Exercise
2-1
Resistance Temperature Detectors (RTDs)
EXERCISE OBJECTIVES
•
•
•
•
To explain how resistance temperature detectors (RTDs) operate;
To describe the relationship between the temperature and the electrical
resistance of the most common types of RTDs;
To define the following terms: nominal resistance, temperature coefficient, and
sensitivity.
To explain how a Wheatstone bridge can be used to measure the voltage
produced across an RTD.
DISCUSSION
Electrical resistance
An important characteristic of all metals is their electrical resistance. Electrical
resistance is the opposition of the metal to the flow of electrical current. Electrical
resistance is measured in ohms (Ω) in both the S.I. and U.S. systems of units.
The electrical resistance of a metal is dependent upon the temperature at which the
metal is. Figure 2-4, for example, shows what happens to the relative resistance of
different metals as their temperature increases. The relative resistance is the ratio
between the resistance at the applied temperature to the resistance at a reference
temperature of 0EC (32EF).
As the figure shows, the relative resistance of the metals increases as their
temperature gets higher. Moreover, the relative resistance increases almost linearly
with temperature, at least over a substantial range of temperatures. Besides, the
relative resistance of nickel increases more sharply with temperature than that of
copper or platinum.
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Resistance Temperature Detectors (RTDs)
Figure 2-4. Relative resistance-versus-temperature relationship of different metals.
Temperature coefficient
All metals have a specific temperature coefficient that indicates their average
change in relative resistance per unit of temperature between 0 and 100EC (between
32 and 212EF).
The temperature coefficient is symbolized by the Greek letter alpha (α). It is usually
measured in ohms per ohm degree Celsius (EC-1) or in ohms per ohm degree Fahrenheit (EF-1).
Figure 2-4, for example, indicates that the temperature coefficient of platinum is
0.00392EC-1 (0.00218EF-1). Consequently, the relative resistance of platinum varies
by 0.392 between 0 and 100EC (32 and 212EF). Beyond 100EC (212EF), the shape
of the platinum curve indicates that the temperature coefficient decreases slightly as
the temperature gets higher.
2-10
Resistance Temperature Detectors (RTDs)
Resistance temperature detectors
A resistance temperature detector (RTD) is a primary element that is used to sense
temperature. The RTD works on the principle that the electrical resistance of metals
changes with temperature.
The RTD consists of a metallic conductor usually wound into a coil. The RTD is to
be connected to an electrical circuit in order to make a constant excitation current
flow through it. As the temperature increases, the electrical resistance of the metallic
conductor increases and, therefore, the voltage across the RTD increases.
Consequently, by measuring the voltage across the RTD, a signal proportional to the
temperature of the RTD can be obtained. This signal can be conditioned into a
current, voltage, or pressure of normalized range that is suitable for instrumentation
and control, the combination of the RTD and the conditioning circuit thus forming a
temperature transmitter.
RTD metals
The selection of a metal for use as an RTD depends on several factors. Among
these, the most important are the capability to follow rapidly changing temperatures,
a good linearity, a good reproducibility, and a relatively high change of resistance for
a given change in temperature (i.e. a high temperature coefficient).
The metals most commonly used for RTDs are platinum, nickel, and copper (refer
to Figure 2-4):
•
Platinum is the preferred metal for RTDs. It has been chosen as the international
standard metal for RTD temperature measurement. Platinum has a nearly linear
resistance-versus-temperature relationship over a wide temperature range.
Platinum offers good stability and reproducibility. It is well-suited for the
measurement of high temperatures up to 650EC (1200EF).
•
Nickel is the second mostly used metal for RTDs. It is less expensive than
platinum and it is more sensitive because of its higher temperature coefficient.
However, nickel has a narrower sensing range than platinum and is limited to the
measurement of temperatures below 300EC (570EF).
•
Copper is the least expensive of the three metals and it has the most linear
relationship. Similar to platinum, copper is well suited for the measurement of
high temperatures. However, copper is subject to oxidation, and it has poorer
stability and reproducibility than platinum.
