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Most handheld methane detectors use one of three sensing mechanisms: catalytic bead combustion, non-dispersive infrared absorption, or a metal oxide semiconductor. Catalytic and infrared sensors are the two you will meet in industrial and trade instruments, and they behave very differently. A catalytic bead burns a sample on a heated element and measures the heat released; it responds to almost any combustible gas and is the basis of the LEL reading on a four-gas monitor such as the NOYAFA 4 in 1 gas detector, which includes LEL alongside H2S, CO and O2. An infrared sensor shines a specific wavelength through the sample and measures absorption at the methane absorption band; it is selective for methane and hydrocarbons but blind to hydrogen and to gases that do not absorb at that wavelength. Neither sensor measures methane concentration in absolute terms. Both produce a signal that the instrument converts to percent LEL using a calibration factor for methane, and that conversion is only valid for the gas the instrument was calibrated on. The number on screen is an estimate of how close the atmosphere is to the lower explosive limit, not a measurement of how much methane is present.
What the sensor is actually responding to, and why it is not methane itself
A methane detector is a transducer plus a calculator. The transducer responds to a physical property that methane happens to change, and the calculator turns that response into a number using a calibration curve. The curve is built by exposing the instrument to a known concentration of methane, usually in air, and recording the signal. Everything the instrument reports afterwards is an interpolation along that curve.
That matters because the physical property being measured is rarely unique to methane. A catalytic bead sensor responds to the heat of combustion. Methane releases heat when it burns, but so do propane, hydrogen, ethanol vapour and the solvent in a can of contact cleaner. The sensor cannot tell them apart. It reports a single number that the instrument labels as LEL, and that label is only true if the gas present is the gas the instrument was calibrated on.
Infrared sensors are more selective. Methane absorbs infrared light at a characteristic wavelength, and a non-dispersive infrared sensor measures how much of that wavelength is absorbed by the sample. The absorption is proportional to the number of methane molecules in the light path, which is a more direct measurement than the heat of combustion. But the selectivity is not perfect, and the instrument still has to convert absorbance into LEL using an assumed methane concentration at the lower explosive limit.
Both mechanisms produce a reading in percent LEL, not percent methane. The distinction is important because the two scales are not interchangeable. Methane’s lower explosive limit is 5 percent by volume in air, so 100 percent LEL corresponds to 5 percent methane. A reading of 10 percent LEL means 0.5 percent methane by volume, not 10 percent methane. The arithmetic is straightforward once you know the LEL, and the conversion between ppm, percent by volume and percent LEL is set out in the conversion guide for methane.
Catalytic bead versus infrared: two mechanisms, two failure modes
A catalytic bead sensor contains two small coils of wire, one coated with a catalyst and one left inert. Both are heated to a temperature at which methane will oxidise on the catalyst. When methane reaches the active bead it burns, raising the bead’s temperature and therefore its resistance. The instrument measures the resistance difference between the two beads and converts it to a concentration. The inert bead compensates for changes in ambient temperature and humidity, which is why the sensor can work outdoors.
The catalytic bead’s strength is that it responds to almost every combustible gas, which is why it is the LEL sensor in most four-gas monitors. Its weakness is that the catalyst is poisoned by substances such as silicone vapours, leaded compounds and some sulphur compounds. A poisoned bead loses sensitivity gradually, and the instrument may not warn you. The bead also needs oxygen to burn the gas, so a catalytic sensor reads low in an oxygen-deficient atmosphere even if methane is present. That is one reason a four-gas monitor such as the Forsensal pumped four-gas monitor measures oxygen alongside LEL: the LEL reading is only trustworthy when there is enough oxygen to support combustion.
