cannot be achieved, then the process control system measurement range should cover the safety instrumented systems measurement range.

The definition of alarm and trip levels should be reviewed between the process, piping and instrumentation specialists to ensure feasibility. Appropriate (minimum) differential between the alarm and the trip should be considered.

The measuring range should be sketched/defined as in Figure 2:

Level Sensor Range selection

Figure 2 Measuring range

4. Material

All materials used for the level measurement should be selected according to the equipment (e.g. piping, vessel, tank…) and the process fluids.

Unless otherwise specified, wet parts of instrumentation devices (displacer, float, diaphragm..) should be minimum AISI 316 or 316 L SS.

Special care should be taken for selection of material (e.g. gold plated membrane) in contact with low molecular mass fluid or if hydrogen permeation is expected.

Material of housing should be AISI 316 or 316 L SS for offshore. Alternatively, other materials such as A365 grade aluminum (epoxy painted) or GRP may also be used.

5. Environment

All devices used for level measurement should be suitable for their environmental conditions. This applies to the temperature, humidity, electromagnetic compatibility, ingress protection as well as the hazardous area. The relevant certification should be provided.

Weight and available space constraints particularly to allow removal of the instrument should be taken into account when selecting a level measurement technology.

Level Instruments should be assessed for the extremes of weather protection including sunshades and protection boxes as required.

6. Standpipes vs. Sensor cages

The terms standpipe and sensor cages are often mixed up. In order to clarify, the following definitions are used:


This is an external extension of the pressure vessel, to which multiple instruments can be connected. A standpipe should follow the pressure vessel code. Usually no instrument is installed inside the standpipe itself. Isolation valves may be used between vessel and standpipe (as per API RP 551 reaffirmed 2007, Figure 12).

Each instrument connected should have its own isolation valves, vents and drain to facilitate maintenance. The distance between the standpipe and the vessel nozzle should not exceed 1 – 1.5 m. Long connections could potentially cause temperature gradients, formation of hydrate and reduction in level coupling between the standpipe and the vessel (refer to API RP 551 reaffirmed 2007, Figure 12).

Sensor cage/chamber

This is an individual cage/chamber in which the level sensor is installed, part of a single level instrument. The sensor cage/chamber can be attached either directly to the pressure vessel or to a standpipe. The sensor cage/chamber should have dedicated process isolation, vent and drain valves provided.

Drain valves should be installed at the bottom connection of the sensor cage and provisions should be made for the appropriate disposal of the drained material. Vent valves are provided to allow depressurization of the instrument prior to draining. In toxic services, drains and vents should be piped away from the instrument to a safe area or disposal system.

Care should be taken to reduce the temperature gradient between the vessel and the Standpipe/bridle/sensor cage/chamber.

Standpipes, bridles, sensor cages and chambers can be reviewed and assessed for thermal insulation and trace heating requirements.

7. Level Sketches

Level sketches should be prepared at an early stage of the engineering. Level sketches should include details related to nozzle sizes and heights, vessel internal and external supports, material, sensor/source location, etc. in line with the Product Manufacturer recommendation.

Level sketches should indicate all level related instruments (transmitters, gauges, switches) for all applications (i.e. BPCS and/or SIS) with tapping connections and normal operating/alarm/trip settings.

Level sketches should describe level threshold in both ‘length’ and ‘%’ measured range.

Note: Sufficient clearance to facilitate level instrument and chamber/cage draining to a safe location/closed drain system should be incorporated into the design.

8. Data

Process data should be carefully addressed with all detailed fluid features as well as the different layers to be measured.

From the design perspectives the BPCS level measurement uncertainty should be better than +/− 5% of the reading and the SIS level measurement uncertainty should be better than +/− 2% of the reading.

For floating facilities (e.g. FPSO), the design should take into account the vessel motion (e.g. pitch, roll) which can influence the measurement range and technology selection. Level sketches should integrate the margins of range and thresholds due to the vessel motion.

Local radiation safety requirements, local radio frequency requirements and operational requirements (radiography) and how such events are managed together with environmental data should be taken into account.

Process data

For each level instrument the minimum following process data range (e.g. min, max, operating, and design) should be defined:

  • process data (e.g. density/SG, temperature, pressure, dielectric, viscosity…)
  • specific service (e.g. corrosive, cryogenic…)
  • level measurement requirements (e.g. safety or control application, alarms and trip values…)
  • presence of other nucleonic isotopes in the fluid
  • presence of salt
  • presence of oil‐film/build‐up
  • presence of sand/water/emulsion/oil/foam
  • the type and name of the substance/process fluid to be measured.

