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Guide · Gas detection · 49 CFR 192.625 · OSHA 1910.146(b) · NIOSH Pocket Guide · RKI Instruments manuals

How do you detect a natural gas or propane leak? The odorant, where each gas collects, what a combustible gas detector reads, what sets one off falsely, and the signs of a leak

Three ways, in the order they happen: your nose, a detector, and the signs around the leak. Natural gas and propane have no smell of their own, so federal pipeline rules require an odorant strong enough that a person with a normal sense of smell notices it at one fifth of the lower explosive limit, well before the gas can burn. A combustible gas detector reads the gas itself, in percent of the lower explosive limit (LEL) or, in leak-check mode, in parts per million, and finds leaks the nose cannot place; it is a catalytic bead or infrared sensor, not the electrochemical cell in a carbon monoxide alarm, which reads zero in natural gas. Where to look depends on the gas: methane is lighter than air and collects high, propane is heavier and pools low. The signs are a hissing at a fitting, dead vegetation or bubbles over a buried line, a persistent odor, and, in a confined space, a monitor reading above 10 percent LEL, which OSHA defines as a hazardous atmosphere.

Bayshore Safety. Industrial safety and PPE supplier since 2017. · Current as of November 2026 · 49 CFR 192.625 Odorization of gas (current e-CFR); NIOSH Pocket Guide to Chemical Hazards, propane (IDLH 2,100 ppm at 10 percent LEL, PEL and REL 1,000 ppm) and methyl mercaptan; 29 CFR 1910.1000 Table Z-1 (propane and L.P.G. at 1,000 ppm) and 29 CFR 1910.146(b) (10 percent LFL as a hazardous atmosphere); the LEL and UEL table on this site’s LEL guides, sourced to the NIOSH Pocket Guide, NIOSH IC 9486 Handbook for Methane Control in Mining and the RKI Instruments FAQ (methane 5 to 15 percent, lighter than air; propane 2.1 to 9.5 percent, density 1.55); RKI Instruments GX-Force Operator’s Manual (Leak Check Mode appendix: 0 to 5,000 ppm methane, T90 in 10 seconds, pump draws from 100 feet) and GX-3R Pro Operator’s Manual (catalytic sensor, methane calibration, 10 and 25 percent LEL alarms, HC gas list)
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RKI Instruments 72-FO-A-B GX-Force Single-Gas Sample Draw Monitor LELRKI Instruments 72-FO-A-B GX-Force Single-Gas Sample Draw Monitor LELRKI GX-Force single-gas LEL monitor with internal pump: Leak Check Mode reads methane 0 to 5,000 ppm through a probe
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Most of the questions in this cluster come from a house (can I detect a gas leak in my house, is it free to have it checked, where does a gas detector go), and the honest answer for a home is that the utility checks a suspected leak, and that is who to call; the detectors we sell are the portable instruments gas fitters, plumbers, utility crews and confined-space entrants carry. The physics is the same in both places, so this page answers the home questions with the published facts and then shows how the trade finds a leak with a combustible gas detector. Facts are from the pipeline odorization rule, the NIOSH Pocket Guide, OSHA’s confined-space rule and the RKI Instruments manuals; the LEL figures are the ones this site’s LEL guides carry from the NIOSH and RKI tables.

Detecting a gas leak, question by question

Each answer names its source. “Gas” below means natural gas (mostly methane) or propane (LPG) unless it says otherwise.