An infrared sensor does not burn the gas. It passes infrared light through the sample and measures absorption at the methane band, typically around 3.3 micrometres. Because nothing is consumed, the sensor is not poisoned by the gases that poison catalytic beads, and it works in oxygen-deficient atmospheres. It is also more selective for methane, which makes it the better choice where methane is the only combustible gas of concern. The trade-off is that it does not respond to hydrogen or to gases that do not absorb at the chosen wavelength, so it is not a general-purpose combustible gas sensor.
| Property | Catalytic bead | Infrared |
|---|---|---|
| What it measures | Heat of combustion on a catalyst | Absorption of infrared light at the methane band |
| Response to methane | Yes, via combustion | Yes, via absorption |
| Response to other combustibles | Broad; responds to most combustible gases | Selective; mainly methane and hydrocarbons |
| Needs oxygen to work | Yes | No |
| Main failure mode | Catalyst poisoning, gradual loss of sensitivity | Optical contamination, drift in the light source or detector |
| Typical use | General combustible gas monitoring, four-gas monitors | Methane-specific monitoring, landfill and biogas |
The table is a general comparison of the two technologies, not a specification of any particular product. The listings for the NOYAFA Gas Detector, the Forsensal Gas Detector, the Testo Gas Leak and the Inficon -718-202-G1 do not state which sensing technology each instrument uses, and you should not assume. A four-gas monitor with an LEL channel is almost always catalytic, but the listing does not confirm it. If the sensing principle matters to your application, ask the supplier for the sensor datasheet before you buy.
The LEL scale, the calibration gas, and why the number is an assumption
Every LEL reading rests on a calibration. The instrument is exposed to a known concentration of methane, the signal is recorded, and the instrument is adjusted so that the signal corresponds to the correct percentage of the lower explosive limit. If the calibration gas is methane, the reading is correct for methane. If the calibration gas is something else, the instrument applies a correction factor, and the reading for methane is an estimate based on that factor.
This is where most confusion arises. A four-gas monitor calibrated on methane will read methane correctly, but if it is exposed to propane it will apply the methane calibration and report a number that is wrong for propane. The size of the error depends on the response factor of the sensor to propane relative to methane, which is a property of the sensor and is not stated in the listing bullets for any of the products here. The practical rule is that a combustible gas monitor is only accurate for the gas it was calibrated on, and the reading for any other gas is indicative rather than quantitative.
The same logic applies to the alarm thresholds. A monitor with a low alarm at 10 percent LEL and a high alarm at 20 percent LEL is not warning you that methane is at 10 percent by volume. It is warning you that the instrument believes the atmosphere is one tenth of the way to the lower explosive limit. That belief is only as good as the calibration and the sensor’s health. The meaning of the LEL scale, and why it is not a percentage of gas, is covered in the explanation of LEL on a gas detector.
None of the listings here publish the calibration gas, the alarm thresholds, or the response factors. The NOYAFA listing states that the instrument responds within 0.5 seconds and alerts continuously until gas levels return to normal, which is a statement about alarm behaviour, not about accuracy. The Forsensal listing states that a calibration certificate is included and that the instrument can be switched between pump and diffusion modes, but it does not state the calibration gas or the alarm setpoints. Those are questions to ask before you rely on the number.
Pump versus diffusion, and what each means for finding the source
A diffusion detector waits for gas to reach the sensor by natural movement. A pumped detector draws a sample through a hose to the sensor, which lets you test a location before you enter it. The difference matters in practice because methane is lighter than air and tends to accumulate at high points, in roof spaces, and in the voids above a leak. A diffusion instrument clipped to your belt will not see methane that has collected at the ceiling.
A pumped instrument lets you sample at the end of a hose, which is why the Forsensal four-gas monitor with pump is supplied with a hose and a natural diffusion cap. The listing states that the instrument can be changed from pump suction to natural diffusion with four screws, which is a useful detail: the pump adds a failure point and a battery load, and some tasks are better served by diffusion. The listing also states a 3000mAh battery with standby time greater than 8 hours, which is a statement about standby, not about continuous pumped operation. Pumped sampling draws more current than diffusion, and the listing does not publish a pumped runtime.
For leak location rather than personal protection, a dedicated leak detector with a flexible probe is often more useful than a four-gas monitor. The probe lets you sniff along a pipe run and watch the reading rise as you approach the leak. The listing for the Testo gas leak detector does not publish its sensing principle, its sensitivity or its response time, so the honest position is that the listing does not give you enough to compare it on specification. The Inficon listing for the Inficon combustible gas detector is similarly thin: it states the country of manufacture and the manufacturer, and nothing about the sensor. Where a listing is silent, the specification cannot be assumed.
