How do I take a thermocouple reading with a pci6025E?

[bradle95]

How do I take a thermocouple reading with a pci6025E? I have seen some postings with scxi-1112 modules which I do not have. I need advice from the ground up. Do I need external circuitry, can I just wire both leads of the thermocouple to Ain0 differential pins, what vis are necessary in the actual programming... If there is an example program or anything else that might seem relavent? I appreciate any help.

[LA 75]

Hello,

You can do thermocouple readings with a pci6025E without using signal conditioning, but your results won't be as accurate as if you were using our Signal Condidioning devices. The way to do a differential measurement for your thermocouple would be simply to wire one end to ACH0 (pin 68) and the other end to ACH8 (pin 34.) Then open Measurement and Automation Explorer (MAX) to see if you are seeing a voltage reading through CH0. Go to Devices and Interfaces, select the board you are using and click on Test Panels. After checking that you effectively have a voltage value through CH0, open LabVIEW and go to Help=> Find Examples. Click on the Search tab and type Temperature. You should see a bunch of examples coming out that are thermocouple related. Choose the one you think is more appropiate for your application, select the device number you are using, CH0 and run it.

Please let me know if you have anu additional questions.

Thanks,

LA

[bradle95]

Hello, thank you for the help. I have seen examples using scxi modules and NI-DAQmx programming to find temperature using a thermocouple. For my application, I do not have a scxi module and my programming is in traditional NI-DAQ. I tried copy and pasting a NI-DAQmx application into my program and it did not work due to the traditional and mx conflict (first I checked it by itself and it did work). My question now is how can I measure temperature using a thermocouple with Traditional NI-DAQ programming on a PCI6025E? Thank you.

[LA 75]

Hello Bradlee,

What version of LabVIEW do you have? I am attaching a code for analog input buffered acquisition made in traditional DAQ. Please let me know if this helped.

Thanks,

LA

[bradle95]

I have Labview 7.0. When I tried to open the sample program that you had attached, I could not do so. "My Data Processing.vi" was not found. I went ahead and opened the file without this vi to take a look at the block diagram, and did not see any thermocouple conditioning. I am assuming that this has to do with the missing vi, but that is my question, how do I program for an application to read a thermocouple voltage and calculate the temperature? I am trying to do this on a PCI6025E DAQ board, with Labview 7.0 with in traditional DAQ. Thank you.

[George M.]

Hi Bradle95,

An example of taking a thermocouple measurement using Traditional DAQ can be found here. This is a fantastic example that should do everything you need it to. Please let me know if you have any questions, and have a Great Day!

George
Applications Engineer
National Instruments

[bradle95]

Thank you. Now it is looking pretty good, but I am not quite there yet. I have two thermocouples attached to Analog Input pins 0 and 1. I have one pin set as my CJC, and the other as my thermocouple that I will be reading the temperature from. When I hold one to increase the temperature, it does increase on the labview reading so I am good up to this point. My question now is why a diffence in my temperature and my CJC is registered when they are acutally both at the same temperature (if I just leave them near each-other in the ambiant air.) Is this just a problem with resolution, or is there something else that I need to do?

Scott

[Quantumbyte]

This is most probably a result of multiple factor that make the measurement imperfect:

- You're not using signal conditioning, so your resolution is definitely very reduced. Any temperature drift in the board could cause a big difference in the reading. I recommend you run your vi for approximately half an hour, unplug the accessory and perform a self calibration. This will have the board at the operational temperature you're expecting and will give you the best results.


-I personally would recommend using a different sensor type (RTD or Thermistor) as the CJC source, in order to obtain an accurate measurement, otherwise you have a bit of a "chicken and egg" problem, because you're using thermocouple 2 as the CJC for thermocouple 1, but what are you using as the CJC source of thermocouple 2?

- Are both your thermocouples of the same type and manufacturer? Due to the nature of the thermocouple itself, you can find slight variations between thermocouples from different vendors, due to the purity of the metals used. While this would usually be almost impossible to detect, it might be driving your voltage just enough to be in the next detectable voltage step (again, due to the reduced resolution).

- I've worked with temperature sensors for a while, and I'd still recommend that you try some sort of signal conditioning in order to increase your accuracy. Even a small amplifier and a passive lowpass filter will do great. I know you said you don't have SCXI, but if you want some lower cost ready to use applications you might consider looking at the SCC Carriers and modules we have.

I hope these suggestions help. Please let me know if you have any other questions.

Daniel