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What is HVAC System ?

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What is an HVAC ?

Acronym HVAC stands for heating ,ventilation and air conditioning. Sometimes Refrigeration “R ” is also added and it becomes “HVACR”.

HVAC is basically climate control of confined space with respect to requirements of persons or goods in it.

HVAC system is not only heating and cooling of air but also concerned with mainatining the indoor air quality (IAQ).

Heating of air is done usually in winter and similarly cooling of air is done in summer season.

HVAC Principle and Theory

HVAC system works on the principles of thermodynamics ,fluid mechanics and heat transfer.

All these fields come into play in various components of HVAC.IAQ Indoor air quality is the quality of air inside the building or structures as mostly related to health and safe keeping of its occupants or items/goods placed.IAQ is changed with inclusiion or contamination with gases and uncontrolled mass & energy transfer.

HVAC systems are used for heating and cooling & air condition in homes, building, industry, vehicles, aquariums and many more.With the passage of time application of HVAC are increasing and more research is in-progress in this field.

HVAC business is also increasing at the same pace as field of application is broadening.

What is a HVAC System?

HVAC system is basically an assembly of various types of equipments installed together to provide heating and cooling alongwith indoor climate control. HVAC systems involve mechanical, electrical and I & C components to provide comfort to the occupants of building/space or to preserve goods,products or items placed in space.

HVAC cooling systems may be integrated with HVAC heating systems or these may be installed separately depending upon HVAC design .HVAC system also serves on industrial scale to keep the machinery running by maintaining the temperature of space/hall/room where machines are installed.HVAC water chillers have become essential for any industry for its various needs.

HVAC System

HVAC System Basic Components

An HVAC System may include the following basic components or units.

  • HVAC water chillers and heaters
  • Hot water generator (if chiller does produce chilled water only) or furnace
  • Chilled water pumps
  • Cooling water pumps
  • Electrical power supply control or Motor control center (MCC)
  • Cooling towers
  • Piping for chilled water and cooling water or condenser side water
  • Valves for chilled water and cooling water sides
  • Air handling units (AHUs) , heating coils and cooling Coils
  • Ducts in ventilation system (supply ducts and return ducts)
  • Fan Coil Units (FCUs) and thermostats
  • HVAC Diffusers and grills
  • HVAC controls (instrumentation & Control components) installed at various locations
  • HVAC software for building HVAC control or building management system (BMS)
  • An Assembly of all above components forms an HVAC system.

HVAC system working Principle

In the background of HVAC system ,an HVAC water chiller produces chilled water which is then circulated throughout the building or space upto cooling coils in air handling units.Blowers move air on cooling coils which is then distributed into various portions of space or building for providing comfort or preserving goods/items as per HVAC design.

Air is distributed through supply ducts and return air is collected in air handling units with the help of return ducts.Chilled water and cooling water pumps provide energy to keep the chilled and cooling water moving.

HVAC Valves are also installed at various points in piping to ease the maintenance of HVAC system or for the sake of system control.Heating of air may be done with the help of HVAC heat pump ,hot water generator or simply by furnace.Some industrial chillers also serve as heaters in winter season.Heating coils take the place of cooling coils in case of heating mode.

HVAC system cost may vary for different applications as heating and cooling space or environment varies. Looking for cheap hvac systems may involve little research in types of HVAC systems and HVAC suppliers otherwise you would be lamenting over waste of millions of dollars for selecting wrong HVAC designer & contractor.

PLC Program for Automatic Parameter Initialization when Power UP

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This is PLC Program for Automatic Parameter initialization when power up.

Problem Description:-

  • In many applications it is necessary to initialize some data when machine is powered up.
  • Sometimes due to power failure, value in some parameters becomes zero.
  • Due to this problem operator has to feed all data again or every time during power failure.
  • When machine will get power up, at that time necessary parameters should be initialized automatically.
  • Here we discuss this issue with some basic ladder logic.