Level data

For each level measurement, the following data should be defined as a minimum:

  • available nozzle diameter and flange connection
  • vessel internal and external arrangements and layouts
  • vessel material composition and wall thickness
  • hook‐up, location in the vessel and installation
  • nucleonic source and detectors calculation note(s)
  • nucleonic source intensity
  • geometry, distance and location of nucleonic sources and detectors
  • operation and maintenance manuals as well as particular instructions (e.g. adjustment and calibration interventions)
  • calibration procedure (in factory and in operation)
  • additional screening around final nucleonic source container installation location
  • dip pipe features (material, thickness, flange diameter…)
  • procedures for handling and storage
  • certificates of licensing and regulatory requirements…

9. Emulsion

An emulsion is a mixture of two or more liquids that are normally immiscible. Emulsions are part of a more general class of two‐phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion should be used when both the dispersed and the continuous phase are liquids. As an example, oil and water can form, first, an oil‐in‐water emulsion, wherein the oil is the dispersed phase, and water is the dispersion medium. Second, they can form a water‐in‐oil emulsion, wherein water is the dispersed phase and oil is the external phase. Multiple emulsions are also possible, including a ‘water‐in‐oil‐in‐water’ emulsion and an ‘oil‐in‐water‐in‐oil’ emulsion.

Particular attention should be paid to any interface measurement in the presence of emulsion. Density of water in oil emulsion will change depending on water fraction. At high water content (approximately 80% of water‐in‐oil) the density is comparable to the water. Then density decreases, not necessary linearly, with the decreasing of the volume fraction of water. At a low water content (approximately 20% of water‐in‐oil), the density drops sharply to the oil value. This means that the emulsion density is not constant. However, the emulsion density can be seen as the average of the oil and the water density. Dielectric or conductivity values of the emulsion follow the same principle of non‐linear dispersion. This implies that the emulsion cannot be ‘seen’ easily as a single two fluids interface.

Note: The foam also is a non‐uniform fluid; density, dielectric and conductivity parameters vary in a stochastic manner.

An emulsion layer at the interface of two fluids may or may not be seen by the instrument depending upon the hook‐up arrangement. When using a sensor cage/stand pipe, the following can be considered:

Figure 3: the interface level device in the sensor cage/stand pipe does not ‘see’ the emulsion in the vessel, so the measured value only represents the average interface level

Figure 4: the interface level device in the sensor cage/standpipe ‘sees’ the emulsion layer so the measured value represents accurately the interface level

Interface level with emulsion

Figure 3 (Left) :  Interface level with no emulsion representativeness

Figure 4 (Right) : Interface level with emulsion representativeness

Note: the accuracy of the interface level measurement in the presence of the emulsion will depend upon the number of nozzles provided, the selected level sensor technology, the correct specification of the technical data, i.e. SG and thorough commissioning and calibration of the instrument. The number of nozzles that can be provided on a tank or vessel is often limited due to the space ant the mechanical integrity of the vessel or tanks. Thus, if accurate level measurement is required in the presence of emulsion, a direct top mounted technology level should be considered.

10. Calibration

Calibration should be performed prior to the factory acceptance test and prior to shipment. A calibration certificate should be provided that detail the traceability of the test equipment used. Site calibration should be performed to ensure the factory calibration has not deteriorated during transportation and any site specific requirements are accommodated. Any shipping stops, seals, plastic fitments, temporary grommet seals or guides to ensure safe shipping should be removed prior to fitting and mechanical completion.

Particular attention should be paid to calibration of Radar, GWR, Capacitance and Nucleonic instruments. The lower range value should be calibrated without any process fluids, but as far as possible with all vessel utilities (e.g. energy for electro dehydrators) present. This should take into account any signal noise/disturbance. The higher range value should be calibrated with the maximum fluid level to be measured. Special tools should be provided. Any special sensor or probe coating should not influence the calibration.

Arrangement for in‐line calibration and flushing of the instrument is recommended.

Onsite verification should be completed for instruments assigned as part of a LOP, e.g. SIS and critical alarms.

11. Stilling wells

A stilling well is a perforated pipe to allow free movement of fluid. This pipe is equipped with a top mounted flange which is supported at the bottom of the vessel. For a long still well, support should be provided along its length; however these supports should not affect the measurement.

Stilling wells provide a stable gauge reference point (limit vertical movement), and provide a relatively ‘quiet’ product surface during filling and emptying of the vessel, especially if ‘swirl’ exists.

Stilling wells may act as a ‘wave guide’ for the radar energy. The well helps to concentrate the emitted signal and minimize the signal loss. The loss of signal is generally due to a low product reflectivity (caused by a low dielectric constant) or surface phenomena like ‘boiling off’ and ‘vapour mist’.

Stilling wells should not be used with viscous fluid, dirty fluid or fluid‐film‐buildup. Stilling well should be one piece from the top to the bottom (i.e. no gaps).

The following features should be considered for stilling well:

  • AISI 316 SS minimum with smooth roughness  ≤  6.3 μm (no welding parts)
  • one piece from the nozzle flange with constant diameter.

Stilling wells slot width/holes diameter should generally be 1/10 of the stilling well diameter with a minimum of 0.635 cm. Spacing between slots/holes should minimum be 15 cm.

Slots/holes should be deburred and their quantity minimized. Holes shape may be slotted or circular. Holes should be on both sides of the stilling well, in order to minimize the risk of plugging especially for waxing service.

