How to calibrate PT100 temperature sensors in the PLC

Why PT100 sensors lose calibration over time

In industrial automation, temperature sensors like the PT100—which are essentially platinum resistors whose electrical resistance changes predictably with temperature—suffer continuous wear and tear. Factors such as excessive mechanical vibration, aggressive thermal cycles, and the natural oxidation of electrical connections cause micro-cracks and deviations from the original calibration. When this happens, the actual process temperature differs from the value displayed on the PLC (Programmable Logic Controller, the rugged computer that runs factory machinery).

To keep production processes within strict quality and safety standards, instrumentation technicians must perform periodic preventive and corrective maintenance. If you have ever needed to test an NTC temperature sensor on a board with a multimeter in smaller electronic circuits, you will notice that industrial logic with the PT100 requires even greater mathematical and electrical rigor due to the critical nature of factory processes.

Identifying deviation in the temperature loop

The first practical step before altering the PLC programming is to isolate the problem to find out whether the error lies in the physical sensor, the extension cables, or the analog input card. Often, operators notice that a boiler is boiling, but the supervisory system shows a lower temperature. This systemic error usually manifests in two main ways: a zero error (a fixed offset across the entire range) or a span error (where the error grows proportionally as the temperature rises).

To diagnose this accurately, disconnect the sensor terminals directly inside the control panel and insert a process calibrator or a resistance decade box. This tool simulates exact ohmic resistance values corresponding to known temperatures, such as zero degrees Celsius. If the PLC continues to display the wrong value even when receiving a perfect standard resistance, the issue is in the analog channel configuration or the input module, rather than the PT100 probe installed in the field.

Step-by-step field calibration procedure

When calibration must be done directly on the industrial loop without dismantling the process, follow a safe and methodical procedure to ensure the error is fully neutralized. Below are the core steps executed by instrument maintenance teams:

1. De-energize the loop section and disconnect the PT100 cables from the PLC analog input module using an insulated flathead screwdriver.
2. Connect a calibrated resistance simulator to the PLC input terminals, injecting the exact value corresponding to 0 °C (100 ohms for the PT100).
3. Access the PLC programming software, locate the scaling blocks or offset and gain parameters, and adjust the reading until the supervisory system displays exactly 0 °C.

After adjusting the zero point, repeat the procedure by injecting the resistance for the upper limit of your operating range, such as 100 °C (138.51 ohms). Adjust the gain or span in the software until the value matches perfectly, ensuring linearity across the entire loop operating scale.

Offset adjustments in PLC logic

It is not always necessary to resort to the manufacturer's configuration software or physically calibrate the field transmitter. In many cases, when the deviation is linear and small, automation engineers prefer to implement a simple mathematical correction directly in the PLC code. This approach saves downtime and centralizes adjustment history within the controller program itself.

To do this, a fixed value (known as an offset) is added to or subtracted from the raw variable read by the analog-to-digital converter, in addition to multiplying the result by a proportional correction factor (gain). While this is a quick fix, it should be used cautiously to mask only minor mechanical drifts, never severe structural insulation failures or partial cable breaks.

Common errors and cabling best practices

One of the biggest enemies of precision in PT100 loops is the parasitic resistance of the copper wires connecting the sensor to the panel. Because the wire has its own resistance, it adds to the platinum resistance, causing the PLC to read a temperature higher than the actual one. This is why critical industrial systems use three-wire or four-wire connections, which can automatically cancel out cabling distance interference.

Additionally, using shielded cables grounded at only one end prevents electromagnetic induction generated by neighboring motors and frequency drives. If there is electrical noise in the loop, the sensor reading will oscillate wildly, making PID control unstable and harming final product quality. To complement this implementation in production environments, see our related technical analyses on How to test a triac with a multimeter on the bench and Slow database? How to optimize queries in Postgres.

Conclusion

Calibrating PT100 sensors and integrating them correctly into the PLC is a task that requires patience, knowledge of electrical physics laws, and technical rigor. A well-calibrated sensor ensures energy efficiency, prevents raw material waste, and protects machinery against catastrophic overheating. Adopting a structured routine of checking with traceable standards elevates the level of industrial maintenance and ensures the reliability of the entire plant.