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Industrial instrumentation Page 1

ReportPressure and Temperature Transmitters

Submitted to

Dr.Ing Naveed Ramzan

Submitted by

Muhammad Jalal Hussain

07-CHEM-98


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Abstract

This report is intended to convey the importance and utilization of Temperature and Pressure

Transmitters in instrumentation used in process industry and in the subject of industrial

instrumentation. In this report theory of transmitter, types of transmitters, pressure

transmitters, temperature transmitters, level transmitters, methods of signal transmission and

transmitters available is the market are discussed. In this report an understanding of

Temperature and pressure transmitter is tried to develop by discussing related commercial

devices.


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Table of Contents

Transmitters .........................................................................................4

Smart and Intelligent Transmitters: ....................................................4

Force Balance Transmitter: ................................................................6

Motion Balance Transmitter: ..............................................................6

Pneumatic Transmitters: .....................................................................6

Pressure Transmitters .........................................................................7

Force-Balance Pneumatic Pressure Transmitter: ...............................7

Absolute Pressure Transmitter: ..........................................................7

Electronic Pressure Transmitters: .......................................................8

Temperature Transmitters .................................................................8

Pneumatic Temperature Transmitters: ...............................................8Level Transmitters ..............................................................................9

Signal Transmission ............................................................................9

Current Signal Transmission: ...........................................................10

Pneumatic Signal Transmission: ......................................................11

Industrial Transmitters ....................................................................11

PX760 Series Differential Pressure Transmitter ..............................11

TX86 Universal Input temperature Transmitter ...............................13

OS1600, OS1700, OS1800 Infrared Temperature Transmitter .......14

PRTXD Series RTD Temperature Transmitters ..............................16

References ..........................................................................................17


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Transmitters

In the process plant, it is impractical to locate the control instruments out in the plant near the process.

It is also true that most measurements are not easily transmitted from some remote location. Pressure

measurement is an exception, but if a high pressure measurement of some a dangerous chemical is to

be indicated or recorded several hundred feet from the point of measurement, a hazard will be created

hazard may be from the pressure or the chemical carried in the line. This problem is solved by a signal

transmission system. In process instrumentation, this system is usually either pneumatic or electrical.

By using transmission system it will be possible to install most of the indicating recording and control

instruments in one location. This helps in decreasing the number of operators in the plant and all the

measurements can be visually seen at one point (control room).

A transmitter is a transducer that responds to a measurement variable and converts that input into a

standardized transmission signal.

The transmitter used in industry are combination of a transducer and signal conditioning circuit that

produces an output current preoperational to the measured variable.

Most common types of Transmitters which are used in industrial control are:

1.

Smart and intelligent transmitters.2. Force balance transmitters.3. Motion balance transmitters.4. Pneumatic transmitters.

Smart and Intelligent Transmitters:The progressed made in design of

transmitters is because of two factors:

1. Improved performance with reduced cost.2. Development in technology.

The most significant advances have resulted from the emergence of low power microprocessors and

analog to digital converts (ADC) which, in junction with the basic junction circuits, can function of

the limited power available at the transmitter in a conventional 4 to 20 mA measurement circuit.

This has led to two different ways for improving the working of transmitters:


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1. By enabling non-linear characteristic to be corrected.2. By enabling secondary sensor to account for any effect on the primary sensor.

Transmitters in which corrections are applied to the primary sensor signal, using microprocessors to

the process information which is embedded in memory; or those in which a microprocessor is used injunction with a secondary sensor to derive for the primary sensor signal are called smart

transmitters.

In simple language a smart transmitter a transmitter in which a microprocessor system is used to

correct non-linearity error of primary sensor through interpolation of calibration data held in memory

or to compensate for the effect of secondary influences on the primary sensor by incorporating a

secondary sensor adjacent to primary sensor and interpolating stored calibration data for both primary

and secondary sensor.

The microprocessor not only manages the smart function but also manages communication. Other

functions which smart sensors can perform are

1. Setting or resetting the zero and span.2. Location and application.3. Running diagnostics and warning incase of malfunction.

Smart sensors incorporating communication system are called Smart and intelligent transmitters.

Intelligent transmitter is the one in which the microprocessor also performs following functions:

1. Deriving the primary measurement signal.2. Processing transmitters information like location etc.3. Managing the communication system.

Smart and intelligent features of such transmitters are summarized below:

Smart Features:

1. Linearization, characterization and Correction of Primary sensor characteristics.2. Expression of the Measurement in engineering units.3. Inclusion of control function and other algorithms.

Intelligent Feature:


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1. Adjustment of span and zero.2. Adjustment of damping, time constant or response time.3. Diagnostic routine and status information.