Can I detect a gas leak in my house, and what are the first signs?The first sign is the smell, by design. Under 49 CFR 192.625 a combustible gas in a distribution line must contain a natural odorant or be odorized so that at a concentration in air of one fifth of the lower explosive limit the gas is readily detectable by a person with a normal sense of smell; the odorant is a mercaptan, a family of sulfur compounds that includes the methyl mercaptan in the NIOSH Pocket Guide (OSHA ceiling 10 ppm), and the smell is the rotten-egg or sulfur odor people describe. Other signs are a hissing or whistling at a meter, valve, appliance connector or buried line; dead or dying vegetation in a strip over a buried pipe; bubbles in standing water or in soapy water brushed on a fitting; a pilot that keeps going out; and dust blowing from the ground near a line. Headache, dizziness and nausea are also reported around leaks, but in a house those symptoms are more often carbon monoxide from an appliance than the gas itself, and CO has no smell at all, so the two are separated by the odor and by which detector responds.
49 CFR 192.625(a); NIOSH Pocket Guide, methyl mercaptan (odorant, ceiling 10 ppm).
Is it free to get a gas leak checked, and who do you call?A suspected natural gas leak is reported to the gas utility, which is the operator of the distribution line the odorization rule applies to; utilities publish an emergency leak number and treat a leak report as an emergency call; whether a utility charges for a leak check is that utility’s policy, and the repair of a customer-owned line or appliance is the owner’s. For propane the supplier is the equivalent call. The instruments on this page are for the people who then do the finding: the technician who walks the line with a leak detector, the plumber pressure-testing a new run, the utility crew bar-holing over a main, the entrant testing a manhole or vault before going in. If you smell gas, the order every utility publishes is the same: leave, do not operate switches or phones inside, and call from outside.
49 CFR 192.625 (the odorization duty sits with the operator of the distribution line); the detector and confined-space facts below.
Is there a detector that detects gas, and how does it work?Yes: a combustible gas detector, called an LEL monitor or gas leak detector depending on the trade. The RKI GX-3R Pro manual describes the sensor: two catalytic elements, a standard one and a poison-resistant one, whose electrical resistance changes as gas reacts with oxygen on the hot element; the circuitry converts the change into a concentration and displays it as a percentage of the lower explosive limit, calibrated to methane as standard. Because methane’s LEL is 5 percent by volume, a reading of 10 percent LEL is 0.5 percent gas in air (5,000 ppm), which is the first alarm on RKI’s monitors (25 percent LEL is the second). For small leaks RKI’s GX-Force has a Leak Check Mode in which the catalytic sensor reads methane in parts per million, 0 to 5,000 ppm in 50 ppm steps with a 10-second response, through a probe on a pump that draws from up to 100 feet of hose; the manual describes it as the mode for pinpointing small leaks from valves, flanges and connections. Infrared sensors read the same gases by light absorption and do not need oxygen to work.
RKI Instruments GX-3R Pro Operator’s Manual (combustible gas sensor description, Table 1 alarm points); GX-Force Operator’s Manual, Appendix C Leak Check Mode and pump description.
Does natural gas rise, and where should a gas detector be placed in a house?Natural gas rises and propane sinks. The LEL table on this site, sourced to the NIOSH Pocket Guide and the NIOSH methane handbook, lists methane as lighter than air and propane as heavier, with a vapor density of 1.55 times air; butane and gasoline vapor are heavier still. So a natural gas detector is mounted high, near the ceiling and near the appliance or meter, and a propane detector low, near the floor, because that is where each gas collects first. In a confined space the same rule becomes a testing order: OSHA’s confined-space standard requires testing at every level, and the trade tests high for methane and hydrogen and low for propane, butane and gasoline vapor, pumping the sample up to the monitor through a hose rather than lowering a person. Fixed home detectors are placed by the manufacturer’s instructions and local code; this page does not track those codes.
LEL and UEL by gas (this site; NIOSH Pocket Guide, NIOSH IC 9486, RKI FAQ); 29 CFR 1910.146(d)(5) and Appendix B (testing at all levels).
What can falsely set off a natural gas detector, and do they actually work?They work, and what sets them off is other combustible vapor, not nothing. A catalytic bead sensor responds to any gas that burns on the bead, so RKI’s manual explains that a monitor calibrated to methane will still detect and respond to a variety of combustible gases, and lists the conversions for propane, hexane, ethanol, isopropyl alcohol, acetone and others; solvent fumes from paint, cleaners, hair spray, aerosol propellants (which are often butane or propane) and alcohol will therefore register on a “natural gas” detector, as will a leak of refrigerant on some sensors. A reading that is not from the gas line is a real combustible reading from something else, which is why the trade confirms a leak with soap solution or a bar-hole survey, not the alarm alone. The GX-Force manual also notes that in Leak Check Mode the instrument does not detect VOC gases like heptane and toluene, and that if the standard catalytic element loses output to a poison such as silicone the monitor switches to its poison-resistant element and warns the user.
RKI Instruments GX-3R Pro Operator’s Manual (standard calibration to methane; HC gas list and conversions; poison-resistant element); GX-Force Operator’s Manual, Appendix C.
