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Appendix B
–
Thermocouple Accuracy
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Appendix B – Thermocouple Accuracy
In order to ensure accurate results when using thermocouple-based devices, certain
installation and testing procedures should be observed. This appendix addresses the
installation of the Model 118 ITM Intelligent Transformer Monitor (ITM) and techniques
required to test its accuracy in a laboratory setting.
7.1. 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 ITM. Thus
sudden
changes in the ambient temperature surrounding the ITM can change the terminal
block temperature faster than the internal ITM 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 ITM 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, to maintain the rated accuracy,
some form of covering is needed over the TC connection terminals to protect them from
stray air currents.
7.2. Method for Testing ITM Accuracy
To test the accuracy of an ITM, a millivolt signal must be applied that 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 add the millivolt equivalent of the degrees
above zero C that are desired as the input to the ITM.
The sketch below indicates one method of doing this. Note that, again, the terminals on the
ITM must be protected from air currents, and the ITM 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 ITM mounted in the enclosure that will be used in the final installation.
Figure 13 – Test setup for applying simulated thermocouple readings to the ITM
THERMOCOUPLE WIRE
COPPER WIRE
Millivolt
Source
ICE BATH
