Measuring voltage from thermocouples

[Marco_cas_84]

Dear all,

I have a doubt about reading voltages from thermocouples.

I've read many discussion on this forum on this topic (for instance http://forums.ni.com/t5/Multifunction-DAQ/measuring-millivolts-cDAQ-NI-9205/m-p/950562/highlight/tru... http://forums.ni.com/t5/Multifunction-DAQ/Thermocouple-channels-stay-differential-once-you-assign-th... http://forums.ni.com/t5/Multifunction-DAQ/Thermocouple-Noise-and-Calibration-SCB-68/m-p/214645/highl... but there is something that is not clear to me.

I'm trying to read voltages (in differential mode) and trasform manually these voltages into temperature from a E-type thermocouple directly connected to my SCB-68 terminal block.

In order to avoid errors I'm following this procedure:

a) manually do a short circuit on PIN ai1-ai9;

b) read voltages @ 1 kHz in order to compute the mean offset voltage (Voff);

c) connect my thermocouple to PIN ai1-ai9 and record voltages (Vmeas) while checking the temperature of the internal temperature sensor for CJC (Visothermal);

d) compute the value of the "corrected" voltage from the following equation: (Vmeas-Voff) + V_isothermal;

e) associate this value to a temperature thanks to the V-T E-type TC curve.

The things seems to be quite good at room temperature, I compute 24.4 °C while the CJC perform 25 °C, and maybe I can improve my software with a filtration in order to avoid the 50 Hz noise.

However I want to know if there is a method to do the Voffset via software and not manually.

Unfortunately I got a NI-PCI-6281 which seems not to support the OTD (open thermocouple detection) which should solve my problem.

Do anybody have some suggestion?

I need to perform all this procedure since I have to do measurements below -200 °C so that I cannot use the NIST polynomial functions and AI temp TC channels (I mean the authomatic temperature measurements of LabVIEW).

Thank you very much for your help.

Regards,

Marco C.

[GiuseppeCioffi]

Hi,

your measure mode is optimal but you can use also MAX configuration to do compensation and give in output (setting as well a custom scale) directly temperature information.

Have you ever performed a thermocouple measure directly by MAX?

Thanks,

Giuseppe.