From Measurement and Control Basics Fourth Edition
The most common method for measuring the resistance of an RTD is to
use a Wheatstone bridge circuit. Figure 7-16 shows the arrangement for a
two-wire RTD. An electrical excitation current is passed through the
bridge, and the RTD and bridge output voltage is an indication of the RTD
resistance. The circuit uses a very stable excitation power source, three
high-precision resistors that have a very low temperature coefficient, and
a high-input impedance amplifier to measure the resistance change of the
RTD with changes in temperature.
The RTD is generally located on process equipment or piping, and the
measurement circuit can be located hundreds of feet from a control room
at the process plant. Since both the slope and the absolute value of resis-
tance of a typical RTD are small numbers, the length of the wire from the
RTD to the Wheatstone bridge circuit can be significant. This is especially
true when we consider that the measurement wires that lead to the sensor
may be several ohms or even tens of ohms. A small lead resistance can
introduce significant error into the measurement of output temperature.
For example, a 10 Ω lead wire on a field-mounted RTD implies a
10 Ω/0.385Ω/°C = 26°C error in a measurement. Even the temperature
coefficient of the lead wire can contribute a measurable error.
The standard method for avoiding this problem has been to use a threewire
connection to the Wheatstone bridge measurement circuit, as shown
in Figure 7-17.
In the circuit shown in Figure 7-17, if wires A and B are perfectly matched
in length, their impedance effects will cancel because each is in an opposite
leg of the bridge. The third wire, C, acts as a sense lead and carries a
very small current (in the microampere range).
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