RTD characteristics
Two important characteristics of RTDs are their nominal resistance and their
temperature coefficient:
•
The nominal resistance is the resistance of the RTD at a given reference
temperature, as specified by the manufacturer. Platinum RTDs, for example, are
usually designed so that their nominal resistance is 100 Ω at the ice reference
point of 0EC (32EF).
2-11
Resistance Temperature Detectors (RTDs)
•
The temperature coefficient is the mean change in relative resistance of the
metal per unit of temperature between 0 and 100EC (32 and 212EF), as
previously explained.
The nominal resistance and the temperature coefficient of an RTD determine the
sensitivity of the RTD within the 0-100EC (32-212EF) temperature range. The
sensitivity is the amount by which the resistance of the RTD will change per unit of
temperature, in Ω/EC (or Ω/EF).
For example, a platinum RTD having a nominal resistance of 100 Ω at 0EC (32EF)
and a temperature coefficient of 0.00392EC-1 (0.00218EF-1) will have a sensitivity of
0.392 Ω/EC (0.218 Ω/EF) within the 0-100EC (32-212EF) temperature range.
Measurement of the voltage across an RTD
As previously mentioned, the voltage produced across an RTD, which is directly
proportional to temperature, can be used for process instrumentation and control.
The traditional method of measuring the voltage across an RTD is to use a
Wheatstone bridge, as Figure 2-5 (a) shows.
•
The RTD and its two lead wires constitute one leg of the bridge. Resistors R1 and
R2 are of equal resistance, while resistor R3 is adjustable and is used as a
reference.
•
A DC voltage source supplies an excitation current to the RTD.
•
A differential amplifier produces a voltage VO proportional to the bridge output
voltage (measured between points a and b).
With the RTD placed in an ice bath at 0EC (32EF), resistor R3 is initially adjusted in
order to obtain a null voltage (0 V) at the output of the differential amplifier. In this
condition, the bridge is said to be null balanced.
Once the bridge has been null balanced, the amplifier output voltage will vary in
direct proportion to the temperature of the RTD.
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Resistance Temperature Detectors (RTDs)
Figure 2-5. Measurement of the voltage across an RTD.
If the two leads that connect the RTD to the bridge are more than a few centimeters
(inches) long, they will introduce an error in the temperature measurement. This
occurs because the resistance of the leads will contribute to the voltage produced
at the output of the bridge, causing the measured temperature to be higher than that
actually measured.
To minimize this error, RTDs are available in a three-wire version. The extra wire is
used to cancel the resistances of lead wires 1 and 2 by balancing the bridge, as
Figure 2-5 (b) shows. This has the effect of removing the error produced by lead
wires 1 and 2 as long as these wires are of equal resistance (i.e. of equal length and
temperature).
Advantages and limitations of RTDs
RTDs have the following advantages: they provide a good sensitivity, a good
reproducibility, and a good stability. They also provide a high accuracy, some
platinum RTDs being able to measure a few thousandths of a degree.
However, RTDs are relatively expensive, and they have a slower response time than
thermocouples. Moreover, the measurement accuracy of RTDs is dependent upon
the thermal stability of the resistors and power supply used in the Wheatstone
bridge.
The RTD probe and the RTD Temperature Transmitter of the Process Control
Training System
The Process Control Training System comes with a three-wire RTD probe that uses
a platinum RTD of 100 Ω at 0EC (32EF). The RTD probe is intended to be used with
the RTD Temperature Transmitter to measure the temperature of the water in the
trainer Column, as Figure 2-6 shows.
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Resistance Temperature Detectors (RTDs)
The tip of the RTD probe, which contains the RTD, is to be inserted into the Column
through the opening of the Float Switch. The other end of the RTD probe, which has
three leads, is to be connected to the "100-Ω RTD" terminals of the RTD
Temperature Transmitter.
The RTD Temperature Transmitter produces an excitation current through the RTD
and it measures the resulting voltage produced across the RTD. This voltage, which
is proportional to the temperature of the RTD, is conditioned into normalized voltages
and current that are available at the transmitter OUTPUTS.
