Thermocouple Noise and Calibration SCB-68

I'm measuring two Thermocouples on an SCB-68. Measuring voltage directly and converting or using MAX I get very steady but inaccurate measurments (they read about 8degC below known values).
If I short a channel and read it, I find that there is an offset in the micro volt range. Measuring this and subtracting it from the TC readings I find that my average readings are now pretty accurate but very noisy (+/- 1degC).
I also notice that if I increase the sampling rate the offset changes (which results in a change in the TC readings).
Is there a better way to eliminate the offset or do I have to tolerate the noise?

Hello Jeramy,
Because thermocouples produce such small signals (mV range), it is best to apply some sort of signal conditioning before measuring the signal. Gain should be applied to amplify the signal and make better use of the range of the ADC. Hardware filtering helps remove noise from the signal. Please see the following tutorial for more information on thermocouples:
http://zone.ni.com/devzone/conceptd.nsf/webmain/806355D26B7378B1862567EE0003A842
National Instruments offers a variety of signal conditioning products. Please see the following Knowledge Base for information on these products as well as the pros and cons of each:
http://digital.ni.com/public.nsf/websearch/F0196F5B6CD8F60286256D4B00728B78?OpenDocument
To correct the offset, I would run a self-calibration on the DAQ device. You could also reduce the noise in your measurements by oversampling and averaging the signal. That way you don't have to buy new hardware.
Let me know if this does not help.
Regards,
Sean C.

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I'm trying to get a more accurate reading from my j-type thermocouple. It is now distorted with noise and jumps rapidly +/- 20 degrees around room temp without any heat being applied. When heat is applied, it climbs as it should but with the same variation. I haven't moved to the labview side of it yet, I'm still trying to get a usable reading in MAX. I have it wired as a differential input with a floating signal source. I've installed 10k ohm resistors for bias current return as instructed. Installing the resistors helped but still leaves me with the readings shown in the attached pic. I've also tried grounding my USB 6009 with no results. Please help. john
Attachments:
thermocouple reading.jpg ‏41 KB

Hi John,
What you are running into is a problem with the actual voltage resolution of your USB-6009. This device has 14-bits of resolution over a minimum range of +/- 1V. That means that the voltage resolution is (1 - (-1))V / (2^14) = 2V / (16384) = 122uV. While this is an excellent resolution for such a low cost USB device, you will still notice problems when trying to make a thermocouple measurement. The reason for that is that Thermocouples make a very small output voltage (see image below).
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J type: 56 microVolts per degree Celsius
T type: 52 microVolts per degree Celsius
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Good: You could read the signal in as a voltage and then manually convert that voltage into a temperature. This would be the least accurate method, but it should allow you to average out your signal and then use the Convert Thermocouple Reading.vi to show the voltage as a temperature.
Better: If you want to keep using the USB-6009 to measure thermocouple, what you really need to do is first amplify your signal before you connect it with your device. This should increase your resolution of the thermocouple allowing you to get more accurate temperature specs. This will require some more complex programming to properly scale your measurements, but it will give you more accurate readings for relatively little cost.
Best: Opt for getting a USB device that is designed for taking temperature measurements such as the USB-9211A. This device would give you a voltage resolution of 160mV/2^24 = 9.54 nV! That's over 12500x the resolution of the USB-6009 that you currently have.
Hopefully the above information helps you better understand what is going on and why. If you have any other questions feel free to post them here.
Message Edited by Otis [DE] on 01-13-2006 11:37 AM
Message Edited by Otis [DE] on 01-13-2006 11:38 AM
Otis
Training and Certification
Product Support Engineer
National Instruments
Attachments:
761029-Thermocouple_Voltages.GIF ‏10 KB

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Thermocouple Noise and Calibration SCB-68

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