Problem Diagram

PLC Program for Automatic Parameter initialization

Problem Solution

  • In this case we need to write logic in PLC program so all parameters will be initialized automatically.
  • We can also set manual initialization button so operator can initialize data during machine operation is running.
  • Here we will consider machine set speed as a data and it will be initialized automatically when machine will turn on.
  • If operator wants to reinitialize set speed during running cycle then he needs do it through initialization button.

Ladder diagram

Here is PLC program Automatic Parameter initialization when power up.

PLC Ladder Logic for Automatic Parameter initialization

List of PLC inputs/outputs

Inputs List:-

  • Parameter Initialization Button – I0.0
  • MW10 : Set Speed form Display

Outputs List:-

Program Description

  • For this application we use S7-1200 PLC and TIA portal software for programming.
  • This logic is used for parameter initialization.
  • For first scan, we used here S7-1200 facilities of system Memory. Every PLC have its own system memory.
  • Always ON bit, always OFF bit, first scan bit, and diagnostic status changed are the system memory for S7-1200 PLC.
  • We can configure any memory address “M” for system memory. Here we configured M1.0 for first scan bit which is used for parameter initialization.
  • We write for parameter initialization in Network 1. Here we use NO contact of First scan bit (M1.0) for moving initial 5 RPM in MW12(Speed for drive).By using MOVE instruction 5 RPM will be moved in MW12 . Add NO contact of Parameter Initialization Button (I0.0) for moving Initial 5RPM in MW12 (Speed for drive) manually.
  • For editing data manually in running cycle we write logic in Network 2. Here operator can enter data in MW10 (SET SPEED) from the display and it will go in MW12(Speed for drive).
  • For Example, Say we need to enter 100 RPM speed from display it will be written in word MW10 (Set Speed from display) and as per logic it will be moved in MW12 (Speed for drive), so motor will run on 100 RPM.

Runtime Test Cases

PLC Program for Parameter initialization Simulation

Article by
Bhavesh Diyodara

Pressure Detection Circuit

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The below Figure shows a block diagram of a typical pressure detection circuit.

Pressure Detection Circuit

Figure : Typical Pressure Detection Block Diagram

The sensing element senses the pressure of the monitored system and converts the pressure to a mechanical signal. The sensing element supplies the mechanical signal to a transducer, as discussed above.

The transducer converts the mechanical signal to an electrical signal that is proportional to system pressure. If the mechanical signal from the sensing element is used directly, a transducer is not required and therefore not used.

The detector circuitry will amplify and/or transmit this signal to the pressure indicator. The electrical signal generated by the detection circuitry is proportional to system pressure. The exact operation of detector circuitry depends upon the type of transducer used.

The pressure indicator provides remote indication of the system pressure being measured.

Why RTD installed after the Orifice Plate ?

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The location of the RTD (thermowell), positioned downstream of the orifice plate so the turbulence it generates will not create additional turbulence at the orifice plate. The American Gas Association (AGA) allows for upstream placement of the thermowell, but only if located at least three feet upstream of a flow conditioner.

A major reason for this is von K´arm´an vortex shedding caused by the gas having to flow around the width of the thermowell. The “street” of vortices shed by the thermowell will cause serious pressure fluctuations at the orifice plate unless mitigated by a flow conditioner, or by locating the thermowell downstream so that the vortices do not reach the orifice.

Also Read : Thermowell Case-Study

Basics of Level Measurement

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Level Measurement

Liquid level measuring devices are classified into two groups: (a) direct method, and (b) inferred method. An example of the direct method is the dipstick in your car which measures the height of the oil in the oil pan. An example of the inferred method is a pressure gauge at the bottom of a tank which measures the hydrostatic head pressure from the height of the liquid.

Level Gauge

A very simple means by which liquid level is measured in a vessel is by the gauge glass method (Figure 1). In the gauge glass method, a transparent tube is attached to the bottom and top (top connection not needed in a tank open to atmosphere) of the tank that is monitored. The height of the liquid in the tube will be equal to the height of water in the tank.

