The Industrial Calibration Process

ISO 9001:2015

EU GMP Annex 15

21 CFR Part 211.68

IQ/OQ/PQ Framework

Identify Instruments Due

Pull from the calibration asset register — instruments due within the calibration period, flagged by due-date tracking. Include any out-of-service instruments that were in use.

Select Reference Calibrator

Confirm the calibrator has a current NABL-traceable certificate and adequate accuracy ratio (4:1 rule). Dry block or liquid bath — based on the sensor type and accuracy requirement.

Stabilise at Set Points

Allow the calibrator to stabilise at each calibration set point. Stabilisation time depends on the calibrator type (5–30 min typical). Record the stabilised temperature from the reference probe.

Record Readings

Record the instrument under test reading alongside the reference at each set point. Minimum 3 set points for most industrial sensors; more for pharmaceutical or metrological applications.

Evaluate and Adjust

Compare the deviation at each point against the allowable tolerance. If in tolerance: issue certificate. If out of tolerance: adjust, then re-calibrate. Document the as-found and as-left readings separately.

Issue Certificate

Generate the calibration certificate with: instrument ID, calibration date, set points, as-found readings, deviations, tolerance, pass/fail, expanded uncertainty, traceability statement, and next due date.

Calibrator temperature uniformity

The temperature uniformity of the dry block insert or liquid bath at the calibration set point — the dominant source for dry block calibrations

Reference probe uncertainty

The uncertainty of the reference probe used alongside the instrument under test — from its NABL calibration certificate

Resolution of the DUT display

The readability of the instrument under test — typically ±0.5 × the last digit of resolution

Repeatability

Variation between repeated readings at the same set point — estimated from a short repeatability study (minimum 3 readings per set point)