I don't sell any of these nor do I receive any remuneration if you buy them, and I have not personally reviewed them, they are listed here to give you perspective
Introduction
gas leak detectors come in a range of types:
carbon monoxide detectors
these are designed to detect CO local leaks from gas heaters, etc and differ from CO alarms which are designed to detect high levels of CO in a room
combustible gas detectors (“LEL”)
particularly used for finding natural gas or LPG leaks from fittings or pipes
refrigerant gas detectors (“freon”/“halogen”/R22/R23/R134a/etc)
designed to find leaks in air conditioner or fridge pipes or fittings
dangerous air, combined gas detectors
not designed specifically for gas leaks but to ascertain if the air in enclosed spaces are safe to enter without breathing apparatus
usually detect CO, H2S, oxygen, combustible gas, and possibly CO2
these detect a range of combustible gases but do not identify which gas it is
they usually provide measures in either ppm or % LEL (lower explosive level)
they usually take 30sec to warm up and need to be auto-calibrated during this period in clean air
there are three main types of sensors:
Catalytic Combustion Sensors:
this is the most common device used in budget detectors
these have a heated catalytic bead coated with a catalyst.
when combustible gas contacts the bead, it reacts with oxygen causing flameless combustion and heating of the bead.
this changes the bead's electrical resistance, measured by a Wheatstone bridge circuit.
the resistance change correlates to gas concentration.
this sensor type responds to many hydrocarbons but cannot distinguish gas types—it only measures total combustible gas concentration
Infrared (IR) Sensors:
these detect gases by measuring the absorption of infrared light at specific wavelengths characteristic to hydrocarbons in the gas sample.
the gas absorbs some wavelengths, distorting the IR beam.
by analyzing absorption spectra, some IR detectors can identify certain gas types or mixtures, giving more selectivity than catalytic sensors.
Semiconductor Sensors:
these sensors have semiconductor materials whose resistance changes when combustible gas molecules interact with the surface. They detect gas presence but have less specificity and are more influenced by environmental factors. They also do not identify gas types but respond generally to combustible gases.
most are calibrated against a methane standard and may use correction factors to adjust for other gases
coal gas “town gas”
produced by destructive distillation (heating in the absence of air) of bituminous coal is an explosive mixture of hydrogen (50-60%), methane (20-35%), carbon monoxide (10-20%), and carbon dioxide (5-10%) and is the cause of coal mine explosions and has now been replaced by natural gas and methane for housing purposes.
lower explosive level (LEL)
the lowest concentration of a combustible or flammable gas or vapor in the air that can ignite or explode if an ignition source (such as a spark or flame) is present
methane (natural gas) has an LEL of about 5%, meaning methane concentrations below 5% by volume in the air are too lean to ignite
propane has a LEL of 2.1% by volume
n-Butane / iso-butane has a LEL of 1.6-1.9% by volume
petrol fumes have a LEL of 1.4% by volume
ethanol vapour has a LEL of 3.3% by volume
hydrogen has an LEL of 4% by volume
coal gas has an LEL of 4-5% by volume
gas detectors display gas concentration as a percentage of the LEL for that gas detected
thus, if it displays 5% LEL and the gas is methane then the concentration of methane by volume is 5% x 5% = 0.05 x 0.05 = 0.25% by volume
NB. to allow humans to smell propane, butane, and natural gas, odorants are added to it
humans can first smell the odorant added to propane when propane is at 0.15% to 2% by volume (range is due to variation in smell between humans), but most can smell this by 20% LEL, and most can smell butane by 0.12% concentration by volume (~7% LEL), and most can smell natural gas at 20% LEL
DO NOT use a naked flame such as a match to detect a gas leak - it may cause an explosion!
you can use soapy detergent water sprayed onto a gas fitting to visually see a leak
examples
Habotest HT601A/B
Catalytic Combustion Sensor
Plus model has a color screen and rechargeable battery
A model has lower accuracy (+/- 30%) and reduced range 50-1000ppm
B model has +/- 10% accuracy for methane and range 0～9999ppm / 0.00～20.00% LEL