Stilling well diameter should be minimum 20 cm (as per API MPMS § 3.1A).

Stilling well design and construction should be approved by the Product Manufacturer.

12. Centring disk

Centring disk used in stilling wells should be compatible with the fluid properties (build‐up, viscosity…) and mounted outside the measuring range. Consideration should be taken into account when using a weighted bottom mounted on the rod instead of using centring disks. Centring disks should be provided as per the Product Manufacturer recommendation.

13. Control and safety

Level measurements should be designed to ensure that the likelihood of common cause, common mode and dependent failures between monitoring, control or safety protection layers are addressed.

This design should consider the following:

  • independency between protection layers
  • diversity between protection layers
  • physical separation between different protection layers
  • common cause failures between protection layers.

As such, differing measurement principles are recommended for control and safety functions.

With reference to ISO 10418 issue 2003, § 6.2.9.

“The two levels of protection shall be independent of, and in addition to, the control devices used in normal process operation” it is suggested to change the recommendation to a requirement for separate nozzles.”

The safety function should provide a reliable and sufficiently fast detection of process upsets. Since the control function can both work as back‐up as well as comparison of the safety function equal performance is recommended for control (accuracy and trip point should be considered).

If shutdown measurements require input from other variables (e.g. temperature and pressure) to calculate the correct value, these inputs should be separate for control and shutdown functions.

It should not be possible to inadvertently isolate instrumentation for shutdown functions from the process.

Level instrumentation used on process vessels, should be designed so that one of the level instruments used for control and safety, should not be affected by radioactive disturbance from tracers, scale and x‐rays.

Level gauge is recommended for the entire measuring range. Level gauges are used for local operation and as reference to level instrumentation.

If multiple level devices are required (e.g. one device for control and second device for alarm, or potentially several devices as part of a SIS), the use of diverse level technologies should be assessed.

Consideration should be given to comparison of different devices used on the same duty, with cross comparison and alarms function from a deviated percentage, i.e. 5%.

14. Heat tracing

All instrument nozzles should be located such that the risk of blockage and solidification in the nozzle is minimized. If there is risk of hydrate formation or freezing in the instrument nozzles or instrument impulse lines, application of heat tracing should be considered.

Note, however, that there may be safety requirements connected with the heat tracing, i.e. hazardous area equipment requirements or over temperature protection.

15. Maintenance access

All level instruments should be designed for long term stability and operation. Intervals for planned production stops are normally two years or longer.

Relief/drainage tubing or pipe should be routed to a safe location according to area requirements, i.e. to a safe location/closed drain system.

Maintenance operation should take into account the hazardous area certification type e.g. Ex ia/ib, Ex d…

Level instruments do not normally require readability from deck. Level gauges or indicators should be readable from deck or permanent platform.

Isolation valves should be available for operation.

16. Service capabilities

During the design phase it is recommended to include the Product Manufacturer in the proper design, construction and installation of the facilities (e.g. level sketch, hook‐up, stilling wells).

At site, the Product Manufacturer should have the capability of assisting in the commissioning and start‐up activities, providing specific training, performing site calibration and issuing specific detailed maintenance procedure for equipment cleaning and replacement.

The Product Manufacturer should provide a comprehensive spares listing, part numbers and the time taken to expedite basic consumable items.

The Product Manufacturer should provide obsolescence plan to indicate spare part availability for each model for users to plan for upgrading or stocking plan as appropriate. Life cycle cost (i.e. total cost of ownership) may be evaluated for selection of measurement technology.

These spares should be added to the maintenance management systems that logs and details the installed device on the specific site and installation.

Abbreviations :

  • BPCS : Basic Process Control System
  • LT Level Transmitter
  • LG Level Gauge
  • DP Differential Pressure
  • ID Internal Diameter
  • SIS Safety Instrumented System
  • SS Stainless Steel
  • FPSO Floating, Production, Storage and Offloading

Source : International Association of Oil & Gas Producers

Acknowledgements : IOGP Instrumentation and Automaton Standards Subcommittee (IASSC), BG Group, BP, Endress + Hauser, Emerson, Honeywell, Krohne, Petrobras, PETRONAS Carigali Sdn Bhd, Repsol, Siemens, Statoil, Total, Vega, Yokogawa.

PLC Program for Alternate Output Circuit


This is PLC Program for an Alternate output circuit (with latched function).

Problem Description

Setting the light ON by pressing a SWITCH on 1st time, 3rd time, 5th time etc. and setting the same light OFF by pressing the SWITCH by 2nd time , 4th time, 6th time etc.

Restore the output status to 0 when system or cycle power up. Output can be START by pressing a BUTTON in ODD number of times and can be STOP by pressing the same BUTTON by EVEN number of times.

Problem Diagram

PLC Program for an Alternate output circuit

Problem Solution

We can solve this problem by using simple Ladder logic. In this we consider one simple example of alternate LED operation.

