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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 tant, because contemporary data-acquisition
systems have built-in linearization correc-
tion features making hardware linearization
unnecessary.
Temperature Measurement
Methods
There are several techniques for meas-
uring temperature with a thermistor, the
most common being the two-wire and four-
wire techniques.
The two-wire technique works by forc-
ing current through the thermistor 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
Basics of Temperature
can cause some error.
In the four-wire technique, a current is
forced through the thermistor and a voltage
Measurement:
is measured. However, the current is sourced
on one set of leads, while the voltage is
sensed on a different set of leads. The voltage
Thermistors is sensed at a different spot from the source
current. This means the test lead resistance
is completely out of the measurement path.
In other words, the test lead resistance is not
a part of the measurement.
Dale Cigoy, Senior Applications Engineer
Keithley Instruments, Inc. Pros and Cons
Thermistors have several distinct advan-
tages over other temperature sensors. For
Introduction temperatures. starters, they are simple to set up and oper-
Whether in industrial applications like Thermistors are also less linear than ate, using a two-wire measurement scheme.
the process industry or in laboratory set- RTDs. This nonlinearity requires a correc- They are also fast, because they can be made
tings, accurately measuring temperature is a tion to linearize it. The Steinhart-Hart equa- small and can respond to temperature chang-
critical part of success. tion helps approximate individual thermistor es quickly.
Temperature measurements are needed in curves for linearity. It describes the resistance On the down side, the non-linear proper-
medical applications, materials research in change of a thermistor as related to tempera- ties of thermistors means they need to be lin-
labs, electrical/electronic component stud- ture. The equation can be written as: earized. They also have a limited tempera-
ies, biology research, geological studies, and ture range and are fragile, because they are
1/T = A + B