Force BalanceTransmitter:In force balance transmitters the measurement produces

a force which tends to move the top of the force bar. This tiny motion moves he ferrite disc closer to

the transformer, changing the output current. This changes the amplitude output of the oscillator,

which is rectified and then amplified to generate a DC mA transmitter signal. This output signal is fed

back through the voice coil on the armature of the force motor which moves the force bar again to its

original by balancing the forces and the amplifier signal stabilizes.

Advantages:

1. By decreasing the motion of the pivot friction is reduced.2. Hysteresis is minimized and greater accuracy can be achieved. No free motion.

Disadvantages:

1. It cannot be used to produce digital signal.

Motion Balance Transmitter:In a motion balance transmitter the process

measurement produces motion against a calibration ring resulting in a change of position

corresponding to a change in the process variable. This change in position is measure d by the

transducer. The output from the transducer is amplified and an electrical signal is used to stabilize the

amplifier.

Pneumatic Transmitters:The device that senses some process variables and transmits

the measured value into an air pressure that is transmitted to various pressure receiver devices for

indication recording, alarm generation and control is known as Pneumatic transmitter

When transmission system is employed, the measurement is converted into a pneumatic signal by the

transmitter scaled from 0 to 100 percent of the measured value. This transmitter is mounted close to

the point of measurement in the process. The transmitter output air pressure for a pneumatic

transmitter is piped to the recording control instrument. The standard output range of pneumatic

transmitter is 3 to 15 PSI. These are the standard signals that are internationally used.


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Suppose we have field mounted pressure transmitter that has been calibrated to a pressure range of

100 to 500 PSI. When the pressure being sensed is 100 PSI the transmitter is designed to produce an

output of 3 PSI air pressure. When the pressure sensed rises to 300 PSI the output will climb to 9 PSI.

When pressure 500 PSI the output signal will be 15 PSI. The signal is carried by tubing to control

room where it is either indicated recorded or feed in to the system. The receiving instrument uses

bellows element to convert this signal in to pointer or pen motion. If the signal is feed to the

controller, a bellow is also used to convert the signal for the use of the controller.

Some basic types of pneumatic transmitters used in industry are:

1. Force balance transmitters.2. Motion balance pressure transmitters.3. Differential pressure transmitters.4. Variable area flow transmitters.5. Temperature transmitters.6. Buoyancy transmitters.7. Speed transmitters.[1]

Pressure Transmitters

Force-Balance Pneumatic Pressure Transmitter:A pneumatic pressure

senses a pressure and converts that pressure into pneumatic signal that can be transmitted over a

reasonable distance. A receiving instrument is then applied to convert the signal into pen or pointer

position or a measurement input signal to a controller. The Foxboro model 11GL force balance

pneumatic pressure transmitter is a simple example. Operation of vital component: the flapper and

nozzle, and the pneumatic amplifier. These two mechanisms are found almost in every pneumatic

instrument. The flapper nozzle system converts the small motion or force into an equivalentpneumatic signal. Flapper movement of only 0.0015 will change the nozzle pressure by 0.75 PSI. This

small change applied to pneumatic amplifier or relay becomes an amplified change of 3 to 15 PSI in

the amplifier output.[2]

Absolute Pressure Transmitter:The pressure being measured is applied at one side

of the diaphragm in a capsule. The space on the other side of the diaphragm is evacuated, thus

providing zero absolute pressure reference.


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The pressure exerts a force on the diaphragm that is applied to the lower end of the bar. The

diaphragm seal serves as the fulcrum for the force bar and as a seal for the pressure chamber. The

force is transmitted through the flexure connector to the range bar, which pivots on the range

adjustment wheel.

Any movement of the range bar causes a minute change in the clearance between the flapper and the

nozzle. This produces a change in the output pressure front the relay to the feedback bellows until the

force in the bellows balances the force on the diaphragm capsule.[2]

Electronic Pressure Transmitters:Atcotran pressure transmitter this device uses a

differential or movable core transformer. The transformer consists of a primary coil and two

secondary coils inter connected with each other. The transformer core is suspended from the tip of the

bourdon tube and moves up and down in response to changes in the process pressure which is being

measured. The primary coil is connected to an alternating current source; the magnetic flux generated

by primary coil is distributed by the core so that voltage is induced in the two secondary windings. If

the core is position in such a way that (in the center of the two secondary coils) the voltage induced in

the each of the two coils is equal, the alternating current output to the indicating instrument is zero

because the two secondary coils are wound in opposite directions and the voltage induced in the one is

180 out of phase with other. So the transformers output corresponds to the difference between the two

voltages induced in the secondary windings. An extremely small movement of the core suffices to

produce a measureable voltage output.[3]

Temperature Transmitters

Pneumatic Temperature Transmitters:There are basically four general

classification of pressure creating thermometers can be employed with pneumatic pressure

transmitters. If the signal is to be transmitted for more than 200 ft or if there are multiple points at

which temperature is to be monitored visually. All the temperature measurements are nothing more

than but a pressure measurement. The pneumatic pressure and temperature transmitters are essentially

alike, differing slightly from each other.