Here we consider one LED and one BUTTON. Press the BUTTON alternately and output should be ON/OFF alternately, here during the Button pressed odd number times then output should be ON and during the button pressing even number of times then the output should be OFF.


Here is PLC program for alternate output circuit (with latched function).

List of Inputs/Outputs

Inputs List:-

Output List:-

M Memory:-

  • M0.0 for LED reset condition
  • M0.1 for counter reset
  • M11.0 & M11.1 – Positive edge

Ladder diagram for alternate output circuit (with latched function)

Program Description

In this application we have used Siemens S7-300 PLC and TIA Portal Software for programming.

Network 1: In network 1 we have used SET instruction to set the LED (Q0.0). Here we have taken NO contact of BUTTON (I0.0) so LED (Q0.0) can be activated by pressing BUTTON (I0.0).

Network 2: Here we used a counter so it will count the switching times of the BUTTON (I0.0). This counter will tell us about the number of times the button is pressed, its value or the value is a EVEN number or ODD number.

Network 3: When counter will reach its preset value (2) or say EVEN number of times, NO contact of the counter will set the M0.0 (LED reset condition).

Network 4: In this network NO contact of the M0.0 will RESET the LED and counter. Here M0.1 (counter reset memory) will RESET the counter.

Network 5: If M0.0 is ON and negative transition (from 1 to 0) of button (I0.0) will be triggered then RESET condition of LED will be OFF.

Note : This example is provided to understand the basic concept of alternate output circuit, it is not full application but we can use this concept in any automation application or any system.

Test Cases

PLC Program Test Cases

Article by
Bhavesh Diyodara

Fire Extinguishers Mock Test – Set 1


Fires that are fueled by ______ require you to use water fire extinguishers in order to fight them.



Pilot Valves and Pneumatic Amplifying Relays


Self-balancing mechanisms consisting solely of a baffle/nozzle detector coupled to a feedback bellows, while functional, are not always practical as field instruments. Nozzles and orifices must be made rather small in diameter in order to minimize compressed air usage (Note 1), but this means the mechanism will require significant time to alter its output pressure (i.e. to fill and empty the bellows and associated air tubing). Such a pneumatic mechanism will be impractically slow if connected to a remote indicating instrument through a long run of tubing, owing to the relatively large volume of that tube.

Note 1 : Compressed air is a valuable commodity because much energy is required to compress and distribute high-pressure air. Every pneumatic instrument’s nozzle is essentially a “leak” in the compressed air system, and the combined effect of many operating pneumatic instruments is that the air compressor(s) must continually run to meet demand.

A practical solution to the compromise between air consumption and responsiveness inherent to simple baffle/nozzle/bellows mechanisms is to boost the nozzle back-pressure (and volume) using a pneumatic “amplifier” device. With a pneumatic amplifier in place, the detector (baffle/nozzle) need not leak great quantities of compressed air, since the amplifier will provide the volume boost necessary to quickly fill (and vent) the bellows and signal tubing.

The design challenge for us, then, is how to construct such a pneumatic amplifier: a mechanism to amplify small pneumatic signal changes into larger pneumatic signal changes. In essence, we need to find a pneumatic equivalent of the electronic transistor : a device that lets a small signal control a larger signal.

First, let us analyze the following pneumatic mechanism and its electrical analogue (as shown on the right):

Pneumatic Instruments Mechanism

As the control rod is moved up and down by an outside force, the distance between the plug and the seat changes. This changes the amount of resistance experienced by the escaping air, thus causing the pressure gauge to register varying amounts of pressure. Moving the control rod up opens the variable restriction formed by the plug and seat, venting air more easily and decreasing the output pressure. Moving the control rod down closes off the vent, causing output pressure to rise. These up-and-down rod motions are analogous to the variable resistor decreasing and increasing resistance, respectively, causing the output voltage to change in direct relation to the variable resistance.

There is little functional difference between this mechanism and a baffle/nozzle mechanism. Both work on the principle of one variable restriction and one fixed restriction (the orifice) “dividing” the pressure of the compressed air source to some lesser value.

The sensitivity of this pneumatic mechanism may be improved by extending the control rod and adding a second plug/seat assembly. The resulting mechanism, with dual plugs and seats, is known as a pneumatic pilot valve. An illustration of a pilot valve is shown here, along with its electrical analogue (on the right):

Pneumatic Instruments Mechanism - 1

As the control rod is moved up and down, both variable restrictions change in complementary fashion. As the control rod moves up, the upper restriction closes off (restricting supply air) while the lower restriction opens up (venting more), causing the output pressure signal to decrease. As the rod moves down, the upper restriction opens (allowing more supply air in) and the lower restriction closes off (venting less), causing the output pressure to rise. The combination of two restrictions changing in opposite direction results in a much more aggressive change in output pressure for a given amount of rod motion than the previous mechanism with its fixed restriction and single variable restriction.

