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Q10023 Cold junction temperatures on large panel

Tags: Help, DAP, DAPL, thermocouple, large system, cold junction, thermistor

Applies to: All termination boards, enclosures

I typically use the "cold junction" reference temperature sensor of a termination board for measuring temperatures with thermocouples. For my next project I need a large number of thermocouple inputs, and will mount an array of thermocouple connectors in a long panel. With relatively large distances between the thermocouple connectors and the CJC temperature sensor, I am concerned that there will be significant temperature gradients across this panel, enough to produce an unacceptable error in the final temperature measurements. Is there a way I can account for these temperature differences more accurately?

The first step to account for the temperature differences is to measure them. Assuming that temperatures tend to vary evenly across the panel, the thermal gradient is approximately constant. The temperature profile can then be characterized by temperatures measured at the two ends of the panel. (If this linear gradient assumption is not true, divide the panel into shorter sections, and use more sensor locations.)

Measure the temperatures at the two ends of the panel section using precision negative temperature coefficient thermocouples. The techniques for doing this are described on this Web site (see references). Thermistors are much quieter and respond more slowly than the thermocouples, so presume that the data preprocessing results in two very slow streams of reference temperature data, CJCleft and CJCright, measured at the two ends of the panel section.

Now you can estimate the reference temperatures at intermediate points along the panel. Suppose that there are 20 columns of 5 thermocouple connectors spaced evenly along the panel. Then you can interpolate the CJC reference temperature for each column using DAPL expression tasks.

     CJC1 = (20*CJCleft + 1*CJCright)/21
     CJC2 = (19*CJCleft + 2*CJCright)/21
     CJC20 = (1*CJCleft + 20*CJCright)/21

Use the appropriate CJC temperature value when computing the voltage to temperature conversions for the thermocouple signals from each column on the panel.


THERMO command, in the Dapl 3000 Reference manual

THERMISTOR command, described in the thermistor calibration page

See the temperature measurement page for background information on temperature measurements.