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Percent by volume answers how much of the air is this gas. Percent LEL answers how close this mixture is to the leanest version of itself that will catch fire. The second scale is built on top of the first by dividing the measured concentration by the lower explosive limit of one named gas, so a reading of one hundred percent LEL is five percent by volume for methane and around two point one percent by volume for propane. Because the divisor changes from gas to gas, the two scales can never be swapped by moving a decimal point, and an instrument calibrated for one gas does not report correct flammability figures in another.
Two scales, one gas, two different denominators
Start with what a percentage is a percentage of. Percent by volume divides the volume of gas by the volume of the mixture it is in, so it is an absolute statement about composition that needs no further information to interpret. Five percent methane in air means five parts of methane for every hundred parts of mixture, and that sentence is complete.
Percent LEL divides the measured concentration by the lower explosive limit of a specific substance. The result is a position on a journey rather than an amount: fifty percent LEL means the mixture has covered half the distance from clean air to the weakest ignitable mixture of that fuel. The number is dimensionless and it is useless without the name of the gas, because the destination it is measured against moves.
That distinction explains most of the confusion in the field. Somebody reads two percent on a display and panics or relaxes depending on which scale they assumed. Two percent by volume of methane is forty percent LEL and is a serious finding. Two percent LEL of methane is a thousand ppm and is a small leak worth chasing. The pillar treatment of ppm, LEL and volume in gas detection works through the same three units side by side.
Why one hundred percent LEL is five percent of the air
Methane will not burn in air below roughly five percent by volume because there is not enough fuel to sustain a flame front, and it will not burn above roughly fifteen percent because there is not enough oxygen. Those two boundaries are the lower and upper explosive limits, and the band between them is the flammable range. Everything outside the band is non flammable, which is why a very rich atmosphere is not safe but is also not currently ignitable, and why letting air into it is exactly the wrong move.
Set one hundred percent LEL at the lower boundary and the arithmetic follows. For methane, one hundred percent LEL equals five percent by volume equals fifty thousand ppm. Ten percent LEL, the conventional first alarm, equals half a percent by volume equals five thousand ppm. The number on a flammability display is therefore a small fraction of a small fraction of the atmosphere, and a space that reads twenty percent LEL still contains ninety nine percent air.
The upper end of the scale is where a flammability instrument runs out of road. Once the mixture passes one hundred percent LEL, a catalytic instrument is outside the region it was designed for, and above the upper explosive limit the sensor can produce a reading that falls as the concentration rises. Volume scale instruments have no such discontinuity because they are not measuring combustion at all.
Where each scale earns its place
Flammability scales belong to personal and area safety. A technician entering a confined space, a plumber checking a boiler room or an inspector walking a plant wants to know how much margin is left before the atmosphere becomes ignitable, and percent LEL states exactly that in a form that is comparable across different fuels. The combustible channel on a four gas instrument such as the BW Technologies GasAlertMicroClip XL, whose bullets describe surveying hydrogen sulphide, oxygen, carbon monoxide and combustibles in natural and industrial environments, exists for that job.
Volume scales belong to process work. Purging, inerting, commissioning, landfill and digester monitoring and product verification all take place at concentrations where a flammability display has long since pegged. A volume reading also lets you confirm the direction of travel through the flammable window, which matters because a vessel being purged with air from a rich state passes through the ignitable band on the way down.
A third family sits underneath both: leak searching in ppm, where the quantities are thousands of times smaller than either scale cares about. The NOYAFA JMS19 four in one detector covers hydrogen sulphide, carbon monoxide, combustibles and oxygen with a response within half a second and three alert modes of light, vibration and sound, which places it firmly in the atmosphere monitoring family rather than the leak searching one.
Lower explosive limits for four common combustibles
The table gives the published flammability constants for four fuels and converts the two scale positions that matter most. Every figure in it is a property of the substance, not of any instrument.
| Gas | Lower explosive limit, percent by volume | 100 percent LEL in ppm | 10 percent LEL in ppm | Upper explosive limit, percent by volume |
|---|---|---|---|---|
| Methane | 5.0 | 50,000 | 5,000 | 15 |
| Propane | 2.1 | 21,000 | 2,100 | 9.5 |
| Butane | 1.8 | 18,000 | 1,800 | 8.4 |
| Hydrogen | 4.0 | 40,000 | 4,000 | 75 |
Two observations follow immediately. The ppm value of a single alarm point varies by nearly a factor of three across these four fuels, so an instrument set to alarm at a fixed ppm figure would be protecting them unequally. And hydrogen has a flammable band so wide that the volume scale stays relevant far past the point where any flammability display has saturated.