Transparent Level Gauge

Figure 1 Transparent Tube

Figure 1 (a) shows a gauge glass which is used for vessels where the liquid is at ambient temperature and pressure conditions. Figure 1 (b) shows a gauge glass which is used for vessels where the liquid is at an elevated pressure or a partial vacuum. Notice that the gauge glasses in Figure 1 effectively form a “U” tube manometer where the liquid seeks its own level due to the pressure of the liquid in the vessel.

Transparent Level Gauge

Gauge glasses made from tubular glass or plastic are used for service up to 450 psig and 400°F. If it is desired to measure the level of a vessel at higher temperatures and pressures, a different type of gauge glass is used. The type of gauge glass utilized in this instance has a body made of metal with a heavy glass or quartz section for visual observation of the liquid level. The glass section is usually flat to provide strength and safety. Figure 2 illustrates a typical transparent gauge glass.

Glass Level Gauge

Figure 2 Gauge Glass

Reflex Level Gauge

Another type of gauge glass is the reflex gauge glass (Figure 3). In this type, one side of the glass section is prism-shaped. The glass is molded such that one side has 90-degree angles which run lengthwise. Light rays strike the outer surface of the glass at a 90-degree angle. The light rays travel through the glass striking the inner side of the glass at a 45-degree angle. The presence or absence of liquid in the chamber determines if the light rays are refracted into the chamber or reflected back to the outer surface of the glass.

Reflex Level Gauge

Figure 3 Reflex Gauge Glass

When the liquid is at an intermediate level in the gauge glass, the light rays encounter an air-glass interface in one portion of the chamber and a water-glass interface in the other portion of the chamber. Where an air-glass interface exists, the light rays are reflected back to the outer surface of the glass since the critical angle for light to pass from air to glass is 42 degrees. This causes the gauge glass to appear silvery-white. In the portion of the chamber with the water-glass interface, the light is refracted into the chamber by the prisms. Reflection of the light back to the outer surface of the gauge glass does not occur because the critical angle for light to pass from glass to water is 62-degrees. This results in the glass appearing black, since it is possible to see through the water to the walls of the chamber which are painted black.

Radiation Level Gauge

A third type of gauge glass is the refraction type (Figure 4). This type is especially useful in areas of reduced lighting; lights are usually attached to the gauge glass. Operation is based on the principle that the bending of light, or refraction, will be different as light passes through various media. Light is bent, or refracted, to a greater extent in water than in steam. For the portion of the chamber that contains steam, the light rays travel relatively straight, and the red lens is illuminated. For the portion of the chamber that contains water, the light rays are bent, causing the green lens to be illuminated. The portion of the gauge containing water appears green; the portion of the gauge from that level upward appears red.

Refraction Level Gauge

Figure 4 Refraction Level Gauge

Ball Float Level Gauge

The ball float method is a direct reading liquid level mechanism. The most practical design for the float is a hollow metal ball or sphere. However, there are no restrictions to the size, shape, or material used. The design consists of a ball float attached to a rod, which in turn is connected to a rotating shaft which indicates level on a calibrated scale (Figure 5). The operation of the ball float is simple. The ball floats on top of the liquid in the tank. If the liquid level changes, the float will follow and change the position of the pointer attached to the rotating shaft.

Ball Float Level Mechanism

Figure 5 Ball Float Level Mechanism

The travel of the ball float is limited by its design to be within ±30 degrees from the horizontal plane which results in optimum response and performance. The actual level range is determined by the length of the connecting arm.

The stuffing box is incorporated to form a water-tight seal around the shaft to prevent leakage from the vessel.

