temperature and pressure transmitters (font 11) 98
TRANSCRIPT
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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]