Measurement Computing DataShuttle - Manual (Page 101)

DataShuttle and DynaRes 11-12-01 Analog Input & Output 13-1
Analog Input & Output 13
Analog Input & Output
Thermocouples
Thermocouples are the most widely used temperature sensor. They are rugged and available in a wide
variety of types and configurations. Some types can be used in temperatures over 2000 degrees Celsius.
Generally, thermocouples are not as accurate as thermistors or RTD’s. Thermocouples output a voltage
proportional to the temperature difference between two separate metal junctions. They require
compensation of the reference, or cold junction, to measure absolute temperature. They are also nonlinear.
The T31 terminal panel has a cold junction sensor, but do not have the isothermal aluminum block. The
thermocouple terminations are not held at exactly the same temperature as the cold junction sensor. The
difference can be as much as several degrees, resulting in an error of several degrees.
Figure 13-1a shows the installation of an ungrounded thermocouple. The COM terminal connection
provides the ground. Use this configuration for probes with isolated thermocouples or whenever the
thermocouple probe is not in contact with grounded metal or other conductors such as water. The shield
connects to one of the separate digital ground terminals on the digital terminal strips.
Thermocouples are commonly grounded as in Figure 13-4b. This is the case for probes with grounded
thermocouples attached to grounded metal or other conductors such as water. Remove the COM
connection for these installations.
Although a properly installed thermocouple is grounded at some point, sometimes this requirement is hard
to satisfy. Sometimes a thermocouple is not in good electrical contact with a conducting surface because of
dirt or corrosion. Sometimes the material is only slightly conducting or the thermocouple may not always
be in contact with it, as in a container of liquid that has been emptied.
For these situations use Figure 13-1c. It is the same as 13-1a except that a resistor has been installed in
place of the jumper wire at auxiliary component location R5. Use a 1 M ohm, ¼ Watt, 5% resistor in
parallel with a 0.1
µ F capacitor. This connection provides a ground reference but doesn’t allow large
currents to flow in the signal wires. This prevents erroneous voltage drops. The modification may be left
in place when you use either ungrounded or grounded thermocouples. The capacitor is optional.