A similar design of pilot valve reverses the directions of the two plugs and seats. The only operational difference between this pilot valve and the previous design is an inverse relationship between control rod motion and pressure:

Pneumatic Instruments Mechanism - 2

Now, moving the control rod up increases pressure while moving it down decreases pressure: precisely opposite the action of the previous pilot valve.

At this point, all we’ve managed to accomplish is build a better baffle/nozzle mechanism. We still do not yet have a pneumatic equivalent of an electronic transistor. To accomplish that, we must have some way of allowing an air pressure signal to control the motion of a pilot valve’s control rod.

If we add a diaphragm to the pilot mechanism, we will create a proper pneumatic relay. The following relay and its electronic analogue are shown here:

Pneumatic Instruments Mechanism - 3

The diaphragm is nothing more than a thin disk of sheet metal, upon which an incoming air pressure signal presses. Force on the diaphragm is a simple function of signal pressure (P) and diaphragm area (A), as described by the standard force-pressure-area equation:

F = PA

If the diaphragm is taut, the elasticity of the metal allows it to also function as a spring. This allows the force to translate into displacement (motion), forming a definite relationship between applied air pressure and control rod position. Thus, the applied air pressure input will exert control over the output pressure.

It is easy to see how the input air signal exerts control over the output air signal in these two illustrations:

Pneumatic Instruments Mechanism - 4

Since there is a direct relationship between input pressure and output pressure in this pneumatic relay, we classify it as a direct-acting relay. If we were to add an actuating diaphragm to the first pilot valve design, we would have a reverse-acting relay as shown here:

Pneumatic Amplifying Relays

The gain (A) of any pneumatic relay is defined just the same as the gain of any electronic amplifier circuit, the ratio of output change to input change:

A = ΔOutput/ΔInput

For example, if an input pressure change of Δ2 PSI results in an output pressure change of Δ12.9 PSI, the gain of the pneumatic relay is 6.45.

Whether or not a pneumatic relay provides a pressure gain, it is guaranteed to provide a volume gain which is necessary to make pneumatic field instruments practical. Note how the diaphragm chamber where the input pressure goes is sealed off: this means there will be no continual draw (or leakage) of input signal air volume. Any pneumatic sensing element sending a pressure signal to the input of a pneumatic relay will not be “loaded” by the relay. The relay, on the other hand, is able to supply or vent a continual flow of air at its output port as needed.

Just as a transistor amplifier circuit presents a light load to the input signal and a comparatively “heavy” source/sink capacity to any load connecting to its output terminals, pneumatic relays similarly boost the volume capacity of a pneumatic signal. Recall that this was precisely our goal for increasing the responsiveness of a baffle/nozzle mechanism: to have a pneumatic amplifier capable of boosting the nozzle’s back-pressure signal.

Adding a pneumatic pressure-amplifying relay to a force-balance system such as our hypothetical laboratory scale improves the performance of that pneumatic system in multiple ways:

Pneumatic Amplifying Relays - 1

The pressure gain of the pneumatic amplifying relay makes the force-balancing bellows respond more aggressively to changes in baffle position than it could on its own. This makes the scale more sensitive, better able to sense small changes in applied weight.

The volume gain of the pneumatic amplifying relay results in greatly decreased response time to changes in applied weight. Without a relay in the system, the rate at which the force-balance bellows fills and empties with compressed air is a direct function of the orifice’s and nozzle’s restrictiveness, respectively. Nozzles and orifices designed for high restriction (small diameters) work well to conserve air usage over time, but they also limit the rate of air flow in or out of the feedback bellows. With an amplifying relay in place, however, we get the best of both worlds: the nozzle and orifice bores may be minimized for minimum air consumption, while the relay’s valves may be made large enough to ensure high flow capacity to and from the bellows for quick response.

It should be noted that the principles of self-balancing mechanisms, baffles and nozzles, amplifying relays, and the like are not limited to pneumatic systems. It is also possible to build self-balancing hydraulic systems using all the same principles, the only difference being the use of liquid (oil) as the working fluid instead of gas (air). An example of a force-balance hydraulic system is the ASCO model NH90 “Hydramotor” linear actuator, which uses a self-contained hydraulic pump and reservoir to provide pressurized oil for the mechanism, a baffle/nozzle mechanism to detect out-of-balance conditions, and a hydraulic amplifying relay to boost the nozzle back-pressure signal to perform useful work through a hydraulic actuating cylinder.

The Foxboro corporation designed a great many of their pneumatic instruments using just one style of (highly sensitive) amplifying relay:

Pneumatic Amplifying Relay Principle

The motion of the diaphragm actuates a pair of valves: one with a cone-shaped plug and the other with a metal ball for a plug. The ball-plug admits supply air to the output port, while the cone-shaped “stem valve” plug vents excess air pressure to the vent port.

The following photograph shows one of these relay units:

FoxBoro Pneumatic Amplifying Relay

Two slotted-drive screws attach the relay to the rest of the controller mechanism, while two smaller (Phillips-drive) screws hold the relay assembly together.

