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A G R E AT E R M E A S U R E O F C O N F I D E N C E where T is temperature, P is the I2R power
generated, and S is degrees C/milliwatt.
Temperature Measurement
Methods
There are several techniques for meas-
uring temperature with an RTD. The first
is a two-wire method. This method works
by forcing current through the RTD and
measuring the resulting voltage. The benefit
is that it's a simple method using only two
wires, making it easy to connect and imple-
ment. The main drawback is that the lead
resistance is part of the measurement, which
can cause some error.
An improvement on the two-wire method
is the three-wire method. Here again, a cur-
rent is forced through the device and a re-
sulting voltage is measured. However, using
Basics of Temperature
a third wire provides compensation for the
lead resistance. This requires either a three-
wire compensating measurement unit or ac-
Measurement: RTDs
tually measuring the contribution from the
third wire and subtracting it from the overall
measurement.
A third technique is called the four-wire
method. Like in the other two methods, a
Dale Cigoy, Senior Applications engineer current is forced through and a voltage meas-
Keithley Instruments, Inc. ured. However, the current is sourced on one
set of leads while the voltage is sensed on
T
another set of leads. The voltage is sensed at
he most widely measured param- RTDs require an external stimulus to the resistive element (RTD), not at the same
eter is temperature. Whether in function properly, usually a current source. point as the source current. This means the
industrial applications such as the However, this current generates heat in the test lead resistance is completely out of the
process industry or in laboratory resistive element, which causes an error in measurement path. In other words, the test
settings, accurately measuring the temperature measurement. The measure- lead resistance is not a part of the measure-
temperature is a critical part of success. ment error is calculated by the formula ment.
Temperature measurements are needed in For example, if the test lead resistance
T = P