Sensor types tie an instrument to one scale
A catalytic bead oxidises the sample on a heated catalyst and measures the resulting temperature rise through a bridge circuit. Its output is close to proportional to percent LEL over the flammable range, it needs oxygen to work, and it can be poisoned by silicones and inhibited by sulphur and halogen compounds. That combination makes it the standard flammability sensor and a poor volume sensor.
Thermal conductivity sensing compares how fast heat leaves a filament in the sample against a reference, which works over the whole volume range and does not need oxygen, but gives a weak signal at the low concentrations where a flammability alarm has to act. Infrared absorption measures how much of a specific wavelength the sample swallows, resists poisoning, functions without oxygen and can be scaled to either display, but cannot see hydrogen because a symmetric diatomic molecule has no infrared absorption band to measure. Those trade offs are the reason an instrument usually publishes one scale rather than both, and the reason sensor lifespan differs so much between technologies.
Reading a detector that publishes only one scale
Most consumer and light trade instruments publish a flammability scale and nothing else, and the honest response is to work out what that leaves undetermined. The TopTes PT205 states detection of the lower explosive limit concentration percentage within a range of one to ten percent LEL, with a parenthetical ppm equivalent based on methane, a thirty second warm up and a bar graph that rises with concentration. It is explicit that the ppm figures are referenced to methane, which is the right way to publish a flammability scale. The Forensics Detectors four gas monitor states channels for oxygen, carbon monoxide, hydrogen sulphide and combustibles with factory calibration and zero and span calibration options, while the units its combustible channel displays are Not stated in the listing bullets.
So the sequence when you pick up an unfamiliar instrument is short. Find the units on the combustible display. Find the gas the scale is referenced to. Find out whether a volume reading exists at all, because if it does not the instrument has nothing useful to say about a rich atmosphere. The companion pages on what LEL means on a gas detector and ppm against LEL cover the two halves of that question in more detail.
Frequently Asked Questions
Is one hundred percent LEL the same for every gas?
As a scale position, yes. As a concentration, no. One hundred percent LEL always marks the leanest mixture of that particular gas in air that will propagate a flame, but the concentration at which that happens differs widely. For methane it is about five percent of the air by volume, for propane about two point one percent and for pentane about one point five percent, so the same scale reading corresponds to very different amounts of gas.
Why would anyone want a percent by volume reading at all?
Because plenty of work happens above the flammable range rather than below it. Purging a pipeline, filling or emptying a storage vessel, checking landfill or digester gas and verifying an inert blanket all involve concentrations far above one hundred percent LEL, where a flammability scale has already pegged. A volume scale keeps counting and lets you confirm that a space has been taken through the flammable window rather than left inside it.
Can a catalytic sensor read percent by volume?
Not meaningfully. A catalytic bead works by oxidising the gas on a heated catalyst and measuring the heat that releases, which requires oxygen and produces a response that flattens and then reverses once the mixture becomes too rich to burn properly. That reversal is dangerous because a very rich atmosphere can display a falling reading. Volume scale measurement is normally done with thermal conductivity or infrared sensing instead.
What does ppm have to do with these two scales?
Parts per million is simply a finer version of percent by volume, with one percent equal to ten thousand ppm. It is the natural unit for leak searching and for toxic exposure, where the quantities of interest are thousands of times smaller than a flammable concentration. Converting ppm to percent LEL needs the same flammability constant as any other conversion between the two scales.
Does an oxygen reading change how a combustible reading should be treated?
Yes, which is why four gas instruments put the two channels side by side. A catalytic bead needs oxygen to burn the sample, so a combustible reading taken in an oxygen deficient space can be low for reasons that have nothing to do with how much fuel is present. An oxygen channel that reads below normal is a warning that the combustible channel beside it is no longer trustworthy.
If a detector only shows a bar graph, which scale is it on?
Often neither in a strict sense. A bar graph with no units is a relative response display that rises as concentration rises and is intended for following a gradient rather than quantifying it. Treat it as a direction finder. If the listing does not state a numeric scale with units, there is no basis for converting what the bars show into either a volume figure or a flammability figure.