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-

Figure 7-16. Two-wire RTD bridge circuit

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 O lead wire on a field-mounted RTD implies a
10 O/0.385O/°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.

Figure 7-17. Three-wire RTD bridge

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).

© 2007 ISA

Products & Services
Pressure Sensors
Pressure sensors include all sensors, transducers and elements that produce an electrical signal proportional to pressure or changes in pressure.
Resistive Temperature Devices (RTD) Elements
Resistive temperature device (RTD) elements are wire windings or other thin-film serpentines that exhibit changes in resistance with changes in temperature. They are usually made of metallic elements or alloys such as copper, nickel, or nickel-iron.
Level Sensors
Level sensors are used to detect liquid or powder levels, or interfaces between liquids. There are two basic level-measurement types: continuous and point or multi-point.
Noncontact Infrared Temperature Sensors
Noncontact infrared temperature sensors absorb ambient infrared (IR) radiation given off by a heated surface. They are used in applications where direct temperature measurement is not possible.
Dial Thermometers
Dial thermometers are bimetal or bi-metallic, liquid or gas-filled, and vapor-tension-based. These devices display temperatures in Fahrenheit, Celsius, or both.

Topics of Interest

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...

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