Instrument calibration, bump testing and functional checks
Instrument calibration, bump testing and functional checks
This domain tests the difference between the three checks and the records that prove they happened.
Definitions
- Calibration is adjustment of the instrument against a traceable standard gas of known concentration, normally zero and span. It confirms accuracy and is usually carried out at manufacturer-defined intervals (commonly every 6 months) or after sensor replacement, poisoning or a heavy drop.
- Bump test (functional test with gas) briefly exposes the sensor to gas above the alarm setpoint. It proves the sensor responds and the alarms (sound, light, vibration) operate. It is not a calibration and does not prove accuracy; it is typically done before each use or at least daily.
- Functional check covers battery, display, pump flow, filters, alarm operation and self-test, sometimes without gas.
Good practice
- Zero the instrument in clean air (20.9% O2, 0% LEL, 0 ppm toxic) or with a zero gas for CO2/O2 instruments.
- Use span gas within the measuring range and matched to the sensor: methane in air for a pellistor, methane in nitrogen for an IR methane sensor where specified, and the correct balance gas for toxic sensors.
- Check cylinder expiry, regulator, tubing and response time; allow the reading to stabilise.
- Record instrument ID, date, gas cylinder certificate, results, adjustments and the tester.
Worked example
A four-gas meter reads 20.9% O2 in fresh air but shows no alarm when exposed to 2.5% methane (50% LEL). Reasoning: the pellistor may be poisoned, the alarm setpoint may be wrong, or the gas cylinder may be empty. The instrument must be removed from service and investigated, not used because it reads normal in air.
Traps
Calling a bump test a calibration; using expired or wrong-balance calibration gas; zeroing in contaminated air; assuming a passed bump test proves accuracy; failing to record the calibration or bump test.