Two different kinds of "not quite right"
Hysteresis is systematic and direction-dependent: at the same input, the reading arriving from below differs from the reading arriving from above — friction in a movement, backlash in a linkage, elastic behaviour in a sensor. Repeatability is random: repeat the identical test the identical way and the readings scatter. The definitions:
Worked example
At the 50% point of a 4–20 mA transmitter: upscale reads 12.010 mA, downscale 11.980 mA, second upscale 12.014 mA:
- Hysteresis = |12.010 − 11.980| = 0.030 mA = 0.030 ÷ 16 = 0.19% of span
- Repeatability (up) = |12.010 − 12.014| = 0.004 mA = 0.025% of span
Field notes
- Approach discipline is everything: overshoot a point and re-approach from the wrong side, and the "hysteresis" you record is fiction. Generate the proper up/down sequence with the calibration table generator.
- Compare against the accuracy spec: combined hysteresis + repeatability should sit comfortably inside the instrument's stated accuracy — translate specs with the error converter.
- Rising hysteresis over successive calibrations is a mechanical wear signature — worth trending, and often the earliest warning an instrument gives.
Frequently asked questions
What is hysteresis in an instrument?
The difference between readings at the same input approached from below (upscale) versus from above (downscale), caused by friction, backlash or sensor behaviour. It is reported as the maximum such difference, usually in percent of span.
What is repeatability?
The spread between repeated readings at the same input, approached from the same direction under the same conditions. It captures random variation, where hysteresis captures direction-dependent offset.
How do I measure hysteresis correctly?
Run all test points upscale without overshooting, then downscale without undershooting. Any overshoot at a point contaminates the measurement — back well off and re-approach if it happens.
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