Electronic Temperature Monitor
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Thermocouple Accuracy
In order to assure accurate results when using thermocouple-based devices, certain
installation and testing procedures should be observed. This appendix addresses
the installation of the Model 118 Electronic Temperature Measuring device
(ETM) and techniques required to test its accuracy in a laboratory setting.
Installation Considerations
The temperature sensed by a thermocouple device is a function of the temperature
differential between the thermocouple (TC) and its point of connection to the
ETM. Thus
sudden
changes in the ambient temperature surrounding the ETM
can change the terminal block temperature faster than the internal ETM
temperature can stabilize. For this reason the point of connection needs to be
protected from moving air.
The most common method of installation for the ETM is to mount it in some type
of enclosure that will protect the unit from air currents such as those caused when
the fans turn ON. If the unit is installed in an enclosure, no further protection is
required.
If the unit is mounted without being installed in an enclosure, then to maintain the
rated accuracy some form of covering is needed over the TC connection terminals
to protect them from stray air currents.
Method for Testing ETM Accuracy
To test the accuracy of an ETM, a millivolt signal must be applied which is the
millivolt equivalent of the temperature between the terminal block and the TC.
Since the precise temperature of the terminal block is not known, a common
method is to use a TC immersed in an ice bath as a reference (0°C) and then
adding the millivolt equivalent of the degrees above zero C that are desired as the
input to the ETM.
The sketch below indicates one method of doing this. Note that again the
terminals on the ETM must be protected from air currents and the ETM must be
given time for the internal temperatures to stabilize before accuracy testing is
performed. The best way to do this testing is with the ETM mounted in the
enclosure that will be used in the final installation.
Test setup for applying simulated thermocouple readings to the ETM