What detector do I need for propane, and does a CO or CO2 alarm detect gas?A combustible gas detector, mounted or carried low. Propane is read by the same catalytic or infrared LEL sensor as methane; RKI’s monitors can be set up and calibrated to propane (the display then shows “HC” for hydrocarbon) or left on methane and converted, and the NIOSH Pocket Guide lists propane’s LEL at 2.1 percent, so 10 percent LEL of propane is 2,100 ppm, which NIOSH also uses as propane’s IDLH; OSHA’s Table Z-1 gives propane and L.P.G. a PEL of 1,000 ppm as an 8-hour average. A carbon monoxide alarm does not detect propane or natural gas: RKI’s CO sensor interference chart lists propane at 0.5 percent by volume and methane at 1.25 percent each reading 0 ppm. A CO2 monitor reads carbon dioxide by infrared at a wavelength methane does not absorb, so it does not read natural gas either. A refrigerant leak detector built for halogenated refrigerants is a different sensor again; whether it responds to methane depends on its sensing method, and the manufacturer’s manual is the only source for that.
NIOSH Pocket Guide, propane; 29 CFR 1910.1000 Table Z-1; RKI Instruments GX-3R Pro Operator’s Manual (HC target gas, Appendix C Table 18).
Are there warning signs before a gas explosion, and how big can one be?A gas can only ignite between its lower and upper explosive limits, so the warning sign is the smell or a monitor reading climbing toward 100 percent LEL: methane burns between 5 and 15 percent in air, propane between 2.1 and 9.5 percent. Below the LEL the mixture is too lean to burn and above the UEL too rich, which is why a tank full of fuel vapor can read over range and still not ignite until air gets in. OSHA’s confined-space rule treats 10 percent of the lower flammable limit as already hazardous, one tenth of the way to ignition, because gas pockets are uneven and the reading at the sensor is not the reading at the ceiling. The size of an explosion is a function of how much gas has accumulated in how much space and is not something a published limit predicts; the odorant rule sets the smell threshold at 20 percent LEL so people notice a leak when the room is still one fifth of the way to the flammable range.
LEL and UEL by gas (this site; NIOSH Pocket Guide and RKI FAQ figures); 29 CFR 1910.146(b) hazardous atmosphere definition; 49 CFR 192.625(a).
What happens if you inhale a small amount of natural gas, and what are the side effects of a leak?Methane is not toxic in the way carbon monoxide or hydrogen sulfide are; the NIOSH Pocket Guide carries no exposure limit for it because it harms by displacing oxygen, a simple asphyxiant, and by burning. Propane has an OSHA PEL of 1,000 ppm and the NIOSH Pocket Guide lists its IDLH at 2,100 ppm with the note that the figure is 10 percent of the LEL, meaning the explosion hazard governs the number, not poisoning. A small whiff of odorized gas is the odorant you notice, at a few parts per million, and the gas itself at a few hundredths of a percent; the headache, dizziness and nausea reported around leaks come when enough gas has built up to push the oxygen down, or from carbon monoxide produced by an appliance burning badly. OSHA’s oxygen-deficient threshold is 19.5 percent; a four-gas monitor shows both the LEL and the oxygen reading so the two causes are told apart.
NIOSH Pocket Guide, propane (IDLH note); 29 CFR 1910.1000 Table Z-1; 29 CFR 1910.146(b) (oxygen-deficient atmosphere below 19.5 percent).
How do they detect gas leaks, and how do you find a very small one?The trade works from the odor to the fitting. A technician with a leak detector in ppm mode (RKI’s GX-Force reads 0 to 5,000 ppm methane in Leak Check Mode with a 50 ppm increment) traces the highest reading along the pipe to the joint, then confirms it with soap solution, which bubbles at a leak too small to register from a distance. Buried mains are surveyed by bar-holing: a rod opens a hole over the pipe and the pump draws a sample up the hose, which the GX-Force manual says can run to 100 feet. Pressure testing finds leaks the nose cannot: a new line is pressurized and watched on a gauge, and a drop locates a leak somewhere in the run before gas is ever introduced. Inside appliances, a connector or valve stem is checked at the fitting with the probe tip. Every method ends the same way: the leak is confirmed by a second means before the fitting is reworked, and the detector is bump tested before the day’s use so a zero reading means something.
RKI Instruments GX-Force Operator’s Manual, Appendix C (Leak Check Mode purpose, range, increment, response time) and pump description (100 feet of hose); GX-3R Pro Operator’s Manual (bump test before each day’s use).
What does a gas leak alarm sound like, and how accurate is a gas leak detector?A portable monitor gives three signals at once: RKI’s manuals describe the buzzer sounding, the LEDs flashing and the vibrator pulsing when any alarm condition occurs, with distinct patterns for the warning and the alarm and a stealth mode that silences the buzzer where that is wanted; fixed home alarms use a loud repeating horn set by their own standard. Accuracy is stated by the manufacturer: RKI lists the GX-3R Pro combustible channel at ±5 percent of reading or ±2 percent LEL, whichever is greater, in a continuous environment (±10 percent LEL at temperature extremes), and the GX-Force Leak Check Mode at ±20 percent of full scale (typically ±10 percent) with a T90 response of 10 seconds; both manuals require a bump test with a known gas before each day’s use and calibration when the bump test fails, because a catalytic sensor drifts and can be poisoned without any visible change.
RKI Instruments GX-3R Pro Operator’s Manual (alarm indications, Table 1 accuracy, bump test); GX-Force Operator’s Manual, Appendix C Table 20.