Where the reading stops being trustworthy
A methane detector is a safety instrument, and like any safety instrument it has conditions outside which its output should not be trusted. The first is oxygen. A catalytic bead needs oxygen to burn the gas, so in a confined space with displaced air the LEL reading will fall even as methane rises. This is why a four-gas monitor measures oxygen as well, and why a low oxygen alarm should be treated as a reason to leave, not as a separate problem.
The second is cross-sensitivity. A catalytic sensor responds to other combustible gases, and some substances that are not combustible can affect the reading. Silicone vapours poison the catalyst, and the loss of sensitivity is gradual and may not trigger a fault. A monitor that reads zero in a known methane atmosphere may be poisoned rather than correct. The third is calibration drift. Catalytic beads drift with use, and infrared sensors drift with contamination of the optical path. A calibration certificate in the box, such as the one the Forsensal listing states is included, is evidence of condition at the point of supply, not a guarantee of accuracy months later.
The fourth is the alarm itself. A monitor is not a controller. It warns you; it does not shut off a gas supply, ventilate a room, or call anyone. The NOYAFA listing describes three alert modes, LED, vibration and sound, and continuous alerting until gas levels return to normal. That is a warning, and the correct response is to leave the area and investigate from a safe place. A detector that is not switched on, not charged, or not calibrated is not a safety device at all. If you are choosing between instruments for a specific job, the comparison of methane detectors sets out the trade-offs; if you are deciding where to put a fixed detector, the guidance on placing a combustible gas detector applies to methane as well, with the difference that methane rises rather than settles.
The number on the screen is a claim, not a fact
A methane detector converts a physical change into a number using a calibration and an assumption about the gas. The physics is well understood, but the number is only as good as the sensor’s health, the calibration gas, and the conditions in which it is used. Treat the reading as a warning that deserves a response, not as a measurement you can rely on without knowing how it was produced. Where a listing does not publish the sensing principle, the calibration gas or the alarm thresholds, ask before you buy, because those are the details that decide whether the instrument is right for the job.
Frequently Asked Questions
Does a methane detector measure methane concentration directly?
No. It measures a physical change that methane causes, such as heat of combustion or infrared absorption, and converts that change into a concentration using a calibration. The conversion assumes the gas present is the gas the instrument was calibrated on. If the gas is different, the number is an estimate rather than a measurement.
Why does my detector read in percent LEL rather than percent methane?
Because the safety question is how close the atmosphere is to exploding, not how much methane is present. Methane's lower explosive limit is 5 percent by volume in air, so 100 percent LEL corresponds to 5 percent methane. A reading of 20 percent LEL means 1 percent methane by volume. The LEL scale makes the alarm threshold meaningful regardless of the gas.
Can a four-gas monitor detect methane if it is calibrated on a different gas?
It will respond to methane, but the reading will be wrong by an amount that depends on the sensor's response factor for methane relative to the calibration gas. The listing does not publish the calibration gas or the response factors, so the honest answer is that you should not rely on the number unless the instrument is calibrated on methane or you have the correction factor from the manufacturer.
Does a catalytic bead sensor work in a confined space with low oxygen?
No. A catalytic bead burns the gas, which requires oxygen. In an oxygen-deficient atmosphere the bead will read low even if methane is present. This is why four-gas monitors measure oxygen alongside LEL, and why a low oxygen alarm means leave the space, not investigate the methane reading.
How often does a methane detector need calibrating?
The listing does not publish a calibration interval for any of the products here. Manufacturer instructions and local regulations set the interval, and it depends on the sensor type and the environment. A calibration certificate in the box, such as the one the Forsensal listing states is included, confirms condition at supply, not ongoing accuracy.
Will a methane detector warn me before the gas reaches a dangerous level?
It will if it is working, calibrated and switched on, and if the alarm thresholds are set below the level you need to act at. The listings here do not publish the alarm thresholds. A detector is a warning device, not a controller: it does not shut off gas or ventilate the space, and the correct response to an alarm is to leave and investigate from a safe place.