Can´t measure temperature with PXI 4351

[Hernar]

Im having troubles measuring temperature with thermocouples. When I measure the temperature of the air or places which are not with potential the card works well. But if I want to measure temperature for example in a transistor that is under a voltage of 13 volts the card shows wrong readings. If I´m not mistaken this card has an input voltage range of +- 15 volts, so the voltage in the transistor is within the input voltage range.

I read that you can configure the input voltage for +-.625mv, +- 1.25, +- 2.5, +- 3.75, +- 7.5, +- 15 volts, but I cannot find the tool or the way to do this.

Do you have any suggestion of what to modify in order to make it work? I attached my code.

Thanks,

Hernar

LabVIEW 8.2

PXI 4351

[SFK]

Hi Hernar,

you are right regarding the maximum voltage range of the PXI-4351, but the issues you are experiencing have other causes.

A) as described on page 2-5 of the NI 435x User Manual, the open thermocouple detection uses a voltage of 2.5V applied to the CH+ input through a high impedance resistor. When you now place the thermocouple on the transistor's head with a +13V potential, the card may be distracted by this voltage level and assume a broken wire, which will void your measurements.

B) in your VI, you activated grounding. If your three thermocouples are all on different cards, this may work, but if they are electrically interconnected, you create a ground loop with this. It might be wiser then to use "real" grounding with the +13V potential as signal ground. This way, you had no common mode voltage on the input amplifiers of the card.

Best regards,

Sebastian