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C.1 - LJTIA Thermocouple Troubleshooting

Following is a basic procedure for troubleshooting and testing thermocouples connected to an LJTick-InAmp.  We will look at the voltages using the test panel in LJControlPanel.  This takes software complications out of the loop.  Make sure all other LabJack related software is closed.

Start by removing all signals from the LJTIA, and jumper all 4 inputs (INA+/INA-/INB+/INB-) to GND.  Turn off all switches on the LJTIA, except #5, to give you a gain of x1 and offset of 0.4 volts.  You should see that both channels measured by the LabJack measure about 0.4 volts.

Now turn on switches 3 and 9 (in addition to 5), to give a gain of x51 and offset of 0.4 volts.  The voltages measured by the LabJack will likely change a little, but should both still be near 0.4 volts.

Now remove the jumpers, connect the +/- of a thermocouple to INA+/INA-, and also connect a 10k resistor from INA- to GND.  Pull on both leads going into INA- to make sure both are clamped securely.

A thermocouple gives you a voltage related to the difference in temperature between the 2 ends.  So if the remote end is at the same temperature as the LJTIA end, the voltage difference created by the thermocouple should be about 0.  That means you should get a reading about the same as when you had jumpers to ground.

Now try changing the temperature of the remote end of the thermocouple. Here are charts where you can look to see what voltage you expect:

http://www.temperatures.com/tctables.html

For example, say you have a Type K thermocouple:

http://www.pyromation.com/downloads/data/emfk_c.pdf

The voltage at the 25 deg C end is 1.000 mV and the voltage at the 35 deg C end is 1.407 mV, so the difference is 407 uV. That is the voltage difference that should be created across the INA+/INA- terminals.  You can confirm it with a DMM if you have one good enough. The LJTIA multiplies this by x51 and adds about 0.4, so your voltage measured on the test panel should be about 0.4208 volts.  That is, about 0.0208 volts higher than the reading with no temperature difference.

You can confirm that 0.421 volts by measuring FIO0 versus GND with a DMM and see if it is the same as the test panel reading.  If not, the reason is usually that the signal is varying.  The DMM will give an average reading over 0.5 or 1.0 seconds, whereas the U3 gives a reading sampled over perhaps 500 microseconds.