Thermocouple Measurements with 9211 drifting

[AndrewO_nyc]

Hi,I am taking thermocouple measurements using the NI USB-9211  with the NI USB - 9162 . I have four unshielded thermocouples connected, 3 J-type and 1 K-type.  When the thermocouples are exposed to the atmosphere, and I take measurements for two minutes, there is a repeatable drift for all four thermocouples. When I start taking data, the temperature readings stay near constant for about 25seconds, and then they all decrease about 0.1 - 0.4 deg C over the course of the next 100 seconds. This happens in both Labview and in MAX. I took data with and without auto-zero mode running, and this still happened. I have attached an image showing this data though I'm not sure if it is going to be uploaded.I understand that this is likely within the accuracy for the thermocouples, however it seems like something is floating which is causing the problem.  Any help would be appreciated. Thanks,Andrew O'Grady


USB_TCdrift2.JPG:
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[Steve_B]

Hi Andrew,
The temperature trend seems so be cooling off. What is the temperature of the atmosphere you are trying to measure? Does this behavior change if you are measuring a different temperature? I am also curious to see what happens if you run your test for longer than 2 minutes? Does this trend continue for a few additional minutes?

[AndrewO_nyc]

Hi Steve,

I should the explain the setup a little bit better:

-Three of the thermocouples are for measuring temperatures of air flow
inside of a long metal duct. The TCs are epoxied inside of small
ceramic tubes so that the tip of the TC is approximately even with the
end of the ceramic tube. I tried to not get epoxy on the tip of the
thermocouple but there definitely is some on the wires. The small
ceramic tubes are then flush mounted inside the wall of the duct. The
idea being that the TC tip is insulated from all temperature changes
except for the flow.

The fourth thermocouple is a surface mount one, placed on the exterior of the duct to help estimate the heat transfer rate.

When I said that i was measuring the "atmosphere", I should have said that
I am running a shakedown test without any flow so that the TCs are
measuring ambient room temperature.  I just took data for 2 consecutive tests, 10 minutes in length, and they again showed decreasing temperature which leveled off after 10 minutes. However both tests started and ended with the TCs close to the same values, so my feeling is that the temperature in the room is not actually cooling, but rather that the voltage is drifting in a repeatable manner.Thanks,Andrew


USB_TCs_10min_T11.JPG:
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USB_TCs_10min_T21.JPG:
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[Steve_B]

Hi Andrew,
 
If you are measuring your thermocouples in a differential mode, you could be experiencing some additional leakage current in your measurements. To correct this it would involve adding a grounding resistor to the negative lead of the thermocouple to discharge this current. I have included links to two KnowledgeBase articles that relate to this issue. The first article relates specifically to thermocouples and the second article explains more about the physical connections that need to be made. I would recommend you look ath the Floating Signal Source Differential Measurement that is addressed in Section 3.
Why Do I Need to Ground My Thermocouples When I Am Already Measuring Them in Differential Mode?<a href="http://digital.ni.com/public.nsf/allkb/862567530005F09C86256522005CD1EF?OpenDocument" target="_blank">http://digital.ni.com/public.nsf/allkb/862567530005F09C86256522005CD1EF?OpenDocument</a>
&nbsp;
Field Wiring and Noise Considerations for Analog Signals<a href="http://zone.ni.com/devzone/cda/tut/p/id/3344#toc0" target="_blank">http://zone.ni.com/devzone/cda/tut/p/id/3344#toc0</a>