PT-104 Data Logger User's Guide
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Background information
3.1
Platinum resistance thermometers (PRTs)
PRTs (Platinum Resistance Thermometers) offer excellent accuracy over a wide
temperature range (from -200 °C to 850 °C). Sensors are interchangeable between
different manufacturers, and are available in various accuracy ratings in packages to
suit most applications. Unlike thermocouples, it is not necessary to use special cables
to connect to the sensor.
The principle of operation is to measure the resistance of a platinum element. The
most common type (PT100) has a resistance of 100 at 0 °C and 138.4 at 100 °C.
The relationship between temperature and resistance is approximately linear over a
small temperature range. For example, if you assume that it is linear over the 0 °C to
100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary
to linearise the resistance to give an accurate temperature. The most recent definition
of the relationship between resistance and temperature is International Temperature
Standard 90 (ITS-90). This linearisation is done automatically with software.
The linearisation equation is:
R
t
= R
0
(1 + A·t + B·t
2
+ C·(t-100)·t
3
)
A = 3.9083 x 10
-3
B = -5.775 x 10
-7
C = (below 0
°
C) -4.183 x 10
-12
(above 0
°
C) 0
For a PT100 sensor, a 1 °C temperature change will cause a 0.384 change in
resistance, so even a small error in measurement of the resistance (for example, the
resistance of the wires leading to the sensor) can cause a large error in the
measurement of the temperature. For precision work, sensors have four wires - two to
carry the sense current, and two to measure the voltage across the sensor element. It
is also possible to obtain three-wire sensors, although these operate on the (not
necessarily valid) assumption that the resistance of each of the three wires is the
same.
The current through the sensor will cause some heating. For example, a sense current
of 245 µA through a 100 resistor generates 6 µW of heat. If the sensor element is
unable to dissipate this heat, it reports an artificially high temperature. This effect can
be reduced by either using a large sensor element, or by making sure that it is in good
thermal contact with its environment.
Using a 1 mA sense current gives a signal of only 100 mV. Because the change in
resistance for a degree Celsius is very small, even a small error in the measurement of
the voltage across the sensor produces a large error in the temperature measurement.
For example, a 100 µV voltage measurement error would give a 0.4 °C error in the
temperature reading. Similarly, a 1 µA error in the sense current would give a 0.4 °C
temperature error.