Hysteresis in temperature sensors
A sensor can read differently at the same temperature depending on where it came from. Calibrations that only approach from one direction hide it.
Hysteresis is the dependence of a sensor's indication on its previous thermal history. Platinum resistance thermometers exhibit it mainly from strain in the element and its encapsulation; thermistors and thermocouples have their own mechanisms.
Detecting and treating it
- Calibrate on both rising and falling sequences and compare readings at common points.
- Treat half the difference as a standard uncertainty contribution with a rectangular assumption if it cannot be corrected.
- Consider whether the service use is one-directional, in which case the calibration direction should match it.
Sensors that have been thermally shocked, dropped or used beyond their range often show increased hysteresis; a change from the previous calibration is a warning sign.
Frequently asked questions
- How much hysteresis does a Pt100 have?
- Good industrial PRTs show a few millikelvin over a 0–100 °C cycle; thin-film and poorly made sensors can show 0.05 °C or more. It is evaluated per sensor by calibration in both directions.
- Can hysteresis be corrected?
- Not reliably, because it depends on history. It is normally treated as an uncertainty contribution, or the calibration direction is matched to the service use.
References
- [1]JCGM 100:2008 — Guide to the expression of uncertainty in measurement (GUM)
- [2]IEC 60751 — Industrial platinum resistance thermometers and platinum temperature sensors
General technical guidance written against the cited sources. It is not regulatory or legal advice and does not replace the applicable standard, guideline or a qualified reviewer's judgement.
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