Equipment Calibration

Onsite and Laboratory Calibration

An instrument drifts out of specification long before anything about it looks wrong. It keeps reading, it keeps displaying a plausible number, and the process is quietly adjusted around it. Calibration places a known reference against the instrument and establishes how far the two disagree, which is the only way to know whether a measurement can be relied upon.

Calibration is performed onsite in the installed position wherever access allows, because that is the condition the instrument actually works in, and in the workshop where the installed position prevents a proper reference being applied. The instrument is first checked for physical damage and for evidence that its case or seals have been opened, and the reference standard and the instrument are allowed to reach thermal equilibrium before readings are taken at the calibration point, normally between 10 and 90 percent of span.

The distinction between as-found and as-left matters commercially as well as technically. An as-found result shows what the instrument was actually reading in service, which is what a quality system is judged on. An as-left result shows it has been brought back within tolerance. We report both, and we will not adjust an instrument unless you ask us to.

Instruments calibrated include:

• Pressure: gauges, differential pressure gauges, transmitters, recorders and switches
• Temperature: thermocouples, resistance temperature detectors, transmitters, indicators and infrared thermometers
• Electrical: digital multimeters, clamp meters, insulation resistance testers, micro-ohmmeters, earth testers and power quality analysers
• Process: flow, level, density and mass flow instrumentation
• Dimensional and mechanical: micrometers, verniers, gauges, torque tools and angle measuring instruments
• Signal path: transducers, transmitters, recorders, alarms and analysers

Each instrument receives a certificate identifying it by asset tag, with its as-found and as-left readings against the reference value, the calculated error for each point and whether it is within the stated tolerance, the adjustment carried out where authorised, the reference standards used with their own calibration status, and the accredited body, certificate number and scope of accreditation. Any instrument found out of tolerance is reported as such, together with what that means for the process it was measuring.