9216 3-wire RTD 100ohm input setup

[dmeissner]

I am using Q1 2023 LabView with windows 10 and the NI-cRIO 9039. I am using a 3-2ire 100ohm RTD that is working correctly when using a multimeter to measure resistance. I have it connected to an NI-9216 with spring terminal pinout with all equal wire lengths. I have the + of the RTD connected to the RTD + on the 9216 the - connected to the RTD minus and the GND connected to the 9216 COM input. My value in labview is reading -490. When I heat up the RTD is goes further negative. I feel like i could just make a generic variable to multiply by -1 and subtract about 22 to room temperature but I will be off by a couple degrees. How do I get labview to read an RTD correctly? The tutorials I am finding are not helping. I found the one that walks through the steps with the 9217 but I can not switch my device to 3-wire. That does not show up in my labview project or NI MAX. I am at a loss. Could someone point me in the right direction? Where would I find the proper tutorial to read an RTD

[Bob_Schor]

The documentation on the NI-9216 seems a bit "scant", but it does say it works in both 3-wire and 4-wire configuration.  This is from the NI-9217 "Getting Started" manual:

Hope this is helpful.

Bob Schor 

[Minions]

Your system may be configured for a 500-ohm RTD as these can use several different varieties of RTDs.  Ensure that the system is set for a 100-ohm RTD and is wired according to the information provided by Bob_Schor above.

The key takeaway here is the information after the schematic and before the note.

The 3-wire RTD works based upon the voltage reading at the RTD- point.  Based on all wiring being equal in length and resistance, the voltage rise recorded at the RTD- input would equate to two lengths of wire [(Ground->RTD) & (RTD->RTD- Input)].  This means that the voltage loss due to the resistance from the RTD+ to RTD- should equal that from RTD+ to GND or RTD- to GND.  That will leave you the voltage across the RTD.  Now all that is left is to find the resistance of the RTD and convert it to temperature.

[dmeissner]

OK, it turned out my RTD labels didn't line up. The GND went to the +, the - went to GND, and the + went to -. Took me too long to figure that out.