How Does a Gas Detector Work? A Complete Guide to Sensor Technologies
Many of the most serious industrial hazards are invisible. Methane, hydrogen, LPG and solvent vapours can form explosive atmospheres; hydrogen sulphide, carbon monoxide, chlorine and ammonia can injure or kill at low concentrations; and a leak of nitrogen or carbon dioxide can silently displace oxygen. A gas detector gives people the warning they need to act before these hazards cause harm.
This guide explains how gas detectors work, the main sensor technologies and where each is used, how alarm levels are set, and the essentials of selecting, installing and maintaining a reliable gas detection system.
What Is a Gas Detector?
A gas detector is an instrument that senses the presence and concentration of specific gases in the air and warns people through audible and visual alarms, and in fixed systems, triggers automatic safety actions such as ventilation, shutdown or isolation. Gas detectors fall into three hazard categories:
- Combustible (flammable) gases: Measured as a percentage of the Lower Explosive Limit (% LEL).
- Toxic gases: Measured in parts per million (ppm), such as H2S, CO, Cl2, NH3 and SO2.
- Oxygen deficiency or enrichment: Measured as % volume oxygen.
How Does a Gas Detector Work?
Every gas detector follows the same basic sequence. Gas reaches the sensor by diffusion or is drawn in by a pump. The sensing element reacts physically or chemically with the target gas and produces an electrical signal proportional to concentration. Electronics amplify and process this signal, compensate for temperature and humidity, and convert it into a reading. If the concentration exceeds preset alarm levels, the detector activates alarms and outputs. In fixed systems, a 4-20 mA, relay or digital signal is sent to a control panel, PLC or DCS.
Main Gas Sensor Technologies
Catalytic Bead (Pellistor) Sensors
Catalytic sensors detect combustible gases. Two heated beads form part of a Wheatstone bridge: one coated with a catalyst and one inert reference. Flammable gas burns on the active bead, raising its temperature and resistance, and the bridge imbalance is proportional to gas concentration. Catalytic sensors respond to almost all combustible gases and vapours and are cost-effective. However, they need oxygen to work and can be poisoned by silicones, sulphur compounds and lead.
Infrared (IR) Sensors
Infrared sensors detect hydrocarbons and carbon dioxide by measuring how much infrared light the gas absorbs at a specific wavelength, compared with a reference wavelength. They are fail-safe, immune to poisoning, work in oxygen-free atmospheres and have long life, often over 10 years. They do not detect hydrogen, which has no infrared absorption.
Electrochemical Sensors
Electrochemical cells are the standard technology for toxic gases and oxygen. The target gas diffuses through a membrane and reacts at an electrode, generating a current proportional to concentration. They are highly sensitive and selective, consume little power and are ideal for H2S, CO, Cl2, NH3, SO2, NO2 and O2. Typical sensor life is two to three years.
Photoionisation Detectors (PID)
PIDs use ultraviolet light to ionise volatile organic compounds (VOCs). The resulting current indicates concentration at ppm or even ppb levels. They are used for solvent exposure monitoring and leak detection.
Semiconductor (Metal Oxide) Sensors
A heated metal oxide film changes resistance in the presence of reducing gases. These sensors are robust and inexpensive but less selective and are more common in commercial and domestic applications.
Open-Path Infrared Detectors
Open-path detectors project an infrared beam between a transmitter and receiver over distances of up to 100-200 metres. They measure gas anywhere along the beam, making them ideal for perimeter monitoring of process units, tank farms and pipelines.
Optical Gas Imaging and Ultrasonic Leak Detection
Gas imaging cameras make hydrocarbon leaks visible on screen, which is valuable for leak surveys and fugitive emission (LDAR) programmes. Ultrasonic detectors respond to the sound of a high-pressure gas leak rather than the gas itself, giving fast warning in well-ventilated outdoor areas.
Understanding LEL and Alarm Levels
The Lower Explosive Limit is the lowest concentration of a gas in air that can ignite. For methane it is about 4.4-5 percent by volume. Flammable gas detectors read from 0-100% LEL, and typical alarm settings are around 10-20% LEL for the first alarm and 20-40% LEL for the second alarm or executive action, giving time to act well before an explosive mixture forms.
For toxic gases, alarms are set relative to occupational exposure limits, such as short-term exposure limit (STEL) and time-weighted average (TWA). Oxygen detectors typically alarm at 19.5% (deficiency) and 23.5% (enrichment).