The transmitter also contains a calibration source that can be used to simulate the
voltage produced across the RTD for any RTD temperature comprised between 0
and 100EC (32 and 212EF). The source eliminates the need to set the RTD at a wellknown temperature when performing calibration of the transmitter OUTPUTS.
The following is a detailed description of the RTD Temperature Transmitter terminals
and adjustments (refer to Figure 2-6):
Î
POWER INPUT terminals: used to power the transmitter with a DC voltage of
24 V.
Ï
CALIBRATION SOURCE adjustment knob: sets the probe temperature to be
simulated by the calibration source signal. This temperature can be adjusted
between 0 and 100EC (32 and 212EF).
Ð
INPUT SELECTOR: selects between the actual probe signal or the simulated
probe signal produced by the calibration source.
Ñ
CALIBRATION SELECTOR switch: places the 0-5 V and 4-20 mA OUTPUTS
in either fixed or variable calibration mode.
Ò
ZERO and SPAN adjustment knobs: used in the variable calibration mode
(CALIBRATION SELECTOR switch at VARIABLE) to set the temperature
range for which the 0-5 V and 4-20 mA OUTPUTS will pass from minimum to
maximum:
– The ZERO knob sets the temperature for which the outputs will be
minimum (0 V and 4 mA), i.e. the minimum temperature to be
detected. The minimum temperature can be adjusted between 0 and
50EC (32 and 122EF).
– The SPAN knob sets the temperature for which the outputs will be
maximum (5 V and 20 mA), i.e. the maximum temperature to be
detected. The maximum temperature can be adjusted between 15 and
30EC (27 and 54EF) above the minimum temperature set by the ZERO
knob.
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Resistance Temperature Detectors (RTDs)
Figure 2-6. The RTD probe and RTD Temperature Transmitter of the Training System.
Ó
CALibrated OUTPUT: provides a voltage proportional to the temperature
sensed by the probe or to the simulated probe signal produced by the
calibration source, depending on the position of the INPUT SELECTOR
switch.
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Resistance Temperature Detectors (RTDs)
This output has a fixed calibration of 100 mV per sensed EC above 0EC (or
56 mV per sensed EF above 32EF). It will pass from 0 to 10 V when the
actual or simulated temperature changes from 0 to 100EC (32 to 212EF).
Ô
0-5 V and 4-20 mA OUTPUTS terminals: provide a voltage and a current
proportional to the temperature sensed by the probe or to the probe
temperature signal simulated by the calibration source, depending on the
position of the INPUT SELECTOR switch.
The calibration of the 0-5 V and 4-20 mA OUTPUTS can be either fixed or
variable, depending on the position of the CALIBRATION SELECTOR
switch:
– In the fixed calibration mode (CALIBRATION SELECTOR switch at
FIXED), the temperature range for which the outputs will pass from
minimum to maximum is fixed and is 0-100EC (32-212EF).
– In the variable calibration mode (CALIBRATION SELECTOR switch at
VARIABLE), the temperature range for which the outputs will pass from
minimum to maximum can be adjusted by means of the ZERO and
SPAN adjustment knobs.
Õ
100-Ω RTD input terminals: used to connect the RTD probe to the transmitter.
Procedure summary
In the first part of the exercise, you will familiarize yourself with the operation of an
RTD Temperature Transmitter in the fixed calibration mode.
In the second part of the exercise, you will familiarize yourself with the operation of
an RTD Temperature Transmitter in the variable calibration mode.
In the third part of the exercise, you will set up and operate a temperature process.
You will use an RTD Temperature Transmitter to measure the temperature of the
water in a column.
EQUIPMENT REQUIRED
Refer to the Equipment Utilization Chart in Appendix A of the manual to obtain the
list of equipment required to perform this exercise.