Finding a gas leak in plain words

  • Natural gas (methane) and propane have no smell of their own. The rotten-egg smell is an odorant the pipeline rule requires, strong enough to notice at one fifth of the flammable level (49 CFR 192.625).
  • The signs of a leak: the smell, a hiss at a fitting, bubbles in soapy water on a joint, dead vegetation over a buried line, a pilot that will not stay lit.
  • A gas detector for these gases is a combustible (LEL) sensor, catalytic bead or infrared. A carbon monoxide alarm reads zero in natural gas; a CO2 monitor does not read it either.
  • Methane rises: detect and test high. Propane, butane and gasoline vapor sink: detect and test low.
  • The monitor reads in percent of the lower explosive limit. Methane burns at 5 percent in air, propane at 2.1 percent; OSHA calls 10 percent LEL a hazardous atmosphere in a confined space and the monitor alarms there.
  • In leak-check mode a pumped monitor reads methane in parts per million through a probe and traces the leak to the fitting; soap solution confirms it.
  • Anything combustible registers: solvents, aerosols, alcohol, paint fumes. A reading is real gas of some kind; the second check tells you which.
  • If you smell gas at home: leave, do not operate switches inside, call the utility (natural gas) or supplier (propane) from outside. The utility treats a leak report as an emergency.
49 CFR 192.625; NIOSH Pocket Guide, propane and methyl mercaptan; 29 CFR 1910.146(b); the LEL and UEL tables on this site; RKI Instruments GX-3R Pro and GX-Force Operator’s Manuals.
Natural gas vs propane vs carbon monoxide: what each is and how it is detected
Natural gas (methane)Propane (LPG)Carbon monoxide
Where it comes fromThe gas line, meter, appliance connectors, buried mainsThe tank, regulator, lines and appliancesAny fuel burning badly: furnaces, water heaters, engines, generators, grills
SmellNone; odorant added (sulfur, rotten egg)None; odorant addedNone, ever
Lighter or heavier than airLighter; collects highHeavier (density 1.55); pools lowAbout the same as air; mixes
Flammable range5 to 15 percent in air2.1 to 9.5 percent in air12.5 to 74 percent in air
Toxic limitNone listed; simple asphyxiantOSHA PEL 1,000 ppm; NIOSH IDLH 2,100 ppm (10 percent LEL)OSHA PEL 50 ppm; NIOSH ceiling 200 ppm; IDLH 1,200 ppm
Sensor that reads itCatalytic bead or infrared (LEL)Catalytic bead or infrared (LEL)Electrochemical CO cell (ppm)
Reads on a CO alarm?No (0 ppm at 1.25 percent, RKI chart)No (0 ppm at 0.5 percent, RKI chart)Yes
First alarm on an RKI monitor10 percent LEL (0.5 percent gas)10 percent LEL of the calibrated gas25 ppm warning, 50 ppm alarm