Chain Float Level Gauge

This type of float gauge has a float ranging in size up to 12 inches in diameter and is used where small level limitations imposed by ball floats must be exceeded. The range of level measured will be limited only by the size of the vessel. The operation of the chain float is similiar to the ball float except in the method of positioning the pointer and in its connection to the position indication. The float is connected to a rotating element by a chain with a weight attached to the other end to provide a means of keeping the chain taut during changes in level (Figure 6).

Chain Float Level Gauge Principle

Figure 6 Chain Float Level Gauge Principle

Magnetic Bond Method

The magnetic bond method was developed to overcome the problems of cages and stuffing boxes. The magnetic bond mechanism consists of a magnetic float which rises and falls with changes in level. The float travels outside of a non-magnetic tube which houses an inner magnet connected to a level indicator. When the float rises and falls, the outer magnet will attract the inner magnet, causing the inner magnet to follow the level within the vessel (Figure 7).

Magnetic Bond Level Measurement

Figure 7 Magnetic Bond Detector

Conductivity Probe Method

Figure 8 illustrates a conductivity probe level detection system. It consists of one or more level detectors, an operating relay, and a controller. When the liquid makes contact with any of the electrodes, an electric current will flow between the electrode and ground. The current energizes a relay which causes the relay contacts to open or close depending on the state of the process involved. The relay in turn will actuate an alarm, a pump, a control valve, or all three. A typical system has three probes: a low level probe, a high level probe, and a high level alarm probe.

Conductivity Probe Level Detection System

Figure 8 : Conductivity Probe Level Detection System

Differential Pressure Level Sensors

The differential pressure (DP) sensor/detector method of liquid level measurement uses a DP detector connected to the bottom of the tank being monitored. The higher pressure, caused by the fluid in the tank, is compared to a lower reference pressure (usually atmospheric). This comparison takes place in the DP detector. Figure 9 illustrates a typical differential pressure detector attached to an open tank.

Open Tank Differential Pressure Level Measurement

Figure 9 Open Tank Differential Pressure Level Measurement

The tank is open to the atmosphere; therefore, it is necessary to use only the high pressure (HP) connection on the DP transmitter. The low pressure (LP) side is vented to the atmosphere; therefore, the pressure differential is the hydrostatic head, or weight, of the liquid in the tank. The maximum level that can be measured by the DP transmitter is determined by the maximum height of liquid above the transmitter. The minimum level that can be measured is determined by the point where the transmitter is connected to the tank.

Not all tanks or vessels are open to the atmosphere. Many are totally enclosed to prevent vapors or steam from escaping, or to allow pressurizing the contents of the tank. When measuring the level in a tank that is pressurized, or the level that can become pressurized by vapor pressure from the liquid, both the high pressure and low pressure sides of the DP transmitter must be connected (Figure 10).

Closed Tank - Dry Reference Leg

Figure 10 Closed Tank, Dry Reference Leg

The high pressure connection is connected to the tank at or below the lower range value to be measured. The low pressure side is connected to a “reference leg” that is connected at or above the upper range value to be measured. The reference leg is pressurized by the gas or vapor pressure, but no liquid is permitted to remain in the reference leg. The reference leg must be maintained dry so that there is no liquid head pressure on the low pressure side of the transmitter. The high pressure side is exposed to the hydrostatic head of the liquid plus the gas or vapor pressure exerted on the liquid’s surface. The gas or vapor pressure is equally applied to the low and high pressure sides. Therefore, the output of the DP transmitter is directly proportional to the hydrostatic head pressure, that is, the level in the tank.

Where the tank contains a condensible fluid, such as steam, a slightly different arrangement is used. In applications with condensible fluids, condensation is greatly increased in the reference leg. To compensate for this effect, the reference leg is filled with the same fluid as the tank. The liquid in the reference leg applies a hydrostatic head to the high pressure side of the transmitter, and the value of this level is constant as long as the reference leg is maintained full. If this pressure remains constant, any change in DP is due to a change on the low pressure side of the transmitter (Figure 11).

