Gas detection sensors protect people, equipment and processes from flammable, toxic and asphyxiant gases. Yet no single sensor technology detects every gas. The three most common technologies in industrial use are catalytic bead, electrochemical and non-dispersive infrared (NDIR). Understanding how they work is the first step to choosing the right detector.
The Three Main Gas Hazards
Before choosing a sensor, identify the type of hazard:
- Flammable gases: methane, propane, hydrogen and solvent vapours. Measured as a percentage of the lower explosive limit (% LEL).
- Toxic gases: carbon monoxide (CO), hydrogen sulphide (H₂S), ammonia (NH₃), chlorine (Cl₂) and others. Measured in parts per million (ppm).
- Oxygen deficiency or enrichment: caused by inert gases, leaks or combustion. Normal air contains about 20.9% oxygen by volume.
Each technology below is suited to one or more of these hazards.
Catalytic Bead (Pellistor) Sensors
How they work
A catalytic sensor contains two small beads, each around a heated platinum wire coil. One bead is coated with a catalyst (the active bead) and the other is not (the reference bead). When a flammable gas reaches the active bead, it burns on the catalyst surface. This raises the bead’s temperature and changes the coil’s resistance. A Wheatstone bridge circuit measures the difference between the two beads.
Strengths
- Responds to a broad range of flammable gases and vapours.
- Proven, well-understood technology.
- Relatively low cost.
Limitations
- Needs oxygen to work. In an oxygen-poor atmosphere it can under-read.
- Can be poisoned by silicones, lead compounds and sulphur compounds, or inhibited by halogens. A poisoned sensor may fail without an obvious sign.
- Very high gas concentrations can damage it.
- Consumes more power than other types.
Catalytic sensors suit general flammable gas monitoring in clean environments, provided they are bump-tested regularly.
Electrochemical Sensors
How they work
An electrochemical sensor is a small fuel cell. Gas diffuses through a membrane to an electrode, where it is oxidised or reduced. This chemical reaction produces a current proportional to the gas concentration. Electrolyte and electrode materials are chosen for a specific target gas.
Strengths
- High sensitivity and good selectivity for toxic gases at ppm levels.
- Low power consumption, ideal for portable and battery-powered detectors.
- Linear output.
- Also the most common technology for oxygen sensors.
Limitations
- Limited lifespan, as the electrolyte and electrodes are consumed or dry out over time.
- Cross-sensitivity: some gases can cause false readings on a sensor designed for another gas.
- Sensitive to extreme temperature and humidity.
Electrochemical sensors are the standard choice for CO, H₂S, oxygen, chlorine, ammonia and many other toxic gases.
NDIR (Non-Dispersive Infrared) Sensors
How they work
Many gas molecules absorb infrared light at specific wavelengths. An NDIR sensor shines infrared light through a sample chamber to a detector fitted with an optical filter. When the target gas is present, it absorbs part of the light and the detector signal drops. A reference channel at a non-absorbed wavelength compensates for lamp ageing and dirt.
Strengths
- Cannot be poisoned in the way catalytic sensors can.
- Works in oxygen-free atmospheres.
- Good long-term stability with low calibration drift.
- Fail-safe: a failed lamp or blocked optics is detected as a fault.
- The standard technology for carbon dioxide (CO₂) measurement.
Limitations
- Only detects gases that absorb infrared light. Hydrogen, for example, has no infrared absorption and cannot be measured by NDIR.
- Higher initial cost than catalytic sensors.
- Response may differ between hydrocarbons, so calibration must match the target gas.
NDIR sensors suit hydrocarbon detection in harsh or poisoning environments, and CO₂ monitoring in buildings, greenhouses, breweries and cold stores.
Comparison Table
| Criterion | Catalytic bead | Electrochemical | NDIR |
|---|---|---|---|
| Main use | Flammable gases (% LEL) | Toxic gases (ppm), oxygen | Hydrocarbons, CO₂ |
| Needs oxygen | Yes | Depends on type | No |
| Poisoning risk | Yes | Low | No |
| Power consumption | High | Very low | Medium |
| Detects hydrogen | Yes | Yes (specific cell) | No |
| Typical lifespan | Several years if not poisoned | Limited, consumable | Long |
| Fail-safe | No | No | Yes |
| Relative cost | Low | Low to medium | Medium to high |
How to Choose Gas Detection Sensors
Ask these questions:
- Which gas, or gases, could be present? This decides the technology. A biogas plant may need methane, H₂S and oxygen sensors in the same detector.
- What concentration range? Leak detection in ppm, flammable risk in % LEL, or process measurement in % volume.
- What is the environment? Temperature, humidity, dust and the presence of poisons such as silicones affect the choice.
- Fixed or portable? Fixed detectors protect an area continuously; portable detectors protect the person wearing them.
- What output is needed? 4-20 mA, RS485 Modbus, relays or wireless, depending on your control or alarm system.
- Is the area classified as hazardous? If so, the detector needs the appropriate explosion protection certification, such as ATEX or IECEx.
Installation and Maintenance Tips
- Mount at the right height. Gases lighter than air, such as methane and hydrogen, rise, so mount near the ceiling. Heavier gases, such as propane and H₂S, collect low, so mount near the floor. CO is close to air density and is often mounted at breathing height.
- Place sensors near likely leak sources, such as valves, flanges, compressors and pumps.
- Bump test regularly by exposing the sensor to a known gas to confirm it responds and alarms.
- Calibrate according to the manufacturer’s schedule.
- Replace electrochemical cells before the end of their stated life.
Getting Gas Detection Right
Catalytic, electrochemical and NDIR gas detection sensors each have a clear role. Catalytic beads cover flammable gases at low cost, electrochemical cells handle toxic gases and oxygen, and NDIR delivers stable, poison-resistant detection of hydrocarbons and CO₂. Many sites combine all three.
Browse our gas detection category to compare fixed and portable sensors, or contact our team for help matching the right technology to your gases and environment.