Cold Junction Compensation; Temperature Measurements - Pt100 Sensors - Intab PC-Logger 2100 User Manual

2.2.2 Cold Junction Compensation

When using thermocouples to measure temperature it is almost always necessary
to know the temperature of the "cold junction". Thermoelectric voltage has to be
"adjusted" with respect to this temperature.
Thermoelectric voltage is the result of the temperature difference between the
"cold junction" and the "hot junction".
A solid state temperature sensor has been placed centrally in the screw terminal
area of the analogue board. Its temperature is automatically sampled every time a
temperature measurement is taken. All temperature measurements are thus
compensated for the actual temperature of the terminals themselves.
Some precautions have to be taken if the "2100" is exposed to rapid temperature
gradients. Since temperature is measured in one point only, a gradient from one
side of the unit to the other may cause misreadings.
By giving the unit time to reach a stable temperature before recordings are
started and by protecting it from sudden changes of ambient temperature a very
high degree of accuracy can be achieved.
2.3

Temperature measurements - Pt100 sensors

The "2100" can also take temperature readings from a Pt-100 sensor. This is
achieved by conventional four wire resistance measurement technique.
A constant current is passed through the sensor via one pair of wires. The
voltage developed across the resistor ( a Pt-100 is a temperature dependent
resistor ) is measured by a channel via the second pair of wires.
The constant current is available at terminal "I-out" on the left-hand side of
channel 1. Several Pt-100´s can be connected in series to the same current
source. ( See next chapter ).
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