Closed Tank Wet Reference Leg

Figure 11 Closed Tank, Wet Reference Leg

The filled reference leg applies a hydrostatic pressure to the high pressure side of the transmitter, which is equal to the maximum level to be measured. The DP transmitter is exposed to equal pressure on the high and low pressure sides when the liquid level is at its maximum; therefore, the differential pressure is zero. As the tank level goes down, the pressure applied to the low pressure side goes down also, and the differential pressure increases. As a result, the differential pressure and the transmitter output are inversely proportional to the tank level.

What is Emergency Shutdown System (ESD) ?

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A critical condition for which immediate shutdown of the gas turbine and compressor is required and delayed shutdown options are not acceptable because of the danger posed to the compressor station, human life or physical damage to the equipment.

Emergency Shutdown System (ESD) is designed to minimize the consequences of emergency situations, related to typically uncontrolled flooding, escape of hydrocarbons, or outbreak of fire in hydrocarbon carrying areas or areas which may otherwise be hazardous.

An emergency shutdown system for a process control system includes an emergency shutdown (ESD) valve and an associated valve actuator. An emergency shutdown (ESD) controller provides output signals to the ESD valve in the event of a failure in the process control system. A solenoid valve responds to the ESD controller to vent the actuator to a fail state. A digital valve controller (DVC) test strokes the ESD valve. An impedance booster device enables the dc powering of the solenoid valve and the DVC over a two wire line while still permitting digital communication over the same two wire line.

Functionality

Traditionally risk analyses has concluded that the Emergency Shutdown system is in need of a high Safety Integrity Level, typically SIL 2 or 3. Basically the system consist of field-mounted sensors, valves and trip relays, system logic for processing of incoming signals, alarm and HMI units. The system is able to process input signals and activating outputs in accordance with the Cause & Effect charts defined for the installation.

Typical Actions from an Emergency Shutdown System

  • Shutdown of part systems and equipment
  • Isolate hydrocarbon inventories
  • Isolate electrical equipment
  • Prevent escalation of events
  • Stop hydrocarbon flow
  • Depressurize / Blowdown
  • Emergency ventilation control
  • Close watertight doors and fire doors
  • Centralized Project Development for Both Safety and Process Needs

Since ESD System is built on the same control platform as the Process Automation System, you can address your process control and functional safety needs with a common control platform. A single Control System can be used to develop both process control and functional safety applications.

Static analysis tools are provided to assist in the verification and validation of the safety strategy. With a single platform and software workbench, Esoteric can provide solutions for Process, Discrete and Functional Safety applications, reducing the need for spares, training and support and providing considerable savings.

Functions of Pressure Detectors

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Pressure Detector Functions

Although the pressures that are monitored vary slightly depending on the details of facility design, all pressure detectors are used to provide up to three basic functions: indication, alarm, and control. Since the fluid system may operate at both saturation and subcooled conditions, accurate pressure indication must be available to maintain proper cooling. Some pressure detectors have audible and visual alarms associated with them when specified preset limits are exceeded. Some pressure detector applications are used as inputs to protective features and control functions.

Detector Failure

If a pressure instrument fails, spare detector elements may be utilized if installed. If spare detectors are not installed, the pressure may be read at an independent local mechanical gauge, if available, or a precision pressure gauge may be installed in the system at a convenient point. If the detector is functional, it may be possible to obtain pressure readings by measuring voltage or current values across the detector leads and comparing this reading with calibration curves.

Environmental Concerns

Pressure instruments are sensitive to variations in the atmospheric pressure surrounding the detector. This is especially apparent when the detector is located within an enclosed space. Variations in the pressure surrounding the detector will cause the indicated pressure from the detector to change. This will greatly reduce the accuracy of the pressure instrument and should be considered when installing and maintaining these instruments.

Ambient temperature variations will affect the accuracy and reliability of pressure detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the pressure detection instrumentation in the proper environment.

The presence of humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. The effects due to humidity are controlled by maintaining the equipment in the proper environment.