Methane has no OSHA or NIOSH exposure limit; its hazards are oxygen displacement and fire.

Source: NIOSH Pocket Guide to Chemical Hazards (propane, carbon monoxide); 29 CFR 1910.1000 Table Z-1; the LEL and UEL table on this site (NIOSH, NIOSH IC 9486, RKI FAQ); 49 CFR 192.625; RKI Instruments GX-3R Pro Operator’s Manual Table 1 and Appendix C Table 18.

RKI GX-Force Leak Check Mode, as the manual states it
ItemFigure
PurposePinpoint small leaks of combustible gas from valves, flanges, connections and other potential leak points
Active sensor by defaultThe catalytic combustible sensor (CO and O2 channels can be enabled in the setup program)
Methane range0 to 5,000 ppm and 0 to 100 percent LEL
Isobutane range0 to 2,000 ppm and 0 to 100 percent LEL
Increment50 ppm
Accuracy±20 percent of full scale (typically ±10 percent), at constant conditions
Response timeT90 in 10 seconds
Sample drawInternal diaphragm pump; sample from as far as 100 feet of hose (longer not advised for flow rate)
Not detected in this modeVOC gases such as heptane and toluene (Normal Mode detects them)

Source: RKI Instruments GX-Force Operator’s Manual, Appendix C Leak Check Mode (overview, Table 20) and Chapter 2 (pump).

Questions people ask

Can I detect a gas leak in my house?

By smell first: the odorant is required to be noticeable at one fifth of the flammable level. Then by the signs (hiss, bubbles, dead grass over a line, a pilot that goes out). A combustible gas detector reads the gas itself; a CO alarm does not.

Is it free to get a gas leak checked?

A suspected natural gas leak is reported to the gas utility’s emergency leak line; propane goes to the supplier. Whether a leak check is charged is the utility’s policy; repairs to customer-owned lines and appliances are the owner’s. Call from outside after leaving.

Is there a detector that detects gas?

Yes, a combustible gas detector (LEL monitor): a catalytic bead or infrared sensor that reads methane, propane and other flammables in percent LEL or, in leak-check mode, in ppm.

Where should a gas detector be placed in a house?

By the gas: natural gas is lighter than air, so a methane detector goes high near the appliance or meter; propane is heavier, so its detector goes low. Follow the detector manufacturer’s instructions and local code for exact placement.

Does natural gas rise?

Yes. Methane is lighter than air and collects at the ceiling; propane (density 1.55), butane and gasoline vapor are heavier and pool at the floor, in pits and in sumps.

What can falsely set off a natural gas detector?

Other combustible vapor: solvents, paint, aerosols (butane and propane propellants), alcohol, cleaners, some refrigerants. The reading is real combustible gas of some kind; confirm the source with soap solution or a second instrument.

