Three-Wire RTD Still Reads High? Check Lead Resistance Balance
Three-wire RTD measurement reduces lead effects under the assumptions of the input circuit. Unequal leads or a poor connection can leave a significant error. A correct-looking three-wire terminal count does not prove correct compensation.
Lead compensation depends on the specific RTD input design.
What is happening?
Lead compensation depends on wiring topology and the measurement implementation. Resistance added asymmetrically by cable or terminals can violate its assumptions. Four-wire measurement uses a different arrangement and must not be improvised on an incompatible input.
Check these five things
-
Verify sensor type, wire count, and supported input mode.
-
Identify which conductors connect to the same sensor end.
-
Inspect approved terminations and cable consistency under isolation.
-
Compare with a suitable reference using the module calibration procedure.
-
Correct the physical cause before adding software offsets.
Worked example
In a teaching installation, one lead acquires extra contact resistance after a terminal repair. The sensor remains stable, but the channel develops an offset because lead effects are no longer balanced as expected. A sensor replacement alone would leave that connection fault in place.
Quick diagnostic reference
| Evidence | Question |
|---|---|
| Offset after repair | Changed lead or contact resistance? |
| Intermittent jumps | Connection stability? |
| Wrong slope | Sensor type or coefficient? |
The mistake to avoid
Shorting terminals to imitate a different wire mode can hide a wiring fault while invalidating measurement accuracy. Use only the connections documented for the actual module.
How to verify the fix
Validate several reference points and check repeatability after reconnecting approved terminals. Record the installed wiring mode and reference results rather than only the final displayed offset.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
OMEGA: RTD two-, three-, and four-wire connections. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.