tidu271

Theory of Operation
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The RTD is generally installed in plastic, food, petrochemical-processing equipment, gas exhausts,
pipelines, stoves, and other places in industry. The measurement circuit inside a control room can be
hundreds of feet away from the processing plant. Under such conditions, the measurement wires that lead
to the sensor can be several ohms. The small resistance of the wire could add to the RTD resistance.
Therefore, errors in resistance measurement can be caused. For example, a wire resistance of 0.5 Ω in
each lead of a field-mounted RTD causes an (0.5 Ω + 0.5 Ω) / (0.385°C/Ω) = 2.6°C error in measurement.
RTDs are available in three different lead wire configurations: 2-wire, 3-wire, and 4-wire. Before discussing
each configuration in detail, the background is given for a ratiometric measurement approach for RTDs, as
this reference design also implements ratiometric measurement.
5.2.3 Self-Heating
Unlike thermocouples, RTDs are not self-powered. Therefore, RTD's need an excitation source. Excitation
current flowing through the RTD produces a useful voltage that can be measured. Maximizing the
excitation current appears desirable by augmenting system performance. The excitation current must be
high enough so that voltage produced is greater than the peak-to-peak noise level of the ADC.
At the same time, care must be taken not to pass a large current through the RTD, since a large current
can cause higher I 2 R losses. I 2 R losses lead to self-heating within the element. If the sensor is unable to
dissipate the self-heat, the sensing element increases above ambient temperature, which can cause
resistance of the RTD element to increase. Therefore, an artificially high temperature will be reported.
The errors due to self-heating cannot be easily corrected. These errors should be kept lower than 25% of
the total error budget. The amount of self-heat generated also depends upon the sensor’s thermal
resistance, surroundings, and placement in an application. An RTD placed in a moving, fluid medium will
not self-heat because the moving fluid will keep the RTD at the temperature of the fluid.
However, in open-air temperature measurements or other applications where the RTD is surrounded by
an insulator, the RTD will self-heat and cause errors. Self-heating error is expressed in mW/°C. The
typical range of RTD self-heating coefficients is 2.5 mW/°C for small elements to 65 mW/°C for larger
elements. Figure 31 displays the self-heating error in °C versus applied excitation current for the range of
RTD self-heating coefficients.
10
1
0.1
Temperature (C)
Error 65mW
( I)
Error 2.5mW
( I)
0.01
1 10 3
1 10 4
1 10 5
1 10 6
1 10 7
1 10 5 1 10 4 1 10 3 0.01
I
Excitation Current (A)
Figure 31. Self Heating versus Excitation Current
Typically, a good compromise is to make sure that RTD and current are scaled so the drift (due to selfheating)
is significantly less than the errors associated with the RTD itself.
28 Temperature Sensor Interface Module for Programmable Logic Controllers TIDU271–May 2014
(PLC)
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