Can't provide Iex for PT100 RTD using NI 9217

[ScienceMonk3y]

Hello, I'm trying to setup a PT100 RTD using the NI 9217 C-Series module but according to my multimeter (probes connected to RTD+ and COM on channel 1), there is no current flow. Also, Labview won't let me toggle from Internal to External or None as the Iex source.

[dmysliwiec13]

What readings are you expecting to see?

Danielle

Applications Engineer

[ScienceMonk3y]

This problem seems to be getting a bit stranger. I changed channels and am getting Iex = 0.5 mA which is fine but setting up the RTD as an 'RTD' in labview gives me a temperature of ~-270C that doesn't update when I immerse the probe in hot water. However I can setup  Labview to read the RTD as a resistance which DOES work (reads ~107 ohms at room temp which is about right considering conversion factor to temperature) but it gives me a negative value. This is easy enough to work around but really bizarre.

[dmysliwiec13]

How are you reading the temperature and resistance? With a DMM?

Are you running code and seeing these results?

When you run the Getting Started example for this module do you see the same unexpected results?

[Lombrozo]

I don't mean to bump an old thread but I just had this problem and noticed that this thread was never fully answered.

The negative number for the resistance measurement (as reported back through the Distributed System Manager or from a shared variable node) is because you plugged the wires in backwards and it's subtracting off the measurement resistance (RTD+ to RTD-) from the lead-wire resistance (RTD- to COM) instead of vice-versa. When done properly, this provides a more accurate value for the temperature-modified resistance of the PT100 sensor.

Using a DMM, figure out which combinations of the 3 wires provides a ~100 ohm or a ~1 ohm measurement. The two leads that provide a minimal resistance between them should be plugged into RTD- and COM (interchangeable). The remaining wire that provides a ~100 ohm measurement to the other two should be plugged into RTD+. If you're using an NI-9217 in 3-wire mode, the excitation current will be internally routed out through the RTD+ output.