Fixed vs Portable Gas Detectors
Fixed gas detectors are permanently installed to monitor process areas, compressor houses, battery rooms and control room air intakes 24 hours a day. They connect to a control panel or safety system that can start ventilation, shut down equipment and activate beacons and sirens. Portable gas detectors are worn or carried by workers for personal protection, confined space entry and leak checks. Multi-gas portable monitors typically measure LEL, O2, H2S and CO together. A comprehensive safety programme uses both.
Selecting the Right Gas Detector
- Target gases: Identify every flammable, toxic and asphyxiant gas that could be present.
- Sensor technology: Choose IR for hydrocarbons in harsh or poisoning environments, catalytic for broad combustible coverage including hydrogen, electrochemical for toxics and oxygen.
- Measuring range and alarm points: Matched to the hazard and regulations.
- Hazardous area certification: ATEX, IECEx or national approvals, intrinsically safe or flameproof.
- Environment: Temperature, humidity, dust, rain, corrosive atmosphere and ingress protection.
- Outputs and integration: 4-20 mA, HART, Modbus, relays, and compatibility with fire and gas panels.
- Functional safety: SIL-rated detectors for safety instrumented functions.
Detector Placement
- Gases lighter than air, such as methane and hydrogen, rise; mount detectors high, near ceilings or above leak sources.
- Heavier gases, such as LPG, H2S and solvent vapours, sink; mount detectors low, near floor level and in pits or trenches.
- Place detectors close to likely leak points: pump seals, compressor packing, flanges, valves and loading points.
- Consider air movement, ventilation patterns and wind direction outdoors.
- Protect control room and building air intakes with dedicated detectors.
- Use a formal gas detector mapping study for complex process facilities.
Common Causes of Gas Detector Problems
- Sensor poisoning: Silicones, lead and sulphur compounds deactivate catalytic sensors.
- Blocked inlets: Dust, insects, paint or ice prevent gas reaching the sensor; use weather guards and keep inlets clean.
- Cross-sensitivity: Some electrochemical sensors respond to other gases; check manufacturer cross-sensitivity data.
- Oxygen deficiency: Catalytic sensors under-read in low-oxygen atmospheres.
- Expired sensors: Electrochemical cells lose sensitivity with age and must be replaced.
Calibration and Maintenance
Gas sensors drift and degrade over time, so regular checks are essential. A bump test exposes the detector to a known gas to confirm it responds and alarms; it is recommended before each day’s use for portable instruments. A full calibration adjusts the reading against a certified calibration gas and is typically performed every three to six months for fixed detectors, or as the manufacturer and site procedures require. Keep sensor inlets clean, replace sensors at the end of their life and record all tests for compliance.
Integration with Fire and Gas Systems
In process plants, gas detectors form part of a wider fire and gas system together with flame detectors, heat detectors, manual call points and alarm devices. Detection signals drive voting logic that initiates ventilation, emergency shutdown, deluge and audible and visual alarms. A clear warning signals system of sirens and beacons ensures everyone on site knows when to evacuate or take action.
Gas Detection Solutions from Bliss Flow Systems
Bliss Flow Systems supplies the complete range of gas detectors, including portable single and multi-gas monitors, fixed catalytic and electrochemical detectors with 4-20 mA outputs, open-path infrared systems for H2S and methane and optical gas imaging cameras for leak detection, with hazardous area certified designs for onshore and offshore facilities.
Frequently Asked Questions
How does a gas detector detect gas?
Gas reaches a sensor that reacts to it chemically or physically, producing an electrical signal proportional to concentration. The detector converts this into a reading and triggers alarms above set levels.
What is the difference between catalytic and infrared gas detectors?
Catalytic sensors burn gas on a heated bead and need oxygen but detect almost all combustibles including hydrogen. Infrared sensors measure light absorption, are immune to poisoning and work without oxygen, but cannot detect hydrogen.
What does % LEL mean on a gas detector?
It is the gas concentration expressed as a percentage of its Lower Explosive Limit. 100% LEL is the point at which the mixture can ignite.
How often should gas detectors be calibrated?
Portable detectors should be bump tested before daily use. Fixed detectors are typically calibrated every three to six months, depending on manufacturer guidance and site procedures.
Where should a gas detector be installed?
Near potential leak sources, high for lighter-than-air gases and low for heavier-than-air gases, taking airflow and ventilation into account.
Conclusion
Gas detectors are a critical line of defence against fire, explosion, toxic exposure and asphyxiation. Understanding how each sensor technology works helps you choose the right detector for every gas and location. With correct placement, regular testing and integration into your fire and gas system, gas detection protects your people, your assets and your operations.
Contact Bliss Flow Systems
For product selection, technical datasheets or a quotation, reach our engineering sales team at the office nearest to you, or send an enquiry through our contact page.