The Fisher corporation used a different style of amplifying relay in some of their legacy pneumatic instruments:

amplifying relay

The following photograph shows one of these relay units (colored black) attached to the back of a model 546 I/P transducer (colored grey):

IP transducer

The pressure gain of this Fisher relay is much less than that of the Foxboro relay, since output pressure in the Fisher relay acts against input pressure by exerting force on a sizable diaphragm. The movable vent seat in the Fisher relay makes this design a “non-bleeding” type, meaning it possesses the ability to close both supply and vent valves at the same time, allowing it to hold an output air pressure between saturation limits without bleeding a substantial amount of compressed air to atmosphere through the vent. The Foxboro relay design, by contrast, is a “bleeding type,” whose ball and stem valves cannot close simultaneously, and thus always bleeds some compressed air to atmosphere so long as the output pressure remains somewhere between saturation limits.

Self-balancing Pneumatic Instrument Principles


A great many precision instruments use the principle of balance to measure some quantity. Perhaps the simplest example of a balance-based instrument is the common balance-beam scale used to measure mass in a laboratory:

Self-balancing Pneumatic Instrument Principles

A specimen of unknown mass is placed in one pan of the scale, and precise weights are placed in the other pan until the scale achieves a condition of balance. When balance is achieved, the mass of the specimen is known to be equal to the sum total of mass in the other pan. An interesting detail to note about the scale itself is that it has no need of routine calibration. There is nothing to “drift” out of spec which would cause the scale to read inaccurately. In fact, the scale itself doesn’t even have a gauge to register the mass of the specimen: all it has is a single mark indicating a condition of balance. To express this more precisely, the balance beam scale is actually a differential mass comparison device, and it only needs to be accurate at a single point: zero. In other words, it only has to be correct when it tells you there is zero difference in mass between the specimen and the standard masses piled on the other pan.

The elegance of this mechanism allows it to be quite accurate. The only real limitation to accuracy is the certainty to which we know the masses of the balancing weights.

Imagine being tasked with the challenge of automating this laboratory scale. Suppose we grew weary of employing a lab technician to place standard weights on the scale to balance it for every new measurement, and we decided to find a way for the scale to balance itself. Where would we start? First we would need some sort of mechanism to sense when the scale was out of balance, and another mechanism to apply more or less downward force on the other pan whenever an out-of- balance condition was detected.

The baffle/nozzle mechanism previously discussed would suffice quite well as a detection mechanism: simply attach a baffle to the end of the pointer on the scale, and attach a nozzle adjacent to the baffle at the “zero” position (where the pointer should come to a rest at balance). Such a mechanism might look like this:

Self-balancing Pneumatic Instrument Principles - 1

Now we have a highly sensitive means of indicating when the scale is balanced, but we still have not yet achieved full automation. The scale cannot balance itself, at least not yet.

Suppose now instead of using precise, machined, brass weights placed on the other pan to counter the mass of the specimen, we employ a pneumatically-actuated force generator operated by the back-pressure of the nozzle. An example of such a “force generator” is a bellows: a device made of thin sheet metal with circular corrugations in it, such that it resembles the bellows on an accordion. Pneumatic pressure applied to the interior of the bellows causes it to elongate, the amount of force applied to the bellows’ end being the product of air pressure and the end surface area:


A photograph of a brass bellows unit appears here, taken from a Foxboro model 130 pneumatic controller:


If the bellows’ expansion is externally restrained so it does not stretch appreciably – and therefore the metal never stretches enough to act as a restraining spring – the force exerted by the bellows on that restraining object will exactly equal the pneumatic pressure multiplied by the cross-sectional area of the bellows’ end.

Applying this to the problem of the self-balancing laboratory scale, imagine fixing a bellows to the frame of the scale so it presses downward on the pan where the brass weights normally go, then connecting the bellows to the nozzle back-pressure:

Self-balancing Pneumatic Instrument Principles - 2

Now the scale will self-balance. When mass is added to the left-hand pan, the pointer (baffle) will move ever so slightly toward the nozzle until enough back-pressure builds up behind the nozzle to make the bellows exert an equal and opposing force to re-balance the mechanism. This balancing action is entirely automatic: the nozzle back-pressure adjusts to whatever value is necessary to maintain the pointer in the balanced position, applying or venting pressure to the bellows as needed to keep the system in a condition of equilibrium. What we have created is a negative feedback system, where the output of the system (nozzle backpressure) continuously adjusts to match and balance the input (the applied weight).

This is all well and good, but how does this help us determine the weight of the specimen in the left-hand pan? What good is this self-balancing scale if we cannot read the balancing force? All we have achieved so far is to make the scale self-balancing. The next step is making the balancing force readable to a human operator.

Before we add the final piece to this automated scale, it is worthwhile to reflect on what has been done so far. By adding the baffle/nozzle and bellows mechanisms to the scale, we have abolished the need for brass weights and instead have substituted air pressure. In effect, the scale translates applied weight into a proportional air pressure: air pressure has now become an analogue for specimen weight. What we really need is a way to now translate that air pressure into a human-readable indication of weight.

