Open Thermocouple Detection (OTD) Feature on C Series Thermocouple Modules
Hardware: CompactDAQ>>Analog Input Modules>>NI 9211
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.
Guidelines on scaling from voltage to temperature readings can be found in the CompactRIO Shipping Examples specific to each module (Hardware Input and Output>>CompactRIO>>Module Specific IO>>Analog Input).
Products and Services: NI 9211 Operating Instructions and Specifications
Products and Services: NI 9212 Datasheet
Products and Services: NI 9213 Datasheet
Products and Services: NI 9214 Operating Instructions and Specifications
Report Date: 01/06/2016
Last Updated: 10/06/2017
Document ID: 755HFPE2