Table of Contents

Introduction

A gas detector is only as good as the gas detector sensor inside it. The enclosure, display and alarms may look identical on two instruments, but the sensing element determines which gases are detected, at what concentration range, how long the instrument stays accurate, and how much the total cost of ownership really is. Selecting the wrong technology is not a minor specification miss — it can mean the alarm never fires for the hazard that actually exists.

This guide compares the four sensor technologies that dominate industrial fixed and portable gas detection: electrochemical, catalytic bead (pellistor), non-dispersive infrared (NDIR) and photoionization detection (PID). It is written for EHS managers, plant engineers and procurement teams specifying detectors for oil & gas, petrochemical and desalination plants across Russia, Central Asia and the Middle East.

Electrochemical Sensors

Electrochemical sensors measure toxic gases and oxygen by oxidising or reducing the target gas at a working electrode inside a wet fuel-cell-style cell. The reaction produces a small current proportional to gas concentration. They are the default choice for parts-per-million (ppm) toxic measurement.

Typical gases: H2S, CO, O2, SO2, NO2, Cl2, NH3, HCN.
Range: ppm-level resolution, good selectivity, low power.
Weaknesses: consumable cells, affected by humidity and temperature, can be poisoned by silicones or by very high H2S exposure; oxygen cells typically last around two years.

Because they are small, low-power and accurate, electrochemical cells are the standard in portable four-gas monitors such as the Hangwei HD620, which combines H2S, CO, O2 and LEL in one instrument.

Catalytic Bead (Pellistor) Sensors

A catalytic bead sensor combusts the target combustible gas on a heated platinum bead; the resulting temperature change shifts the bead resistance, which the electronics measure. This is the classic technology for measuring the lower explosive limit (LEL) of flammable gases and vapours.

Typical gases: methane, propane, butane, LPG, solvent vapours — nearly all combustibles that burn in air.
Weaknesses: requires oxygen to combust; can be poisoned by silicones, lead compounds and high H2S; loses calibration if the bead is exposed to very high concentrations.
Service life: typically around five years in clean service, shorter in poisoned or corrosive environments.

Infrared (NDIR) Sensors

Non-dispersive infrared (NDIR) sensors pass infrared light through a sample chamber and measure how much light a specific gas absorbs at a characteristic wavelength. A dual-wavelength reference design compensates for dirt, window fouling and aging.

Typical gases: hydrocarbon combustibles (methane, propane, butane), CO2, refrigerant gases.
Strengths: no oxygen required, resistant to catalyst poisoning, long service life (commonly five to ten years), stable zero and repeatable readings.
Weaknesses: cannot detect hydrogen or gases that do not absorb IR; higher unit cost than catalytic beads.

For continuously monitored hydrocarbon hazards in demanding climates — the heat, dust and sand of Gulf-region plants, or the wide -40°C to +70°C swings handled by the Hangwei HD9300 fixed detector — NDIR is often the lower-maintenance choice.

Photoionization Detection (PID)

A PID uses a high-energy UV lamp to ionise volatile organic compounds (VOCs); the resulting ions produce a current proportional to concentration. PIDs excel at detecting a broad range of VOCs at very low, ppm to ppb levels.

Typical gases: benzene, toluene, xylene, solvents, aromatics and other VOCs.
Strengths: extremely sensitive, wide VOC coverage, fast response.
Weaknesses: not gas-specific (the reading is a sum of ionisable compounds), the UV lamp and electrode stack are consumables, and response factors must be set per gas.

Technology Comparison

The table below summarises the typical operating envelope of each technology. Exact figures depend on the manufacturer and application; treat these as engineering planning values and confirm against datasheets.

Technology	Best for	Typical range	Typical life*	Key advantage
Electrochemical	Toxic gases & O2	ppm level	1.5–3 years	Low power, high accuracy
Catalytic bead	Combustibles (LEL)	0–100 % LEL	~5 years	Low cost, broad fuel response
NDIR infrared	Hydrocarbons, CO2	ppm to % volume	5–10 years	No O2 needed, poison-resistant
PID	VOCs	ppb to ppm	1–2 years (lamp 6–12 mo)	Highest sensitivity to VOCs

*Typical manufacturer-rated values; actual service life depends on concentration of target gas, humidity, temperature extremes and exposure to poisons such as silicone vapours.

Cross-Sensitivity and Interference

No sensor is perfectly selective, and real-world procurement failures often come from ignoring interference. Electrochemical CO cells can respond to hydrogen; catalytic beads are affected by silicone vapours that permanently poison the catalyst; PIDs cannot distinguish benzene from toluene without a chosen response factor, and the reading reflects the sum of all ionisable gases present. A sensor spec sheet lists cross-sensitivity values for a reason — review them against the actual process chemistry before ordering, and involve the manufacturer early if several gases are present together.

Response time is another practical factor. Electrochemical and NDIR elements settle within seconds on a well-designed instrument, but dusty sample-draw systems and flooded or fouled beads slow real-world response. In harsh climates, specify an IP-rated enclosure and, where dust is present — as in grain, cement or powder handling — consider a dedicated dust-rated detector such as the Hangwei HD9100.

How to Choose the Right Sensor

Match the sensor to the actual gas — toxic ppm hazards need electrochemical; combustible atmosphere needs catalytic or NDIR; VOC leaks need PID.
Consider the environment: oxygen-deficient locations favour NDIR over catalytic beads; dusty or silicone-laden areas favour poison-resistant NDIR.
Plan consumables into the budget: electrochemical cells and PID lamps are replacement items, not one-time purchases.
Check cross-sensitivity against the full process gas list, not only the primary hazard.

Explore configurable sensing elements: Modular Gas Sensor

FAQ

Which sensor is used for H2S and CO detection?

Electrochemical sensors. They offer the ppm-level accuracy and low power required for toxic gas measurement and are used in four-gas portables such as the Hangwei HD620.

Can one sensor detect both combustible and toxic gas?

No single element does both well. Multi-gas instruments combine separate elements — for example an electrochemical cell for H2S/CO/O2 and a catalytic bead for LEL — in one housing.

Why choose NDIR over catalytic bead for methane?

NDIR does not need oxygen, resists catalyst poisoning and lasts longer, which lowers maintenance in continuously monitored outdoor hydrocarbon applications.

What does a PID detect that other sensors miss?

Volatile organic compounds and low-concentration aromatic/solvent vapours, at levels far below LEL sensors. It is not gas-specific, so a response factor must be chosen.

How long does a gas detector sensor last?

Electrochemical cells roughly 1.5–3 years, oxygen cells about 2 years, catalytic beads around 5 years, and NDIR elements 5–10 years. Treat these as planning values.

Get Expert Guidance

Hangwei Sensors (Henan Hangwei Sensing Technology Co., Ltd.) has more than 20 years of gas-detector manufacturing experience, 20 patents, 10 production lines and a full OEM/ODM service. We help buyers in Russia, Central Asia and the Middle East match the right sensor technology to the process — and supply detector-plus-controller packages certified to ATEX, IECEx and EAC (TR CU 012/2011) where required.

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