Measuring voltage from thermocouples

[kenlimch]

How do i measure voltage from a thermocouple which is joined to another one. Meaning that they have a common point. I have tried reading in a voltage from 1 point and then subtracting it with the other but it does not give me the reading the multimeter is showing

[Annette Perez]

Hello,

For troubleshooting bad readings, probably the first thing to do would be to check the board's configuration.


Is it configured in differential mode? Differential mode is the most recommended mode for thermocouple readings. When using differential mode, just make sure to connect the positive lead of the thermocouple to ACH0 (for example), and the negative to ACH8.


Once you have all the signals connected to your DAQ board, try running a MAX Test Panel. I usually run it using Continuous Mode. Before starting the acquisition, check the input limits in MAX, adjust them according to the input signal so that the board applies the appropriate gain.


Finally, create a Virtual Channel (by taking into consideration the thermocouple type). Are
you reading the correct temperature? Is the CJC in your terminal block enabled? It's always easier to compare the temperature than the millivolts readings.


I hope these tips help. Good luck with your application

[kenlimch]

Hi,
Is it fine if I used single ended refernce to measure the voltage? I've actually tried using differential mode before but my input voltage readings and others go haywire after the change. Please advise because I'm new to Labview

[membruce]

When you tie 2 thermocouples together, you will have a problem of setting up a 3rd thermocouple in the junction.

Also, thermocouple voltages are non linear in respect to temperature, this means that if the temperatures are different, you can't just add the voltages without some error. You also have to use a "cold junction coupler" (CJC) to attach the voltmeter leads to the thermocouple. This is to prevent a thermoelectric effect in the lead connections.


Let me know if I can help you,
Bruce

[kenlimch]

Hi, Bruce
Thanks for attention to this message. Currently my main problem is getting a reading from the thermocouple using Labview. I've tried using differential mode but it gets worse such that my voltage readings for the RTD and other inputs go haywire. I know I must have made a mistake somewhere but as I''m still new to Labview, I can't help it. Attached is a manual of the thermal probe I'm working on. Please advise.

[membruce]

Unfortunately I am new to LabView also. Have you tried to simulate the TC reader? Maybe use a function generator set to low voltage and low frequency. Make shore you check your voltage before hooking up. That way you can divide the system and narrow the location of the problem.

You say it gets worse. How? Over time? As the temperature go?s up?

Bruce

[Anonymous]

Hello,

For troubleshooting bad readings, probably the first thing to do would
be to check the board's configuration.


Is it configured in differential mode? Differential mode is the most
recommended mode for thermocouple readings. When using differential
mode, just make sure to connect the positive lead of the thermocouple
to ACH0 (for example), and the negative to ACH8.


Once you have all the signals connected to your DAQ board, try running
a MAX Test Panel. I usually run it using Continuous Mode. Before
starting the acquisition, check the input limits in MAX, adjust them
according to the input signal so that the board applies the
appropriate gain.


Finally, create a Virtual Channel (by taking into consideration the
thermocouple type). Are you reading the
correct temperature? Is the
CJC in your terminal block enabled? It's always easier to compare the
temperature than the millivolts readings.


I hope these tips help. Good luck with your application

[gbush]

Looking at the manual, it looks like you are doing a couple of things, loading the circuit and creating a ground loop. Thermocouples are touchy in that they only like certain connections. The pt1000 that is connected as your second thermocouple is the cold junction conpensation value. When you connect anything else to those terminals, strange things happen. There are many thermocouple testers on the market which will test your thermocouple, and can be connented to your system to simulate the thermaocouple to test your system. If you are trying to remotely gather the readings into a computer for display/datalog, this is not a good way to do this. Either add a second thermocouple, or get a controller that re-transmits the readings(typically a 0-5volt signal that reflects the
scale of the device)