PROCEDURE
Operation of the RTD Temperature Transmitter in the fixed calibration mode
G
2-16
1. Get the RTD Temperature Transmitter and 24-V DC Power Supply from
your storage area. Mount these components on the Main Work Surface.
Resistance Temperature Detectors (RTDs)
G
2. Power up the RTD Temperature Transmitter.
G
3. Get the RTD probe from your storage location and connect it to the
100-Ω RTD input of the RTD Temperature Transmitter.
Let the probe tip lie on the Work Surface.
G
4. Make the following settings on the RTD Temperature Transmitter:
INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RTD
CALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . FIXED
This selects the RTD probe signal as the transmitter input signal and places
the transmitter OUTPUTS in the fixed calibration mode.
G
5. Connect a DC voltmeter to the 0-5 V OUTPUT of the RTD Temperature
Transmitter.
Since this output is in the fixed calibration mode, it generates a fixed voltage
of 50 mV per sensed EC above 0EC (or 28 mV per sensed EF above 32EF).
According to the voltmeter reading, what is the ambient temperature?
G
6. Further experiment with the operation of the transmitter in the fixed
calibration mode:
–
Fill a suitable container with ice water (a mixture of ice cubes and
water).
–
Immerse the tip of the RTD probe into the ice water. The 0-5 V
OUTPUT voltage should decrease and stabilize at about 0.0 V, which,
in the fixed calibration mode, corresponds to an RTD temperature of
0EC (32EF).
–
Fill a suitable container with boiling water heated by an electric kettle or
a microwave oven.
–
Immerse the tip of the RTD probe into the boiling water. The 0-5 V
OUTPUT voltage should increase and stabilize at about 5.0 V, which,
in the fixed calibration mode, corresponds to an RTD temperature of
100EC (212EF).
Note: The 0-5 V OUTPUT of the RTD Temperature Transmitter
will stabilize at a voltage lower than 5.0 V if the atmospheric
pressure is lower than 101.3 kPa, absolute (14.7 psia).
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Resistance Temperature Detectors (RTDs)
Record below your observations.
Operation of the RTD Temperature Transmitter in the variable calibration mode
Note: In the following steps, you will use the calibration source
of the RTD Temperature Transmitter to calibrate its 0-5 V
OUTPUT so that the voltage at this output passes from 0.0 to
5.00 V when the probe temperature simulated by the calibration
source passes from 25 to 55EC (77 to 131EF), respectively.
G
7. Make the following settings on the RTD Temperature Transmitter:
INPUT SELECTOR . . . . . . . . . . . . . . . . . . . . . . . . . . CAL. SOURCE
CALIBRATION SELECTOR . . . . . . . . . . . . . . . . . . . . . . . VARIABLE
ZERO adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.
SPAN adjustment knob . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MAX.
This selects the calibration source signal as the transmitter input signal and
places the transmitter OUTPUTS in the variable calibration mode.
G
8. Set the probe temperature to be simulated by the calibration source of the
transmitter at 25EC (77EF).
To do so, adjust the CALIBRATION SOURCE knob of the transmitter until
you obtain a voltage of 2.5 V at the CAL. OUTPUT of the transmitter.
G
9. While monitoring the voltage at the 0-5 V OUTPUT of the transmitter, turn
the ZERO adjustment knob counterclockwise and stop turning it as soon as
the voltage ceases to decrease, which should occur around 0.01 V. Then
very slowly turn the knob in the clockwise direction and stop turning it as
soon as the voltage starts to increase.
This sets the minimum temperature to be detected at 25EC (77EF)
approximately.
G 10. Now set the probe temperature to be simulated by the calibration source of
the transmitter at 55EC (131EF).
To do so, adjust the CALIBRATION SOURCE knob of the transmitter until
you obtain a voltage of 5.5 V at the CAL. OUTPUT of the transmitter.
G 11. Adjust the SPAN knob in order to obtain a voltage of 5.00 V at the
transmitter 0-5 V OUTPUT.
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Resistance Temperature Detectors (RTDs)
This sets the maximum temperature to be detected at 55EC (131EF)
approximately.
