Measuring temperature Via Thermocouple and USB-6009?

Hi,
I am trying to perform temperature measurements using a k type thermocouple and the USB-6009 DAQ. I tried to use the Cont Acq Thermocouple Sample.vi from LabVIEW's example list, but unfortunately I don't know how to fit my DAQ and thermocouple in the vi code. I don't know how to create a thermocouple temperature measurement channel or how to get all the channels in the task, etc.
Any help is appreciated.
Thanks.

bilbeigg,
It should be noted that the USB-6009 is not well suited for thermocouple measurements. It has 14 bits of precision with a minimum input range of +/-1V, which means it can only detect a voltage change as small as about 122 microvolts. This is much too large a change to get a good temperature reading with most types of thermocouples. Here is a discussion thread on the same topic (Note that the USB-6008 and 6009 are very similar):
http://forums.ni.com/ni/board/message?board.id=170&message.id=294080#M294080
Here is a knowledgebase that explains what you are seeing:
http://digital.ni.com/public.nsf/allkb/8166630BD6965EEA86256FDD0052E89C?OpenDocument
Chris M
Message Edited by CMal on 03-12-2008 12:17 PM

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I'm using NI-DAQmx 7.5, Labview 7.0, and a USB-6009.
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).
Common thermocouple types have the following response:
K type: 41 microVolts per degree Celsius
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

Measuring temperature Via Thermocouple and USB-6009?

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