Connection of PT100 to NI9217

[SteveR_]

So I've got a couple of PT100 RTD's connected as in the attached image and I'm struggling to get both of them to read correctly at the same time. When I just connect the Gauge PRT at a 3 wire with the blue, yellow and white wires it works, and the same with the Fluid PRT using the white and two red wires. When I connect them both to two channels on the ni9217, one reads as expected but the other is reading as if it is short circuiting the PRT (-244 degrees C which is about 0 ohms from a PT100). I cant figure this out and have tried several different orientations. Also, surenrly given the orientation of my two RTD's, using a 1mA excitation for both would give me a 2mA excitation for the second gauge given current laws (1mA and 1mA into a node gives 2mA out). Also, Labview will only let me use internal excitation and complains every time I try to select external or no excitation. 

Any ideas of how I could make this read as expected?

[GerdW · Accepted solution]

Hi Steve,

according to manual you can connect all the COM inputs of the module.

So I suggest to put the common connection between your 2 Pt100 to COM of your two channels.

Then wire according to the specs as shown for 3-wire-RTDs, (you might connect RTD- and COM too)…

[SteveR_]

Not sure why I never thought to just use a common ground between the two RTD's to solve the problem but there we are. Thank you for spotting my idiocy and solving the problem.