What a gas leak detector is
A gas leak detector is carried along a pipe, a joint, a valve or a hose until the reading rises, and the place where it rises is the leak. That single purpose separates it from the other instruments in this family: it is not worn to protect the person carrying it and it is not installed to watch a plant, but moved deliberately through a space in order to localise a fault that is already suspected.
The method is what decides the specification. The probe travels, so it is normally on a flexible tube that can be bent into a confined space and held at the point of interest; the reading changes as the probe moves, so the display has to respond quickly and often also present the trend; and the concentration at a leak is far above the surrounding atmosphere, so the lower part of the range matters more than the top. What the instrument reports is a concentration near the source rather than an exposure level.
Most instruments of this kind respond to combustible gas as a group rather than to one named substance, because the sensor is built to detect the presence of anything that burns. That makes them useful across methane, natural gas, liquefied petroleum gas, hydrogen and organic vapours, and it also means the reading is expressed against a calibration gas and will differ from the true concentration of a different gas by a factor. Where one specific substance has to be quantified, this is the wrong family of instrument.
How a gas leak detector measures
The sensing element in most combustible leak detectors is a catalytic element that burns a small quantity of gas on its surface and reports the heat that results, or a semiconductor element whose resistance changes as gas adsorbs on it. Either way the response follows how much combustible gas is present rather than which one it is, which is why the reading is scaled as a percentage of the lower explosive limit, the fraction of the concentration at which the gas will burn in air.
Reading against the lower explosive limit rather than in concentration is deliberate. It puts every combustible gas on one scale without the operator converting between them: ten per cent of the lower explosive limit means the same thing for methane as for propane, and a threshold set there is a safety statement rather than a measurement. The cost is that the figure is not the gas concentration, and an instrument calibrated on one gas reads a different gas with a known but non-unity factor.
The last part of the chain is the alarm. Close to a source the reading climbs quickly, so the notification has to work without being looked at: a lamp, a sounder and often a vibration element, so that an operator whose attention is on the pipe still knows when the probe has found something. Some instruments add a trace of the recent reading, which shows the trend as the probe approaches and is easier to act on than a number that is still moving.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Sensing principle | A catalytic element, a semiconductor element, or a specific electrochemical cell | Whether the instrument responds to combustible gas as a group or to one named substance. The group response is what lets one instrument cover methane, propane, hydrogen and organic vapours. |
| Detection range | A narrow band near the alarm point, or a wide span from parts per million up to the lower explosive limit | Whether the instrument can search a dilute atmosphere as well as confirm a strong one. A narrow band is quicker to read and less useful for tracing a faint leak. |
| Alarm threshold | A fixed point, or an adjustable one | How much gas has to be present before the instrument announces it. The threshold should sit below the level at which the operator has to act, not at it. |
| Alarm channels | Lamp, sounder and vibration in any combination, sometimes individually switchable | Whether a notification is noticed in a noisy plant or in daylight. Where the operator is watching a joint rather than the display, an audible and tactile alarm is what carries the message. |
| Probe form | A sensor fixed in the body, or a flexible tube with the sensor at its end | Whether a confined space, a duct or the back of a manifold can be reached at all. This is the choice that decides which leaks are findable. |
| Display and trend | A bar or a lamp only, or a numeric display with a recent trace | Whether the operator can see which way the reading is moving. A trend makes the approach to a leak visible before the threshold is reached. |
| Power and rating | Cells or a rechargeable pack, and whether the enclosure is approved for a hazardous area | Whether the instrument can be taken into the zone where the leak is, which is an approval question rather than a convenience one. |
The probe form and the approval for the area decide whether the instrument can be used at the leak at all; the rest decides how quickly it finds it.
How to choose a gas leak detector
| Decision | What settles it | What to ask |
|---|
| Which gas, or whether any | The installation being checked | A combustible group response covers natural gas, liquefied petroleum gas, hydrogen and solvent vapour without a change of instrument. A named toxic gas needs a different sensor. |
| Whether the area is classified | The zone the leak is in | Where the atmosphere may be explosive, the instrument has to be approved for that zone; an unapproved instrument is itself a possible source of ignition. |
| How far the probe has to reach | The geometry of the equipment | A manifold, a duct, a pit or the underside of a tank decides whether a flexible probe is needed or a body-mounted sensor will do. |
| Whether a figure is needed or only an indication | Whether the finding has to be reported or only acted on | A simple instrument that alarms is faster to use; one with a display and a trend is what produces a figure for a report. |
| How long a shift runs | The interval between charges or cell changes | A search that runs all day needs a battery life quoted for that duty rather than for a bench test. |
| Whether the readings are kept | Whether a maintenance record is required | Stored readings turn a visit into a record; without them the finding exists only in the operator’s memory. |
| Whether it is also used for personal protection | Whether the carrier has to be warned of their own exposure | A leak detector is a search tool and does not take the place of a personal detector with the appropriate certification. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a gas leak detector usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a gas leak detector |
| Portable gas detectors | Are worn on the person and alarm on the atmosphere around the wearer, with a certified response to named substances. A leak detector is carried along the installation to find where gas is escaping and is not a personal protective instrument. |
| Fixed gas detectors | Are installed permanently where a leak would be expected and run without an operator. A leak detector is what is taken out after such a system has reported something, to find the source. |
| Toxic gas detectors | Respond to a named hazardous substance at concentrations dangerous to health rather than to any combustible gas as a group. A leak of a toxic gas that will not burn is outside what a combustible leak detector can find. |
| Single-gas detectors | Are configured for one named gas and worn or placed for it. A leak detector trades that specificity for a broad response that covers whichever combustible gas the installation carries. |
| Air quality monitors | Report the general condition of an occupied space over time. They do not localise a leak and are not built to be moved along a pipe. |
The boundary worth settling first is who the instrument protects. If the answer is the person carrying it, it has to be a personal detector with the right certification; a leak detector protects the installation.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The gas or gases involved, or whether a general combustible response is sufficient
- The zone classification where the leak is, and the approval the instrument therefore needs
- The geometry of the equipment, and how far the probe has to reach
- Whether a numeric reading and a trend are required, or an alarm indication is enough
- The alarm threshold, and where it comes from
- Whether a calibration certificate and calibration gas are required with the instrument
- The length of a working shift, and whether the instrument must last it on one charge
- Whether readings are stored or reported
