Measurement Computing CIO-EXP-GP - Manual (Page 18)

5 CONFIGURATION FOR THERMOCOUPLE MEASUREMENT
Thermocouples are temperature sensors constructed of wires of two dissimilar metals fused together at a
point. This junction of two metals produces a voltage that varies relative to temperature. Thermocouple
voltages require several manipulations in order to be useful. These are:
1. A very low voltage is produced and so must be amplified by a factor of between 100 and 1,000.
2. The voltage produced by the thermocouple is not linear with respect to temperature, so it must be
linearized. Linearization in this case is calculated by software after the voltage is acquired.
3. A voltage-producing junction is also created at the screw terminal where the thermocouple is
connected to the CIO-EXP-GP. The temperature at this “cold junction” must be measured and the
voltage calculated and subtracted from the total measured from the thermocouple. This is also calculated
by software. The circuit that measures this temperature is the Cold Junction Compensation (CJC) circuit.
4. Thermocouples are subject to EMI and RFI noise due to the very low level of the voltage and the large
amplification factor. These affects can be reduced through averaging and filtering. There is a 70Hz low
pass filter on the CIO-EXP-GP. Averaging may be done in software.
Thermocouples are not as accurate as RTDs or other precision temperature sensors, but they are much
less expensive. Sometimes, an attempt is made to make a measurement beyond the accuracy of the
thermocouple such as measuring 1/10th of a degree over the full scale. Read the accuracy and
repeatability specification of the thermocouple, and consider the effects of linearization on the reading
before choosing thermocouples.
The CIO-EXP-GP is not the optimum choice for thermocouple-only applications. The CIO-EXP32 and
EXP16 are less expensive and just as accurate for thermocouple measurements. The CIO-EXP-GP has
extra circuitry devoted to bridge and RTD sensors.
5.1 Selecting The Output Channel
The General Configuration section describes the channel selection, setting the jumper and verifying the
installation and operation of the CIO-EXP-GP with your data acquisition board. Configure your boards
as described in that section before continuing with this section.
5.2 Selecting The CJC Output Channel
There is a set of jumpers near the 37-pin connectors labeled “CJC SEL”, which stands for cold junction
compensation select. These jumpers connect the on-board measurement of the cold junction temperature
to one of the A/D board channels for use in temperature calculations.
The CJC temperature reference is universally used by software to compensate for the voltage induced at
the cold junction (the screw terminal). The software package you are using will determine which channel
you need to set this jumper on. The default is channel 7 (the channel used by default by the Universal
Library). If you are not using the Universal Library, check your software documentation before selecting
a channel. Failure to supply the CJC reference by installing the jumper on the correct channel will result
in inaccurate temperature calculations by the software.
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