To make this air pressure readable to a human operator, all we must add to the system is a pressure gauge. The gauge operates on air pressure, but now the air pressure is proportionately equivalent to specimen weight. In honor of this proportionality, we may label the face of the pressure gauge in units of ounces or grams instead of PSI or kPa:

Self-balancing Pneumatic Instrument Principles - 3

It is very important to note how the pressure gauge performs an entirely different function with the feedback bellows in place. With just a baffle-nozzle mechanism at work, the pressure gauge was hyper-sensitive to any motion of the scale’s balance beam – it served only as a highly sensitive indicator of balance. Now, with the addition of the feedback bellows, the pressure gauge actually indicates how much weight is in the specimen pan, not merely whether the scale is balanced or not. As we add more mass to the specimen pan, the gauge’s indication proportionately increases. As we take away mass from the specimen pan, the gauge’s indication proportionately decreases.

Although it may seem as though we are done with the task of fully automating the laboratory scale, we can go a step further. Building this pneumatic negative-feedback balancing system provides us with a capability the old manually-operated scale never had: remote indication. There is no reason why the indicating gauge must be located near the scale. Nothing prevents us from locating the receiver gauge some distance from the scale, and using long lengths of tubing to connect the two:

Self-balancing Pneumatic Instrument Principles - 4

By equipping the scale with a pneumatic self-balancing apparatus, we have turned it into a pneumatic weight transmitter, capable of relaying the weight measurement in analog pneumatic form to an indicating gauge far away. This is the basic force-balance principle used in most pneumatic industrial transmitters to convert some process measurement into a 3-15 PSI pneumatic signal.

Pneumatic Sensing Elements


Most pneumatic instruments use a simple but highly sensitive mechanism for converting mechanical motion into variable air pressure: the baffle-and-nozzle assembly (sometimes referred to as a flapper and- nozzle assembly). A baffle is nothing more than a flat object obstructing the flow of air out of a small nozzle by close proximity:

baffle-and-nozzle assembly

flapper and Nozzle relation

The physical distance between the baffle and the nozzle alters the resistance of air flow through the nozzle. This in turn affects the air pressure built up inside the nozzle (called the nozzle back-pressure). Like a voltage divider circuit formed by one fixed resistor and one variable resistor, the baffle/nozzle mechanism “divides” the pneumatic source pressure to a lower value based on the ratio of restrictiveness between the nozzle and the fixed orifice.

This crude assemblage is surprisingly sensitive, as shown by the graph. With a small enough orifice, just a few thousandths of an inch of motion is enough to drive the pneumatic output between its saturation limits. Pneumatic transmitters typically employ a small sheet-metal lever as the baffle. The slightest motion imparted to this baffle by changes in the process variable (pressure, temperature, flow, level, etc.) detected by some sensing element will cause the air pressure to change in response.

The principle behind the operation of a baffle/nozzle mechanism is often used directly in quality control work, checking for proper dimensioning of machined metal parts. Take for instance this shaft diameter checker, using air to determine whether or not a machined shaft inserted by a human operator is of the proper diameter after being manufactured on an assembly line:

baffle nozzle mechanism principle

If the shaft diameter is too small, there will be excessive clearance between the shaft and the inside diameter of the test jig, causing less air pressure to register on the gauge. Conversely, if the shaft diameter is too large, the clearance will be less and the gauge will register a greater air pressure because the flow of air will be obstructed by the reduced clearance.

The exact pressure is of no particular consequence to the quality-control operator reading the gauge. What does matter is that the pressure falls within an acceptable range, reflecting proper manufacturing tolerances for the shaft. In fact, just like the 3-15 PSI “receiver gauges” used as pneumatic instrument indicators, the face of this pressure gauge might very well lack pressure units (such as kPa or PSI), but rather be labeled with a colored band showing acceptable limits of mechanical fit:

pneumatic pressure signalsThis is another example of the analog nature of pneumatic pressure signals: the pressure registered by this gauge represents a completely different variable, in this case the mechanical fit of the shaft to the test jig.

Although it is possible to construct a pneumatic instrument consisting only of a baffle/nozzle mechanism, this is rarely done. Usually the baffle/nozzle mechanism is just one of several components comprising a “balancing” mechanism in a pneumatic instrument. It is this concept of self-balancing that we will study next.

Pneumatic Instrumentation


In Olden days, before the usage of 4-20mA transmitters in industries, we were using pneumatic transmitters for our process measurement applications. Briefly, in pneumatic transmitter, the compressed instrument air supply is the source to the pneumatic transmitter and the output of the pneumatic transmitter also in the form of air signal like 3 to 15 psi. For example, a pneumatic pressure transmitter indicates zero input pressure as 3 PSI output and 100% input pressure signal as 15 PSI instead of 4-20mA as it is not yet invented. Here we are discussing about olden Pneumatic Transmitters.

