Calibration is checking and adjusting an instrument against a known standard so its measurements stay accurate within a stated tolerance.
Calibration is the process of comparing an instrument’s readings against a known reference standard and adjusting it so its measurements stay within a stated tolerance. Every measuring device drifts over time - torque wrenches, gas detectors, scales, thermometers, pressure gauges - and calibration is the routine that catches the drift before it produces bad readings, failed work, or unsafe decisions. It sits alongside the inspection schedule as one of the recurring obligations attached to an asset.
Why instruments drift
Springs fatigue, sensors age, electronics heat and cool, and kit gets knocked about in vans and toolboxes. None of this announces itself: a torque wrench that reads low still clicks, a gas detector that has lost sensitivity still switches on, a scale that has crept off zero still shows a confident number. Drift is invisible at the point of use, which is exactly why calibration runs on a schedule rather than waiting for symptoms.
How calibration works
A calibration compares the instrument against a reference standard whose own accuracy is traceable back up a chain to a national measurement standard. The typical sequence:
- As-found readings are taken first, recording how the instrument performed before anyone touched it.
- The instrument is adjusted (or assigned a correction factor) to bring it back within tolerance.
- As-left readings confirm the result.
- Everything is documented in a calibration certificate, with the next due date.
The as-found readings carry the real weight. If the instrument arrived out of tolerance, everything it measured since the last calibration is now in question - which can mean re-checking completed work, not just fixing the tool.
Certificates and records
A calibration certificate states the instrument’s identity (usually its serial number), the standard used, the as-found and as-left results against tolerance, the date, who performed the work, and when the next calibration falls due. Customers, accreditation bodies, and auditors ask for these, and they ask per instrument: “show me the current certificate for this exact detector”. A folder of PDFs named scan0041.pdf does not survive that conversation, so the certificate needs to live with the asset’s record.
Setting calibration intervals
The manufacturer’s recommended interval is the starting point, not the answer. Tighten it for instruments that are heavily used, transported daily, or have a history of arriving out of tolerance; consider stretching it for stable instruments with a clean record. Criticality trumps convenience - kit used for safety decisions, like safety equipment and gas detection, deserves the conservative interval. A lapsed calibration has a habit of surfacing at the worst moment, turning into downtime when an instrument is pulled from service mid-job or fails an audit.
Calibration in practice
The common failure is not bad calibration work - it is the instrument nobody realised was due. The habits that prevent it: every measuring instrument gets its own asset record, the next due date is recorded as a field on that record rather than in someone’s head, the current certificate is attached alongside it, and out-of-service instruments are clearly marked so nobody grabs one from the shelf. In AMPthilly, teams add a calibration-due custom field to the asset record, attach each certificate as a document, and log the work through the service desk so the instrument’s full history stays on its record permanently.
Related terms
- Inspection Schedule - the planned calendar of recurring checks that calibration dates slot into
- Downtime - what a lapsed calibration causes when an instrument is pulled from service
- Maintenance Backlog - the queue overdue calibrations quietly join
- CMMS - the software category built around scheduling recurring maintenance work
- Asset Uptime - the share of time an asset is actually available for use