Learn Automation with MINATA #70: Sensor Calibration and Scaling — From a Signal to a Value You Can Trust
Sensor Calibration and Scaling: From a Signal to a Value You Can Trust
A tidy number on the HMI does not prove the number is right. The path from the physical quantity to the PLC tag passes through several links, and the error can sit in any of them. Correct calibration and scaling are the condition for trusting the figure.
Draw the measurement chain
Draw the whole chain: physical quantity → sensor → transmitter → wiring → isolator, if fitted → analogue input module → scaling in the PLC → the unit shown on the HMI, together with the calibration history. When an error appears, that chain tells you where to measure, rather than replacing the sensor on a hunch. The fault may lie in the transmitter, in the scaling or in the module, and not in the sensor at all.
Declare the scaling completely
Scaling has to state the module's raw range, the engineering range, any offset, the limits and the out-of-range behaviour. The linear formula is only correct when it uses the real raw range of that module. Record the three ranges — sensor, module and display — together, so the HMI never claims one range while the PLC scales another.
Handling under-range and fault
With 4–20mA, 4mA may be engineering zero, but a value below 4mA has to be treated as a fault or under-range according to the datasheet — never confused with a genuine zero. A transmitter with a broken wire gives roughly 0mA. If the PLC reads that as 0 bar rather than "signal lost", the machine can make a wrong decision. Define the threshold and the response explicitly.
Calibrate at several points
Check the scaling at several points across the range — 0%, 50%, 100% — against a reference, not only at midscale. Non-linear error only shows up when several points are checked. Record the date, the reference instrument used and the result, so drift can be followed over time.
A worked engineering situation
A 4–20mA pressure transmitter covers 0–10 bar. During commissioning, the technician applies reference pressures at several points, compares them with the PLC value, confirms the scaling and configures anything below 4mA to raise a fault. So when a wire breaks later, the PLC reports a lost signal instead of showing 0 bar and letting the machine carry on.
The ranges, the thresholds and the calibration procedure all follow the equipment datasheet and the process requirements.
Common mistakes
- Trusting the HMI number without checking the measurement chain.
- Scaling against the wrong raw range for that module.
- Reading anything below 4mA as a genuine zero, losing broken-wire detection.
- Checking only one point, and missing non-linear error.
- Keeping no calibration history, so drift goes unnoticed.
Calibration and scaling checklist
- [ ] The full chain from quantity to PLC tag is documented.
- [ ] Scaling declares raw range, engineering range, offset and limits.
- [ ] Under-range and fault are handled separately from a genuine zero.
- [ ] Several points across the range are verified against a reference.
- [ ] Date, reference instrument and result are recorded.
A value that is trustworthy at its source is the foundation of correct control and monitoring. #71 turns to pneumatics and pressure monitoring, so a machine never quietly runs short of force.
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