For carbon monoxide, hydrogen sulphide, chlorine and ammonia, electrochemical sensors are the usual choice. They are specific, sensitive and use almost no power. They wear out in two to three years and need regular calibration, because the electrolyte slowly dries.

Catalytic-bead sensors burn a small amount of gas on a heated bead and detect the extra heat. They read in percent of the lower explosive limit (LEL). They can be poisoned by silicones and sulphur compounds, so keep hair products and sealants away. Infrared sensors detect hydrocarbons by absorption, last longer, and do not need oxygen. Choose them for methane and propane in demanding sites.

A photoionisation detector uses UV light to ionise vapours and can find them at parts-per-billion level. MOS sensors are cheap for air-quality monitoring but are not selective.

Electrochemical oxygen sensors check for low oxygen in confined spaces. Zirconia sensors work hot and measure oxygen in boilers and engine exhaust to tune combustion.

Hydrogen is light, fast and burns easily. Detectors should be placed high, because the gas rises. Use sensors designed for hydrogen, since many standard sensors respond poorly.

• Heavy gases (propane, chlorine, H2S) collect low. Light gases (hydrogen, methane) rise.
• Place sensors near likely leak points, not in a corner of the room where air barely moves.
• Bump-test before use and calibrate on schedule with the right test gas.

Each card below shows the sensor in use, what it is and how it works, and where you will find it.

SENSOR 1 OF 12

Electrochemical Gas Sensor

An electrochemical gas sensor has electrodes in an electrolyte. When the target gas reaches the electrode, a chemical reaction produces a small electric current, proportional to the gas concentration. It is very good for toxic gases such as CO and H2S and uses very little power.

USED IN

toxic gas detection

SENSOR 2 OF 12

Catalytic-Bead Sensor

A catalytic-bead sensor has a heated bead coated with a catalyst. When flammable gas touches it, the gas burns on the bead and makes it hotter, which changes its resistance. This is compared with a reference bead. It is widely used to detect flammable gas as a percentage of the lower explosive limit.

USED IN

flammable gas detection

SENSOR 3 OF 12

Infrared (IR) Gas Sensor

An infrared (IR) gas sensor passes infrared light through a gas sample. Some gases, like CO2 and methane, absorb a specific wavelength, and the drop in light shows the gas concentration. It is stable, long-lasting and not poisoned by other gases.

USED IN

CO2methane

SENSOR 4 OF 12

Metal-Oxide Semiconductor (MOS) Gas Sensor

A metal-oxide semiconductor (MOS) gas sensor has a heated metal-oxide layer. When certain gases touch it, the layer's resistance changes. The change shows the presence of the gas. It is low cost, small and sensitive, but not very selective, so it is used in air-quality sensors and basic alarms.

USED IN

air qualityalarms

SENSOR 5 OF 12

Photoionization Detector (PID)

A photoionization detector (PID) uses ultraviolet light to ionise gas molecules. The ions create a small electric current that is measured, and it shows the concentration. It detects many VOCs at very low levels (ppb), so it is used for leak detection and air monitoring.

USED IN

VOCleaks

SENSOR 6 OF 12

Thermal-Conductivity Gas Sensor

A thermal-conductivity gas sensor compares how well a gas conducts heat with how well a reference gas does. A heated element cools differently in different gases, and the change in resistance shows the gas concentration. It is used for gases such as hydrogen and in gas analysers.

USED IN

gas analyserschromatography

SENSOR 7 OF 12

Zirconia Oxygen Sensor

A zirconia oxygen sensor uses a heated ceramic made of zirconium dioxide. At high temperature the ceramic allows oxygen ions to pass, and a voltage is made between the two sides, depending on the oxygen difference. It is used in vehicle exhausts, boilers and furnaces to control combustion.

USED IN

enginesboilers

SENSOR 8 OF 12

pH Sensor

A pH sensor measures how acidic or basic a liquid is on a scale of 0 to 14, where 7 is neutral. It has a thin glass bulb and a reference electrode, and a voltage develops that depends on the hydrogen-ion level. It must be calibrated regularly and is used in water treatment and food.

USED IN

water treatmentfood

SENSOR 9 OF 12

Ion-Selective Electrode (ISE)

An ion-selective electrode (ISE) has a special membrane that reacts to only one type of ion, such as sodium, nitrate or fluoride. The voltage that develops is proportional to the concentration of that ion. It is used in labs, water testing and process control.

USED IN

labswater testing

SENSOR 10 OF 12

Conductivity Sensor for Chemical Solutions

A conductivity sensor for chemical solutions measures how well a liquid carries electric current. More dissolved salts or ions mean higher conductivity. It is used to check concentration, purity and mixing in boilers, cooling towers and chemical processes.

USED IN

boilerscooling towers

SENSOR 11 OF 12

Flame Detector (UV / IR)

A flame detector (UV/IR) looks for the ultraviolet or infrared radiation produced by flames. When it sees the typical flame pattern, it gives an alarm. It reacts in a fraction of a second, so it is used on burners, in oil and gas plants and in fire protection systems.

USED IN

burnersoil and gas

SENSOR 12 OF 12

Hydrogen Gas Sensor

A hydrogen gas sensor detects hydrogen leaks. Hydrogen is very light and burns easily, so early detection is important. Different types are used, such as catalytic, thermal or semiconductor, to give an alarm at a low concentration. It is used in battery rooms and fuel-cell systems.

USED IN

battery roomsfuel cells