How do I measure thermocouple with SCB-68 and Traditional NI-DAQ?

[AB-SP]

I am using the SCB-68 with the AT-MIO-16XE-50 multifunction board. I have two thermocouples connected to the SCB-68. I have configured the dip switchs on the SCB-68 for "Temp Sensor Enabled" and "Accessory Power On". In MAX I have set the range from "0-10 V". I am using Traditional NI-DAQ functions to read the values in LabWindows/CVI.

The problem is that my readings are jumping around by about 4C. Is there something I can do to get more accurate readings in LabWindows/CVI with this hardware?

[bushman]

Is your MIO configured for differential inputs?

[AB-SP]

Yes, the MIO is configured for DIFF

[LA 75]

Hello,

I have a couple of questions please before we determine where the problem is coming from.

1. Are your readings jumping when you make your measurements in the Test Panels in MAX? Or does it only happen on CVI?

If it is happening also in MAX, it may be more of a hardware problem. On the other hand, if it only appears on CVI, it may be more of a programming problem. If it is happening in MAX, let me know how you are making your connections (what pins you are using) and also try a DC voltage coming from a battery or a power supply just to be sure that the 'jumping' is not coming from the thermocouple measurements but it is originated in the board. If the problme is just happening in CVI, try running one of our shipping examples that come with the NI-DAQ drivers or that are available on the web.

Please let me know if you have any additional questions.

Thanks,

LA

[AB-SP]

Here are the answers to your questions.

1) The measurements are jumping in CVI and in MAX.

2) I have measured voltages (around 0.2 V) and the readings are very stable. Maybe my problem with the thermocouples is the low voltage levels (typically 10mV)???

[George M.]

Hi,

What you are seeing (4C jumps) is more than likely noise in the signal. The first thing you should do is tighten up your range. You could use a range of 0 to 100mV if you know your signal won't be larger than that. If that doesn't help completely, you can just take an average to get a better reading. Or, you can get some of the SCXI products that will condition a thermocouple signal for you. I hope this helps, and have a great day!

George