3-wire RTD Wiring for SCXI1322/1122

[C_M_W]

Hi, I am trying to wire a 3-wire RTD to an SCXI 1322 connected to an 1122. I wired it using the following: RTD Black 1: CH0 +, RTD Black 2: CH8+, RTD Red: CH0-, Jumper wire: CH0- to CH8-. I am using the multiplex configuration to give the excitation to the RTD. The temperature it is reading equates to around 2 ohms too high to what it should be measuring. The resistance between the two black RTD wires is around 2 ohms. I believe that the DAQ system is not compensating for the resistance between the two black RTD wires and that is why it is reading a higher temperature than it should. Is there a way to have NI Max compensate for this resistance? or should the wiring be different? The RTD I am using is an Omega PR-11-2-100-1/16-6-E. Which is a PT-100 ohm RTD, 3-wires, and an alpha value of 0.00385. Thank you for the help.

[wsimpson0050]

In a proper 3‑wire RTD, the two same‑color leads are tied together at one end of the RTD element. Their individual resistances should be equal (or very close), so the DAQ can subtract one lead’s resistance from the other and cancel lead resistance. If you measure about 2 Ω between the two same‑color wires, that usually means they’re not actually bonded together at the element (or there’s damage/miswiring), so the system can’t cancel the lead resistance and your reading skews high. A PT100 at room temp is ~110 Ω; adding ~2 Ω of uncompensated lead resistance will look like a warmer temperature. NI’s configuration expects the two “same‑end” wires to be effectively shorted at the sensor so their resistances match; it does not numerically compensate large mismatches between them.

NI MAX might fix the issue because:

• Three‑wire math assumes symmetry: The compensation relies on the two same‑end leads having equal resistance. If one is ~2 Ω different, the subtraction won’t cancel properly. NI‑MAX’s built‑in RTD measurement uses that assumption; it doesn’t include a per‑channel manual offset to “subtract 2 Ω” from one sense lead in hardware mode.

• What NI‑MAX can do: It lets you select RTD type (PT100, α=0.00385), excitation, and wiring (2/3/4‑wire) so the device applies the standard conversion. It doesn’t correct a wiring or sensor lead fault

Read:

Making an RTD or Thermistor Measurement in NI-MAX - NI

Your jumper between the lows is correct in multiplexed setups where channels share excitation/return. The key is ensuring the two “same‑end” leads truly are the matched pair at the sensor. NI’s RTD wiring instructions in NI‑MAX and the 1122 documentation walk through this topology; follow the module’s RTD example for 3‑wire and make sure the pair goes to the two “+” inputs for the selected channel group, with the single lead to the “–” input.

Read:

NI SCXI-1122 Datasheet | ArtisanTG

You can also try to check Omega’s datasheet for PR‑11‑2‑100 color code. The two same‑end wires should be the same color and should measure ~0 Ω between them (a near short), not ~2 Ω. If they’re not near 0 Ω, the probe or extension cable is miswired or defective. Try to re‑terminate or replace the cable/probe by fixing the junction so the two same‑end leads are truly tied together at the element with equal resistance.