G 12. Now that the RTD Temperature Transmitter is calibrated, proceed to the
next part of the exercise.
Measuring temperature with an RTD
Preliminary setup
G 13. Get the Expanding Work Surface from your storage location and mount it
vertically (at an angle of 90E) to the Main Work Surface, if this has not
already been done.
G 14. Connect the system shown in Figure 2-7, being careful not to modify the
calibration settings just made on the RTD Temperature Transmitter.
Figure 2-8 shows the suggested setup.
The speed of the variable-speed drive of the Pumping Unit will be controlled
with a controller, FIC1, placed in the manual (open-loop) mode. The Heating
and Cooling Units will be controlled manually. (This is the reason why there
is no temperature controller, or "TC" instrumentation symbol illustrated next
to these units in the flow diagram of Figure 2-7.)
The Column will first be operated in the pressurized mode in order to purge
air from the components downstream of the Column. Consequently, let the
tip of the RTD probe lie on the Work Surface for now.
Note: Make sure to mount the Heating Unit at the highest
possible location on the Expanding Work Surface, in order for this
unit to be above the other process components, as Figure 2-8
shows. Failure to do so may result in water entering the Heating
Unit upon disconnection of the hoses, which in turn might cause
damage to the Heating Unit.
Moreover, mount the 24-V DC Power Supply and the RTD
Temperature Transmitter in such a manner that water cannot
enter these components and their electrical terminals when hoses
are disconnected.
The Heating Unit must be connected for the direction of flow
indicated by the arrow heads in the symbol on its front panel.
On the other hand, the Cooling Unit will operate regardless of the
direction of water flow through it. However, to minimize the risk of
cavitation caused by air suction within the pump when the water
becomes hot, connect the Cooling Unit as indicated in Figure 2-8,
that is, with the upper unit port used as the hot water inlet and the
lower unit port used as the cooled water outlet. For the same
reason, mount the Column at the highest possible location on the
Expanding Work Surface in order to create a substantial head of
water upstream of the Cooling Unit.
2-19
Resistance Temperature Detectors (RTDs)
Note: If the controller you are using as flow controller FIC1 is the
Lab-Volt Process Control and Simulation Software (LVPROSIM),
model 3674, you can refer to Figure B-1 of Appendix B for details
of how to connect the LVPROSIM computer to the variable-speed
drive (SC1) of the Pumping Unit.
Figure 2-7. Measuring temperature with an RTD temperature transmitter.
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Resistance Temperature Detectors (RTDs)
Figure 2-8. Suggested setup for the diagram of Figure 2-7 (see table next page for the detail of the
components).
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Resistance Temperature Detectors (RTDs)
Î : Column
Ò : Cooling Unit
Ï : Heating Unit
Ó : RTD probe
Ð : Paddle Wheel Flow Transmitter
Ô : RTD Temperature Transmitter
Ñ : Pumping Unit
Õ : DC Power Supply
G 15. Make the following settings
On the Heating Unit:
S1 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Manual control knob . . . . . . . . . . . . turned fully counterclockwise
On the Cooling Unit:
S1 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Manual control knob . . . . . . . . . . . . turned fully counterclockwise
S2 switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
On the RTD Temperature Transmitter:
SELECTOR switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . RTD
Note: The 0-5 V OUTPUT of the RTD Temperature Transmitter
should still be calibrated for a temperature measurement range of
25-55EC (77-131EF) from the first part of the exercise.
G 16. Power up the Heating Unit:
–
–
Connect the AC line cord of this unit to a wall outlet.
Set the POWER switch at I.
G 17. Power up the Cooling Unit and the Paddle Wheel Flow Transmitter by
connecting their POWER INPUT terminals to the 24-V DC Power Supply.
Purging air from the components downstream of the Column
G 18. Make sure flow controller FIC1 is in the manual (open-loop) mode. Set the
output of this controller at 0% (0 V).
G 19. On the Column, make sure the cap of the insertion opening of the Float
Switch is tightened firmly.
