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9213 not returning actual temperatur values
ceskyhlas
05-06-2011 04:52 PM
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Hello,
I am using the NI USB 9162 carrier with a NI 9213. I need to read the temperature of some J type thermocouples and graph them for a period of 4 hours. After I start MAX I use Labview Signal Express. The step setup seems easy enough, but the values the thermocouple are returning seem off. I start it and it goes to about 1.376k in Celsius or 2. something in fahrenheit. I have applied heat with a hot air gun and let the thermocoule sit in ice, but it constantly shows the same value. Even when I try to calibrate it the numbers don't vary much.
I have a thernoneter with me that I connected the thermocouple to and it reads all the changes in temperature. Please help, I have tried everyhting I can think of.
I am brainstorming other options, like misplaced thermocouple connections inside the 9213 module, or my USB cable is too long to send a proper signal, it is only 6 feet though.
Please let me know if anyone can think of anything.
Brandon_G.
05-10-2011 08:47 AM
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Hi ceskyhlas,
It sounds like you may have an open thermocouple across your input. Each channel has an open thermocouple detection (OTD) circuit, which consists of a current source between the TC+ and TC– terminals. If an open thermocouple is connected to the channel, the current source forces a full-scale voltage across the terminals.
Here are a few suggestions to help narrow in on the problem.
1. Perform a self test on the 9213 in MAX and ensure it passes.
2. Use test panels in MAX to test your 9213 to see if you can read a voltage level on your thermocouple. At 25 degrees C you should get a voltage of 1.277 mV on a J type thermocouple.
3. See if you can independently measure a voltage on your thermocouple using a DMM.
4. Ensure that your leads on your thermocouple are connected properly- often times the red wire on the thermocouple is the (-) terminal.
5. Do you see the same readings on different channels?
Please let me know if you run into any problems after trying the above troubleshooting steps.
Thanks!
Brandon G
Precision DC Hardware Engineer