While electricity is commonly used as a medium for transferring energy across long distances, it is also used in instrumentation to transfer information. A simple 4-20 mA current “loop” uses direct current to represent a process measurement in percentage of span, such as in this example:

Pressure Transmitter Working Principle

The transmitter senses an applied fluid pressure from the process being measured, regulates electric current in the series circuit according to its calibration (4 mA = no pressure ; 20 mA = full pressure), and the indicator (ammeter) registers this measurement on a scale calibrated to read in pressure units. If the calibrated range of the pressure transmitter is 0 to 250 PSI, then the indicator’s scale will be labeled to read from 0 to 250 PSI as well. No human operator reading that scale need worry about how the measurement gets from the process to the indicator – the 4-20 mA signal medium is transparent to the end-user as it should be.

Air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure transmitter designed to output a variable air pressure according to its calibration rather than a variable electric current. Such a transmitter would have to be supplied with a source of constant pressure compressed air instead of an electric voltage, and the resulting output signal would be conveyed to the indicator via tubing instead of wires:

Pneumatic Pressure Transmitter Principle

The indicator in this case would be a special pressure gauge, calibrated to read in units of process pressure although actuated by the pressure of clean compressed air from the transmitter instead of directly by process fluid. The most common range of air pressure for industrial pneumatic instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process measurement scale and an output pressure of 15 PSI represents the high end of the measurement scale. Applied to the previous example of a transmitter calibrated to a range of 0 to 250 PSI, a lack of process pressure would result in the transmitter outputting a 3 PSI air signal and full process pressure would result in an air signal of 15 PSI. The face of this special “receiver” gauge would be labeled from 0 to 250 PSI, while the actual mechanism would operate on the 3 to 15 PSI range output by the transmitter. As with the 4-20 mA loop, the end-user need not know how the information gets transmitted from the process to the indicator. The 3-15 PSI signal medium is once again transparent to the operator.

Typically, a 3 PSI pressure value represents 0% of scale, a 15 PSI pressure value represents 100% of scale, and any pressure value in between 3 and 15 PSI represents a commensurate percentage in between 0% and 100%. The following table shows the corresponding current and percentage values for each 25% increment between 0% and 100%. Every instrument technician tasked with maintaining 3-15 PSI pneumatic instruments commits these values to memory, because they are referenced so often:

3-15 PSI pneumatic instruments

Pneumatic temperature, flow, and level control systems have all been manufactured to use the same principle of 3-15 PSI air pressure signaling. In each case, the transmitter and controller are both supplied clean compressed air at some modest pressure (20 to 25 PSI, usually) and the instrument signals travel via tubing. The following illustrations show what some of these applications look like:

Example : Biodiesel “wash column” temperature control

wash column temperature control

Example : Flow Control System

Flow Control Loop

Example : Two-element boiler steam drum level control

Two-element boiler steam drum level control

Instruments operating on compressed air, and process measurement signals transmitted as air pressures through long runs of metal tubing, was the norm for industrial instrumentation prior to the advent of reliable electronic instruments. In honor of this paradigm, instrument technicians were often referred to as instrument mechanics, for these air-powered devices were mechanically complex and in frequent need of adjustment to maintain high accuracy.

Back in the days of control room panels populated by rows and rows of pneumatic indicators, recorders, and controllers, clean and organized routing of all the instrument signal tubes was a significant concern. By contrast, electrical wires are relatively easy to organize through the use of marshaling panels and terminal blocks – bundles of tubes (especially metal tubes!) are not. A photograph taken of the upper rear portion of an old control room panel shows a portion of a marshaling board where dozens of bulkhead-style 1/4 inch instrument tube fittings are organized in neat rows (Note 1), where a multitude of pneumatic instrument signal lines once attached:

Note 1 : the staggered layout of the tube fittings, intended to improve access to each one. Remember that the technician used a 9/16 inch wrench to loosen and tighten the tube fitting nuts, so it was important to have working room between fittings in which to maneuver a wrench.

pneumatic instrument signal lines

Each bulkhead fitting bears a numbered tag (Note 2), for easy identification and documentation of tube connections. Loop diagrams of pneumatic control systems documented each bulkhead fitting where an instrument signal passed, in the same way that modern loop diagrams document each terminal block where an electrical signal connection is made.

Note 2 : The numbers are difficult to see here, because the entire panel has been painted in a thick coat of grey paint. This particular panel was stripped of all pneumatic instruments and outfitted with electronic instruments, so the rows of bulkhead fittings no longer serve a purpose, but to remind us of legacy technology.

Pneumatic instruments still find wide application in industry, although it is increasingly rare to encounter completely pneumatic control loops. One of the most common applications for pneumatic control system components is control valve actuation, where pneumatic technology still dominates. Not only is compressed air used to create the actuation force in many control valve mechanisms, it is still often the signal medium employed to command the valve’s position. Quite often this pneumatic signal originates from a device called an I/P transducer, or current-to-pressure converter, taking a 4-20 mA control signal from the output of an electronic controller and translating that information as a pneumatic 3-15 PSI signal to the control valve’s positioner or actuator.