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Resistance Temperature Detectors (RTDs)
G 20. Make sure the reservoir of the Pumping Unit is filled with about 12 liters
(3.2 gallons US) of water. Make sure the baffle plate is properly installed at
the bottom of the reservoir.
G 21. Turn on the Pumping Unit by setting its POWER switch at I.
G 22. On the Pumping Unit, adjust valves HV1 through HV3 as follows:
–
–
–
Open HV1 completely;
Close HV2 completely;
Set HV3 for directing the full reservoir flow to the pump inlet (turn handle
fully clockwise).
G 23. Set the variable-speed drive of the Pumping Unit to the maximum speed:
with controller FIC1 in the manual (open-loop) mode, set the controller
output at 100% (5 V).
G 24. Allow the level of the water to rise in the pressurized Column until it
stabilizes at some intermediate level. This will force air out of the
components downstream of the Column.
Note: If the cap of the insertion opening of the Float Switch on
the Column has not been tightened firmly, air will be allowed to
escape from the Column and the water level will not stabilize in
the Column. Should this case occur, stop the variable-speed drive
of the Pumping Unit. Open valves HV1 and HV2 of the Pumping
Unit in order to drain the Column to the reservoir. When the
Column is empty, tighten the cap of the insertion opening of the
Float Switch on the Column with more force. Then resume the
procedure from step 22.
Placing the system in the water recirculating mode
Note: In the following steps, you will place the system in the
water recirculating mode by setting the Pumping Unit valves so as
to direct the return flow directly to the pump inlet, not to the
reservoir. This will reduce the time required to raise or decrease
the temperature of the process water. For the same reason, the
water level in the Column will be set at a low, minimum level of
7.5 cm (3 in).
G 25. On the Pumping Unit, close valve HV1, which will cause the water level to
rise further in the Column. Then set valve HV3 for directing the full return
flow directly to the pump inlet (turn handle fully counterclockwise).
G 26. On the Column, remove the cap of the insertion opening of the Float Switch
to depressurize the Column. (The water level in the Column will remain
stable).
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Resistance Temperature Detectors (RTDs)
G 27. On the Pumping Unit, open valve HV2 in order to decrease the water level
in the Column to 7.5 cm (3 in), then close this valve.
G 28. Readjust the output of controller FIC1 until you read 4.0 V approximately at
the "F (cal.)" output of the Paddle Wheel Flow Transmitter. This will set the
flow rate at about 4 l/min (1.1 gal US/min).
Note: Small, continuous variations of a few tenths of volts around
the adjusted mean value of 4.0 V are normal at the flow
transmitter output.
However, large variations of one volt or more are abnormal, and
indicate that air has entered the system through an untight
connector or component on the suction side of the pump.
Should that case occur, stop the variable-speed drive of the
Pumping Unit in order to drain the column to the reservoir. When
the Column is empty, check the inside of the connector on the
Pumping Unit return line hose for any dirt or particles. Also, check
the o-rings on the two hose connectors of the Cooling Unit for any
fissure or crack. Once you have located and eliminated the cause
of the leak, reconnect the system as in Figure 2-7 and resume the
procedure from step 19.
Measuring temperature with the RTD
G 29. Insert the RTD probe all the way into the Column in order for its tip to be
submerged in the water.
G 30. Have the signal at the 0-5 V OUTPUT of the RTD Temperature Transmitter
plotted on the trend recorder of controller FIC1.
Adjust the update rate of the trend recorder (sampling interval) in order to
be able to monitor the transmitter signal over a period of 10 minutes
approximately.
Note: If the controller you are using as controller FIC1 is the LabVolt Process Control and Simulation Software (LVPROSIM),
model 3674, refer to Figure B-5 of Appendix B for details of how
to connect the LVPROSIM computer to the RTD Temperature
Transmitter. On the I/O Interface, make sure the RANGE switch
of ANALOG INPUT 1 is set at 5 V.
