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Temperature Q&A with JR Madden: Our 3-Wire RTDs Drift and Require us to Recalibrate our Temperature Transmitters

Temperature Q&A with JR Madden: Our 3-Wire RTDs Drift and Require us to Recalibrate our Temperature Transmitters

Q. We have been using 3-wire RTDs in our process for years and are required to check the calibration once every 6 months. Each time we have checked the sensors we’ve noticed they have drifted. This requires us to re-calibrate our temperature transmitters. Do you have something you can suggest to help us reduce the drifting problem we regularly see in these sensors?

A. The problem with 3-wire RTDs is that the lead wires build up small amounts of corrosion causing a lead wire imbalance which results in measurement errors. If these are Platinum 100 ohm RTDs, just one ohm of resistance change on any lead wire can represent a +4.7°F degree error. We suggest using 4-wire RTDs which compensate for unequal resistance in the lead wires. For this to be an option your temperature transmitters need to accept a true 4-wire sensor input. Some temperature transmitters, PLCs, and DCS input cards only accept 3-wire RTDs. Others indicate a place to connect a fourth wire, but do not use it for measurement.

You can also use Class A RTDs with 4 wires and a transmitter that can accept and be calibrated with a 4-wire RTD. Our Class A element RTDs with a .06% accuracy have been cycled for 1000 hours, and will hold their accuracy for a 5 year period.

You can learn more about using 3-wire and 4-wire RTDs in the white paper: A Practical Guide to Improving Temperature Measurement Accuracy. 

These Moore Industries temperature transmitters are designed to accept true 4-wire RTDs: RIY, STZ, TCS, TDY, TFZ, THZ, THZ3/TDZ3, TPZ, TRY, TRX. For these and other Moore Industries temperature solutions, visit our Temperature Sensors, Transmitters and Assemblies page.

Published: April 13, 2016
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