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A very simple device in which the pointer is directly moved by a bourdon tube with the tube d device

is connected which controls the position of the flapper. Let us consider an example; temperature is

rising of the measured stream. This increase in temperature will move the flapper close to the nozzle.

This will increase the pressure on the orifice, as a result an increase in pressure output of the

pneumatic relay .The increase in output of the relay is sent in two ways. One to the remote gage and

second to the transmitter feedback bellows which cause the flapper to move away from the nozzle

until a new condition is established.

Another method can be used for accomplishing the Pneumatic temperature transmittance. This

method is used for the filled system. The pressure created by the liquid in the temperature sensing

bulb tries to push the diaphragm up and the motion lever with it. Any movement of the motion lever

will move the baffle in accordance with the nozzle. The movement of the baffle in the vicinity of the

nozzle will be 40 to 50 times as greater as that of the motion of lever; the lever amplifies the motion

of the diaphragm. The full range of the device can be obtained with a baffle movement of about 0.001

inch, it may be vary from instrument to instrument as per requirement. As the baffle moves away from

the nozzle, the pressure in the tubing of the nozzle decreases. Because the relay is of the reverse

acting type, this decrease of the pressure at the nozzle will create an Increase in output pressure of the

relay.[4]

Level Transmitters

Level can be measured by using a pressure transmitter. The principle in level measurement by using a

pressure transmitter is based on the measurement of the hydrostatic pressure produced by a column of

liquid at a given height.[5]

Signal Transmission

For control purpose the there is requirement of a scheme for the transmission of data from instrument

(sensor) to the controller system (PLC, DCS etc). Following are the methods used for the transmission

measured variable.

1. Current Signal Transmission2.

Pneumatic signal transmission


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Current Signal Transmission:When a current loop is implemented using analog

type electrical signal, the most common and frequently used method to transmit the analog signal is in

the form of a current loop. When a process current loop is designed, an operating range is specified

for the control variable to regulate according to the conditions. Thats wh y, it is necessary of define a

set point anywhere within that range. According to one standard the signal conditioning is so that 4-20

mA current range on signal transmission wire represents the specified range of the variable, according

to the given condition.

Following are the point regarding the significance and importance of using the current

transmission to represent the controlled variable:

1.Load impedance:We can avoid different errors introduced by attaching different

loads to transmit the current while using current to carry analog information about the variable. Hence

the delivered current is not changed by any change in the lead resistance or by insertion of a serried

resistance in the circuit. Mostly the transmitting circuits are designed to work in to any load from 0

ohm to about 1000 ohm.

2. Interchangeability:When we use a specified current to represent the variable

range, we provide for interchangeability of the controller in the process control loop. Once we have

changed all the dynamic variable range to a fixed current range then all control can be based on a set

point and deviation as some percentage of this range. Thus the controller only gets a 4-20 mA signal,

according to the process condition.

3.Measurement and power supply:The current signal lines are also the power

supply lines to energize the transducer and local signal conditioning. Only two wires are required to

connect a transducer and signal conditioning measurement system to the rest of the loop. The signal

conditioning is designed such that the circuit draws more or less current from the power source in

proportion of the value of dynamic variable according to the process condition.


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Pneumatic Signal Transmission:The final control element in a process

control loop (whether PLC or DCS) is a pneumatic device or element. In most cases, an entire process

control loop may be pneumatic, basically it depends on the plant and the process whether the wholecontrol system is pneumatic or electrical (current driven). The standard transmission signal, mostly

used in industry, is a pressure level of a range 3-15 PSI. A current to pressure converter is used to

scale the 4-20 mA signal to 3-15 PSI pressure signal.[6]

Industrial Transmitters

Below are mentioned few Pressure and Temperature transmitters which are used in industries with

their detail:

PX760 Series Differential Pressure Transmitter

PX760 series transmitter is a two wire microprocessor based

device. It works on a digital signal from 4 to 20 mA. It has

ceramic sensor. This transmitter measures and locally displays

differential pressure and level from 1.8 to 2100 mbar (1 to 8430 inof H2O). It is smart sensor; it has build in temperature

compensator, linearization software and displays the sensor

temperature.

A basic Block for diagram, which displays how this differential

transmitter works, is given below.

