When I disconnect my thermocouple sensor from my NI 9211, NI 9212, NI 9213 or NI 9214, I notice that a positive temperature is read. What causes this to happen and is there a way that I can force the module to read a negative value instead?
When a thermocouple is disconnected, C Series Thermocouple Modules use an internal source to pull the input to a higher positive voltage value than the allowable input range. The positive voltage of the module results in a positive temperature reading. When sampling at the maximum, allowable sample rate of the module, you will see the Open Thermocouple Detection (OTD) feature cause a rise in the temperature or voltage reading. The image below depicts the NI 9212 acquiring room temperature readings from a Type K thermocouple. During acquisition, the thermocouple was disconnected. Note the rise in temperature from 25 degrees to full scale.
There are several applications that may require OTD to pull the input to a negative value. This can be accomplished by following the steps listed below.
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