Summary

The three functions of pressure monitoring instrumentation and alternate methods of monitoring pressure are summarized below.

Pressure detectors perform the following basic functions:

If a pressure detector becomes inoperative:

  • A spare detector element may be used (if installed).
  • A local mechanical pressure gauge can be used (if available).
  • A precision pressure gauge may be installed in the system.

Environmental concerns:

  • Atmospheric pressure
  • Ambient temperature
  • Humidity

Functions of Temperature Detectors

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Functions of Temperature Detectors

Although the temperatures that are monitored vary slightly depending on the details of facility design, temperature detectors are used to provide three basic functions: indication, alarm, and control. The temperatures monitored may normally be displayed in a central location, such as a control room, and may have audible and visual alarms associated with them when specified preset limits are exceeded. These temperatures may have control functions associated with them so that equipment is started or stopped to support a given temperature condition or so that a protective action occurs.

Detector Problems

In the event that key temperature sensing instruments become inoperative, there are several alternate methods that may be used. Some applications utilize installed spare temperature detectors or dual-element RTDs. The dual-element RTD has two sensing elements of which only one is normally connected. If the operating element becomes faulty, the second element may be used to provide temperature indication. If an installed spare is not utilized, a contact pyrometer (portable thermocouple) may be used to obtain temperature readings on those pieces of equipment or systems that are accessible.

If the malfunction is in the circuitry and the detector itself is still functional, it may be possible to obtain temperatures by connecting an external bridge circuit to the detector. Resistance readings may then be taken and a corresponding temperature obtained from the detector calibration curves.

Environmental Concerns

Ambient temperature variations will affect the accuracy and reliability of temperature detection instrumentation. Variations in ambient temperature can directly affect the resistance of components in a bridge circuit and the resistance of the reference junction for a thermocouple. In addition, ambient temperature variations can affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the temperature detection instrumentation in the proper environment.

The presence of humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. The effects due to humidity are controlled by maintaining the equipment in the proper environment.

Detector Uses Summary

1.Temperature detectors are used for:

  • Indication
  • Alarm functions
  • Control functions

2.If a temperature detector became inoperative:

  • A spare detector may be used (if installed)
  • A contact pyrometer can be used

3.Environmental concerns:

  • Ambient temperature
  • Humidity

Bridge Circuit Construction

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Figure 1 shows a basic bridge circuit which consists of three known resistances, R1, R2, and R3 (variable), an unknown variable resistor RX (RTD), a source of voltage, and a sensitive ammeter.

Bridge Circuit Construction

Figure 1 Bridge Circuit

Resistors R1 and R2 are the ratio arms of the bridge. They ratio the two variable resistances for current flow through the ammeter. R3 is a variable resistor known as the standard arm that is adjusted to match the unknown resistor. The sensing ammeter visually displays the current that is flowing through the bridge circuit. Analysis of the circuit shows that when R3 is adjusted so that the ammeter reads zero current, the resistance of both arms of the bridge circuit is the same. The below Equation 1 shows the relationship of the resistance between the two arms of the bridge.

Bridge Circuit Equation

Since the values of R1, R2, and R3 are known values, the only unkown is Rx. The value of Rx can be calulated for the bridge during an ammeter zero current condition. Knowing this resistance value provides a baseline point for calibration of the instrument attached to the bridge circuit. The unknown resistance, Rx, is given by below Equation 2.

Bridge Circuit Final Equation

Bridge Circuit Operation

The bridge operates by placing Rx in the circuit, as shown in Figure 1, and then adjusting R3 so that all current flows through the arms of the bridge circuit. When this condition exists, there is no current flow through the ammeter, and the bridge is said to be balanced. When the bridge is balanced, the currents through each of the arms are exactly proportional. They are equal if R1 = R2. Most of the time the bridge is constructed so that R1 = R2. When this is the case, and the bridge is balanced, then the resistance of Rx is the same as R3, or Rx = R3.