Figure 1

PX760 Series Differential Pressure

Transmitter


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Figure 2

In this transmitter the inputs are sampled and digitized by an Analog to Digital Converter (ADC). The

sensor signal integrity is readily checked by a diagnostic routine. The data obtained from the sensor is

linearised and converted to selected engineering units. The measurements are then adjusted to

according to the lower and upper range values. This value is finally converted to a 4 -20 mA analog

output signal. The configurations are kept in a permanent memory bank of the instrument. The

transmitter also performs internal diagnostics to give maximum reliability and helps the user to

identify any problem.[7]


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TX86 Universal Input temperature Transmitter

Figure 3

TX86 Universal Input temperature Transmitter

The TX86 is a two wire looped power temperature transmitter. It is a very versatile instrument which

converts, indicates and transmits a signal from a thermocouple, RTD, Potentiometer sensors into a

linearised 4-20 mA looped current needed by DCS, PLC, PC, SCADA system or recorder.

TX86 has a microprocessor based advanced digital and analog electronics circuitry and a temperature

measuring method which enables it to provide a high degree of performance and flexibility at

reasonable cost and ease of use. The transmitter incorporates galvanic isolated solid state rely alarm

output which can be programmed as high and low alarm trip system with normally open or close

contacts for variety of control application.[8]


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OS1600, OS1700, OS1800 Infrared Temperature Transmitter

Figure 4

OS1600, OS1700, OS1800 Infrared Temperature Transmitter

The OS1600, OS1700, OS1800 Infrared Temperature Transmitter is sensitive to radiation or infrared

energy given off by the object whose temperature is to be measured. When the instrument is aimed

towards the target the radiation focuses on the infrared detector causing it to generate an electrical

signal this signal is converted by the system electronics into a DC output current which varies with

intensity of target radiation, a direct relation exists. The temperature transmitter produces a linear DC

current electronic signal ranging from 4 to 20 mA which is directly proportional to the infrared energy

emitted by the target. A readout device causes the meter to display the temperature of the target. A

typical basic measurement loop is shown below.


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Figure 5

Instrument loop

The OS1600, OS1700, OS1800 Infrared Temperature Transmitter family are non contact infrared two

wire transmitters with temperature range from -45 to 2500oC . These transmitters are very versatile

and have high performance. The linear 4 20 mA output signal allows the sensor to be interfaced

with a variety of remote devices like indicators, recorder, controller or computers. The sensors have

an adjustable time of response of 0.2 to 5.0 seconds an emissivity adjustment of 0.1 to 0.99 and

adjustable peak hold position. [9]


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PRTXD Series RTD Temperature Transmitters

The PRTXD series RTD temperature transmitters are microprocessor controlled industrial

temperature indicator with a digital temperature

display and 4-20 mA electrical signal device in a

rugged housing. The PRTXD has a 2-wire

connection and all operating power is supplied by

the 4-20 mA current loop. The RTD temperature

reading is linearized for both the digital display and

the 4-20 mA output. It can display in both F or C

scale and the 4-20 mA output may be set to

correspond to a desired temperature range.

It has a range from -50 to 200oC. Its accuracy

0.1% of span plus maximum sensor error of

0.3C at 0C, 1.1C at 150C. Any DC

supply/loop resistance that maintains a direct

current of 8 V to 32 V at gauge terminals. Erraticoperation may occur if loop voltage falls below

specifications.[10]

Figure 6

PRTXD Series RTD Temperature Transmitters


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References

1. Singh, S.K, Industrial Instrumentation and Control, Tata McGraw Hill, India, 3rd edition, Page551 to 557, 2009.

2. Anderson, N.A, Instrumentation for Process Measurement and Control, Chilton Company,USA, 3

rdedition, Page 60 to 65.

3. Holzbock, W.G, Instruments for Measurement and Control, Reinhold Publishing corp., USA,2

ndedition, Page 84 to 85.

4. Fribance, A.E, Industrial Instrumentation Fundamentals, McGraw Hill company, USA,International Students Edition, Page 463 to 465.

5. Macmillan, G.K, Considine, D.M, Process/Industrial instruments and control Handbook, TataMcGraw Hill, 5

thedition.

6. Jhonson, C, Process Control Instrumentation Technology, Prentice Hall, India, 4th edition,Page 24 to 25.

7. http://www.omega.com/pptst/PX760.html [ Retrieved on October 13th 2010]8. http://www.omega.com/pptst/TX86.html [ Retrieved on October 13 th 2010]9. http://www.omega.com/pptst/OS1600_1700_1800.html [ Retrieved on October 13th 2010]10.http://www.omega.com/pptst/PRTXD.html [ Retrieved on October 13th 2010]

temperature and pressure transmitters (font 11) 98

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