Last reviewed 30 Sept 2026 · 8 min read
Calibration
Calibration is the comparison of an instrument's reading with a reference of known, higher accuracy (traceable to a national standard) and the determination of the correction — the calibration factor — needed to convert its output into engineering units. Without calibration, a reading is only a number.
What is checked
| Item | Meaning and how it is checked |
|---|---|
| Zero | The reading with no input; corrected by the zero offset |
| Span / sensitivity | The output per unit input (mV/V per kN, mV per mm, Hz² per µε); found by applying known inputs |
| Linearity | The deviation of the readings from the straight line through zero and full scale (as % of full scale) |
| Hysteresis | Difference between readings on the loading and unloading paths |
| Repeatability | Agreement of repeated readings at the same input |
| Resolution and noise | Smallest detectable change and the noise floor |
| Temperature effect | Change of zero and span with temperature |
| Cross-sensitivity | Response to inputs in other directions (a transverse acceleration on an accelerometer) |
Methods
- Laboratory calibration — the load cell is loaded in a calibrated testing machine or dead-weight machine; the LVDT is moved with a micrometer; the accelerometer is compared with a reference on a shaker; thermometers are compared in a temperature bath. Points are taken at increasing and decreasing inputs, at 10–20 % steps, and the calibration certificate records the coefficients and the uncertainty. Laboratories are accredited (in India, NABL) and certificates are traceable to national standards.
- Shunt calibration (strain gauges and bridge sensors) — a precise resistor is switched across one arm of the bridge to simulate a known strain, and the recorded output is compared with the calculated value; the check includes the gauge, cable and amplifier.
- In-situ (field) calibration — using known loads (a truck of known weight), a comparison with a portable reference instrument, zero checks under no load, and cross-checks between sensors.
A 1000 kN load cell has a rated sensitivity of 2.0 mV/V. With an excitation of 10 V the full-scale output is 20 mV.
At 500 kN the expected output is 10.0 mV. The instrument reads 9.8 mV — the error is mV, that is .
If the calibration certificate gives a linearity of ±0.25 % of full scale, the reading is out of tolerance: the cell, cable or amplifier is checked and re-calibrated before use.
Calibration interval and records
Instruments are recalibrated at regular intervals (typically once a year, or before and after a major test), after any shock, overload or repair, and whenever a result looks doubtful. Each sensor has a calibration record (serial number, date, factor, uncertainty, the next due date), which is kept with the channel map of the system.