In LVPROSIM, select Analog Input 1 from the Trend Recorder
selection list to have the RTD Temperature Transmitter signal
plotted on the trend recorder. Set the LVPROSIM sampling
interval at 1500 ms. Access the Configure Analog Inputs window
and set the minimum and maximum range values of Analog
Input 1 at 25 and 55EC (77 and 131EF), respectively, which
corresponds to the current measurement range of the RTD
Temperature Transmitter. Set the filter time constant of this input
at 0.5 second. Make sure the square root extracting function is
unselected. Accept setup and return to main screen.
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Resistance Temperature Detectors (RTDs)
G 31. On the trend recorder, observe the RTD Temperature Transmitter output
signal.
Since no electrical power is applied to the heating element of the Heating
Unit, theoretically, the water in the Column should be at ambient
temperature.
Assuming that the ambient temperature is below 25EC (77EF), the level of
the RTD Temperature Transmitter signal should be at 0% of span on the
trend recorder, since the minimum temperature the transmitter can detect
has been adjusted to 25EC (77EF).
Yet, you may observe that the RTD Temperature Transmitter signal is at
some higher level, thermal energy being transferred to the recirculated
water mainly from frictional resistance of the pump internal parts.
G 32. On the Heating Unit, set the manual control knob to the mid position. On the
trend recorder, observe what happens to the temperature of the water in the
Column.
Now that electrical power is applied to the heating element of the Heating
Unit, thermal energy is transferred from this element to the recirculated
water.
Consequently, the temperature of the water should increase in the Column.
Is this your observation?
G Yes
G No
G 33. Let the temperature of the water in the Column increase to about 45EC
(113EF), or 67% of span, then turn the manual control knob of the Heating
Unit fully counterclockwise to remove electrical power from its heating
element.
According to the RTD Temperature Transmitter output signal on the trend
recorder, did the temperature of the water in the Column increase linearly
over time?
How long did it take for the temperature to increase from the initial
temperature to the final temperature of 45EC (113EF)?
G 34. On the Cooling Unit, turn the manual control knob fully counterclockwise.
What happens to the temperature of the water in the Column?
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Resistance Temperature Detectors (RTDs)
G 35. Allow the temperature of the water in the Column to cool down.
According to the RTD Temperature Transmitter output signal, does the
temperature of the water in the Column decrease linearly over time?
Explain.
G 36. Stop the variable-speed drive of the Pumping Unit by setting the output of
controller FIC1 at 0%.
G 37. Turn off the Pumping Unit, the Heating Unit, and the 24-V DC Power Supply
by setting their POWER switch at O.
G 38. Open valve HV1 of the Pumping Unit completely and let the water in the
Column drain back to the reservoir. The Column can also be drained by
disconnecting the end of the hose connected to the Cooling Unit inlet port
and reconnecting it to either of the auxiliary return ports on the Pumping
Unit.
G 39. Disconnect the system. Return all leads, hoses, and components to their
storage location.
CAUTION!
Hot water may remain in the hoses and components. Be
careful not to allow water to enter the electrical components
and their terminals upon disconnection of the hoses.
G 40. Wipe off any water from the floor and the Process Control Training System.
CONCLUSION
In this exercise, you familiarized yourself with the operation of an RTD temperature
transmitter in the fixed and variable calibration modes. You learned that, in the fixed
calibration mode, the temperature measurement range is fixed and is equal to 0100EC (32-212EF). In the variable calibration mode, the temperature measurement
range can be adjusted, and a maximum span of 30EC (54EF) can be obtained. Since
this span is narrower than the 100EC (180EF) span of the fixed calibration mode, the
variable calibration mode provides a greater measurement accuracy for any given
transmitter output range.
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Resistance Temperature Detectors (RTDs)
REVIEW QUESTIONS
1. What is an RTD? How does an RTD work?
2. What are three metals commonly used for RTDs? What are the advantages and
limitations of each metal?
3. Name and describe two important characteristics of RTDs.
4. How is the voltage produced across an RTD traditionally measured?
5. Why are RTDs available in three-wire version? Explain.
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