Do natural gas detectors actually work?

Yes, within their stated accuracy (RKI: ±5 percent of reading or ±2 percent LEL on the GX-3R Pro combustible channel), provided they are bump tested with a known gas before each day’s use, because catalytic sensors drift and can be poisoned.

What detector do I need for propane?

A combustible (LEL) detector calibrated to or converted for propane, placed low. Propane’s LEL is 2.1 percent, so 10 percent LEL is 2,100 ppm, which NIOSH also uses as its IDLH.

Does a carbon monoxide detector detect natural gas or propane?

No. RKI’s CO sensor interference chart shows methane at 1.25 percent and propane at 0.5 percent each reading 0 ppm. CO is read by an electrochemical cell; fuels by a combustible sensor.

Does a CO2 alarm detect gas?

No. A CO2 monitor reads carbon dioxide by infrared at its own wavelength; it does not respond to methane or propane. It is also not a carbon monoxide detector.

What does a gas leak alarm sound like?

On a portable monitor, a buzzer, flashing LEDs and a vibrator together, with different patterns for the warning (10 percent LEL on RKI units) and the alarm (25 percent LEL). Fixed home alarms use a repeating horn set by their own standard.

Are there warning signs before a gas explosion?

The smell and a rising LEL reading. Gas ignites only inside its flammable range (methane 5 to 15 percent, propane 2.1 to 9.5 percent); OSHA treats 10 percent of the LEL as already hazardous, and the odorant is set to be smelled at 20 percent LEL.

What happens if you inhale a small amount of natural gas?

Methane is not a poison; it displaces oxygen and burns. A whiff of odorized gas is mostly odorant at a few ppm. Symptoms around a leak come from oxygen displacement when a lot has accumulated, or from carbon monoxide if an appliance is burning badly.

How to detect a very small gas leak?

A pumped monitor in ppm mode (GX-Force Leak Check Mode: 0 to 5,000 ppm methane, 50 ppm steps, 10-second response) traced along the pipe to the highest reading, then soap solution on the fitting, or a pressure test of the whole run.

What is the LEL of natural gas, and what is 10 percent LEL in ppm?

Methane’s LEL is 5 percent by volume (50,000 ppm), so 10 percent LEL is 0.5 percent gas, 5,000 ppm. That is where RKI’s monitors give the first alarm and where OSHA’s confined-space rule calls the air hazardous.

Is methane highly flammable?

Yes, between 5 and 15 percent in air. Below 5 percent the mix is too lean and above 15 too rich, which is why a monitor reports the reading as a percentage of that lower limit.

Will my refrigerant leak detector detect natural gas?

Only if its sensing method responds to hydrocarbons; detectors built for halogenated refrigerants often do not. The detector manufacturer’s manual is the only source; a combustible (LEL) monitor is the instrument built for methane and propane.

Related guides

What is LEL?And what 10 percent LEL means on a gas monitorLEL and UEL by gasMethane, propane, hydrogen and the restHow do LEL sensors work?Catalytic bead and infraredWhat does 20 percent LEL mean?The 10, 25 and 50 percent set pointsWhat is the carbon monoxide exposure limit at work?OSHA, NIOSH and the ppm numbersGas detection for confined space entryWhat to test and the order

The ratings, test results and standards shown here are the manufacturers' published values, reproduced as published. Nothing on this page is a recommendation by Bayshore Safety that any product is suitable for a particular task, hazard or workplace. Selecting safety products and protective equipment is the responsibility of the end user and the employer, based on their own hazard assessment and the regulations that apply to their work. Confirm every value on the manufacturer's own documentation before use.

Need monitors matched to the gases on your permit?Call (925) 215-7337 or email us the gases in your hazard assessment and where people work, and we will quote the monitors, calibration gas and docks with the manufacturer figures listed. We service and calibrate portable gas instruments and sell nationwide.
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