Terminology and Measurement Error
Accuracy, repeatability, range and the words that matter.
12 min
Instrumentation has a precise vocabulary, and using it loosely leads to the wrong instrument being bought and the wrong conclusion being drawn from a reading.
Core terms
- Range — the lower and upper limits the instrument is calibrated to measure, for example 0 to 10 bar.
- Span — the difference between them; in that example, 10 bar.
- Zero — the lower range value. Elevated or suppressed zero applies where the measured variable at 4 mA is not actually zero, which is common on level measurement with wet legs.
- Accuracy — how close a reading is to the true value, usually quoted as a percentage of span or of reading. Note the difference: a percentage of span figure means the absolute error is the same across the whole range, so the relative error at the bottom of the range is much worse.
- Repeatability — how consistently the same input produces the same output. Often more important than accuracy for control, because a control loop can be tuned around a consistent offset but not around a wandering one.
- Resolution — the smallest change the instrument can detect.
- Hysteresis — a different reading on rising input than on falling input.
- Deadband — the input change that produces no output change at all.
- Drift — a change in output over time with no change in input. The reason calibration intervals exist.
- Turndown — the ratio of maximum to minimum measurable value with acceptable accuracy.
Sources of error
- Ambient temperature effects on the sensor and electronics.
- Static pressure effect on differential pressure transmitters.
- Installation effects — insufficient straight pipe run on a flow meter, a tapping in the wrong place, a filled or drained impulse line.
- Process effects — density and composition changes, coating, plugging and erosion.
- Electrical — noise pickup, earth loops, poor terminations.
An instrument's specification is achieved under reference conditions. The error that matters is the total installed error under real conditions, which is usually several times the headline figure.