When balance exists, R3 will be equal to the unknown resistance, even if the voltage source is unstable or is not accurately known. A typical Wheatstone bridge has several dials used to vary the resistance. Once the bridge is balanced, the dials can be read to find the value of R3. Bridge circuits can be used to measure resistance to tenths or even hundredths of a percent accuracy. When used to measure temperature, some Wheatstone bridges with precision resistors are accurate to about + 0.1°F.

Two types of bridge circuits (unbalanced and balanced) are utilized in resistance thermometer temperature detection circuits. The unbalanced bridge circuit (Figure 2) uses a millivoltmeter that is calibrated in units of temperature that correspond to the RTD resistance.

Unbalanced Bridge Circuit

Figure 2 Unbalanced Bridge Circuit

The battery is connected to two opposite points of the bridge circuit. The millivoltmeter is connected to the two remaining points. The rheostat regulates bridge current. The regulated current is divided between the branch with the fixed resistor and range resistor R1, and the branch with the RTD and range resistor R2. As the electrical resistance of the RTD changes, the voltage at points X and Y changes. The millivoltmeter detects the change in voltage caused by unequal division of current in the two branches. The meter can be calibrated in units of temperature because the only changing resistance value is that of the RTD.

The balanced bridge circuit (Figure 3) uses a galvanometer to compare the RTD resistance with that of a fixed resistor. The galvanometer uses a pointer that deflects on either side of zero when the resistance of the arms is not equal. The resistance of the slide wire is adjusted until the galvanometer indicates zero. The value of the slide resistance is then used to determine the temperature of the system being monitored.

Balanced Bridge Circuit

Figure 3 Balanced Bridge Circuit

A slidewire resistor is used to balance the arms of the bridge. The circuit will be in balance whenever the value of the slidewire resistance is such that no current flows through the galvanometer. For each temperature change, there is a new value; therefore, the slider must be moved to a new position to balance the circuit.

Temperature Compensation

Because of changes in ambient temperature, the resistance thermometer circuitry must be compensated. The resistors that are used in the measuring circuitry are selected so that their resistance will remain constant over the range of temperature expected. Temperature compensation is also accomplished through the design of the electronic circuitry to compensate for ambient changes in the equipment cabinet. It is also possible for the resistance of the detector leads to change due to a change in ambient temperature. To compensate for this change, three and four wire RTD circuits are used. In this way, the same amount of lead wire is used in both branches of the bridge circuit, and the change in resistance will be felt on both branches, negating the effects of the change in temperature.

Summary

Temperature detection circuit operation is summarized below.

The basic bridge circuit consists of:

  • Two known resistors (R1 and R2) that are used for ratioing the adjustable and known resistances
  • One known variable resistor (R3) that is used to match the unknown variable resistor
  • One unknown resistor (Rx) that is used to measure temperature
  • A sensing ammeter that indicates the current flow through the bridge circuit

The bridge circuit is considered balanced when the sensing ammeter reads zero current.

A basic temperature instrument is comprised of:

  • An RTD for measuring the temperature
  • A bridge network for converting resistance to voltage
  • A DC to AC voltage converter to supply an amplifiable AC signal to the amplifier
  • An AC signal amplifier to amplify the AC signal to a usable level

An open circuit in a temperature instrument is indicated by a very high temperature. A short circuit in a temperature instrument is indicated by a very low temperature.

Temperature instrument ambient temperature compensation is accomplished by:

  • Measuring circuit resistor selection
  • Electronic circuitry design
  • Use of three or four wire RTD circuits

Capacitive Type Pressure Transducers Principle

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Capacitive-type transducers, illustrated in Figure, consist of two flexible conductive plates and a dielectric.

In this case, the dielectric is the fluid.

Capacitive Pressure Transducer

As pressure increases, the flexible conductive plates will move farther apart, changing the capacitance of the transducer.

This change in capacitance is measurable and is proportional to the change in pressure.