Chapter 7 - Temperature Measurement: Four-Wire Resistance Measurement

The Wheatstone bridge method of measuring the resistance of an RTD has
certain problems associated with it. These problems are solved by the
technique of using a current source along with a remotely located DVM,
as shown in Figure 7-18. The output voltage read by the DVM is directly
proportional to RTD resistance, so you only need one conversion equation
to convert from resistance to temperature. The three bridge resistors are
replaced by one RTD. The digital voltmeter measures only the voltage
dropped across the RTD and is insensitive to the length of the lead wires.

One disadvantage of a four-wire system is that, obviously, it requires one
more wire than a three-wire bridge. However, this is a small price to pay
given the increased accuracy of the temperature measurement it provides.

Resistance to Temperature Conversion

The RTD is a more linear device than the thermocouple, but it still requires
curve fitting to yield a more precise reading. The Callendar-Van Dusen
equation has been used for years to approximate the platinum RTD curve:

(7-10)

You determine the exact values for coefficients α, β, and δ by testing the
RTD at four temperatures and then solving the resulting equations. Typical
values for platinum RTDs are as follows:

Example 7-7 illustrates a typical calculation to obtain the resistance ratio
for a platinum RTD.