How to Verify RTD Lead-Wire Compensation in an Installed Loop

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What You Need

Bring a decade resistance box or RTD simulator, a good ohmmeter, the loop drawing showing the wiring scheme, and the transmitter configuration tool. Know what the loop claims to be: the theory of why the schemes differ is covered in 3-wire vs 4-wire RTDs and RTD lead-wire resistance compensation, and this procedure is the field test of whether that theory is being honored.

Match the Wiring Scheme to the Configuration

At the transmitter or input card, physically count the conductors landed and compare against the connection type in the configuration. The classic faults are a sensor wired 3-wire while the input is configured 2-wire, which throws away the compensation entirely, and a 4-wire input jumpered down to two conductors at the terminals, which silently converts a compensated scheme into an uncompensated one while the configuration still says 4-wire.

Also confirm the third and fourth conductors run the full distance to the element, not just to a junction box where the cable changes. Compensation only cancels the resistance of the path the sense wires actually share with the measurement path; a 3-wire scheme that collapses to 2-wire for the last fifty meters leaves that last stretch uncompensated.

Measure and Match the Lead Resistances

Lift the sensor connections and measure each lead's resistance end to end; on a 3-wire sensor, the two conductors commoned at the element should read nearly identical values to each other. Three-wire compensation works by measuring one lead and assuming the other matches it, so any mismatch between the two passes straight into the reading. A corroded terminal or a crimp splice on one leg is enough to unbalance an otherwise healthy loop.

The arithmetic makes the stakes concrete: a Pt100 changes about 0.385 ohm per degree Celsius, so if each lead measures 2.6 ohm and none of it is compensated, the reading sits about 13 degrees high. That is why a 2-wire scheme is only defensible on very short runs, and why a mismatch of even half an ohm between supposedly matched leads is worth chasing.

Prove It with a Decade Box at the Field End

The decisive test: disconnect the element and connect a decade box at the field end of the actual installed cable, wired in the same scheme as the sensor. Set a known table value - 100.00 ohm for zero Celsius on a Pt100 - and read the loop. If the compensation is working, the indicated temperature matches the table value closely despite the cable between the box and the input; if the reading sits high by a stable offset, you are seeing the uncompensated lead resistance directly.

Repeat at a second resistance near the top of the range. A fixed offset at both points is lead resistance or a scheme mismatch; an error that grows with the reading points at the transmitter's interpretation of the signal instead, which is calibration territory rather than wiring - see calibrating a temperature transmitter for that split.

Verifying the Result and Common Mistakes

Reconnect the element, confirm the reading returns to a plausible process value, and compare the loop against its historical trend; a compensation fault you just fixed should appear as a step change matching the offset you measured. If the element itself has never been proven against the standard table, close the loop with the procedure in bench-testing an RTD.

Mistakes to avoid: testing with the decade box at the transmitter terminals instead of the field end, which bypasses the very cable you are trying to prove; ignoring self-heating by leaving high measuring current on a small element, a separate effect covered in RTD self-heating error; and correcting a lead-resistance offset with a configuration bias, which hides the fault until the next cable repair changes it.

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