A Gas Detector That Reads Zero Is Not Necessarily Working

A gas detector showing zero and a green light is telling you one of two things. Either there is no gas present, or the instrument has stopped being able to tell. From the outside, the two look identical.

Sensors drift, and nothing announces it

Electrochemical and catalytic sensors lose sensitivity gradually. Exposure to the gas they detect consumes them. So does heat, humidity, dust, silicone vapours from sealants and lubricants, and simple age. A sensor at the end of its life still powers up, still self-tests, and still reads zero in clean air. What it no longer does is respond properly when gas arrives.

This is the difference between a detector that is switched on and a detector that is working, and it cannot be seen. It has to be tested.

Bump test and calibration are not the same job

• A bump test applies a known gas above the alarm threshold and confirms the instrument alarms. It answers one question: does this thing still react at all? It takes under a minute and should be routine — for portable instruments used in confined space or hot work, before each use.
• A calibration applies a certified span gas of known concentration and adjusts the instrument so its reading matches. It answers a different question: is the number it shows correct? Typically every six months for fixed systems, or per the manufacturer, and always after a sensor replacement or a serious over-range exposure.

A bump test does not correct drift. It only catches a detector that has failed completely. An instrument can pass every bump test for a year while reading thirty per cent low, which is exactly the error that matters when the number is being used to decide whether people can enter.

The parts that get forgotten

The span gas cylinder has an expiry date and a certificate, and both matter — calibrating against out-of-date gas gives you a confident wrong answer. Fixed systems need their alarm path tested, not just the head: the sensor may detect while the relay, horn, beacon or shutdown it is supposed to trigger does nothing. And detector placement should be reviewed when a process changes, because a head positioned for a leak point that moved two years ago is monitoring empty air.

Keep the records

Every bump test and calibration should be recorded with the date, the instrument serial number, the gas used and its certificate reference, the as-found reading and the as-left reading. The as-found reading is the valuable one: a pattern of instruments drifting low is telling you the sensors are ageing out and should be replaced before they fail rather than after.