| Gas Sensor | Measures the concentration of a target gas in air. | Electrochemical, catalytic bead, infrared, photoionization, or metal-oxide semiconductor sensing. | Oxygen, toxic gases, combustible gases, volatile organic compounds, and carbon dioxide. | The sensor converts a chemical or physical interaction with the gas into an electrical signal that represents gas concentration. | Live concentration reading, warning alarm, high alarm, low oxygen alarm, or fault signal. | Sensor selection must match the gas, expected concentration range, environmental conditions, and required response time. |
| Electrochemical Sensor | Detects many toxic gases and oxygen at relatively low concentrations. | Electrochemical cell. | Carbon monoxide, hydrogen sulfide, chlorine, ammonia, nitrogen dioxide, and oxygen. | The target gas reacts at an electrode, producing a current that is approximately proportional to the gas concentration within the sensor's operating range. | Concentration display, visual alarm, audible alarm, and relay or control-system signal. | Consumable components can have a limited service life. Temperature, humidity, cross-sensitivity, and sensor poisoning may affect accuracy. |
| Catalytic Bead Sensor | Detects combustible gases and vapors that may create a fire or explosion hazard. | Catalytic oxidation, also called a pellistor method. | Methane, propane, hydrogen, and other combustible gases. | Combustible gas oxidizes on a heated catalyst, changing the temperature and electrical resistance of the sensing bead. Readings are commonly expressed as a percentage of the lower explosive limit. | Combustible-gas concentration, low alarm, high alarm, and shutdown or ventilation signal. | Requires oxygen for catalytic combustion. Silicone compounds, sulfur compounds, lead, and other contaminants can reduce sensitivity or damage the sensor. |
| Infrared Sensor | Measures combustible gases or carbon dioxide without consuming the target gas. | Non-dispersive infrared absorption. | Methane, propane, other hydrocarbons, and carbon dioxide. | The gas absorbs infrared light at characteristic wavelengths. The reduction in transmitted light is used to calculate concentration. | Continuous concentration reading, alarm relay, analog output, digital communication, or control-system input. | Generally resistant to catalytic-sensor poisons and suitable for oxygen-deficient environments, but optical contamination, condensation, and gas-specific calibration can affect performance. |
| Photoionization Detector | Detects and estimates concentrations of many volatile organic compounds. | Ultraviolet photoionization. | Solvents, fuels, aromatic hydrocarbons, and other ionizable volatile organic compounds. | Ultraviolet photons ionize gas molecules with ionization energies below the lamp energy. The resulting electrical current is related to vapor concentration. | Parts-per-million display, exposure alarm, trend data, or data-logging output. | Response varies by compound. The method does not detect all gases and may require compensation for humidity and lamp condition. |
| Metal-Oxide Semiconductor Sensor | Provides broad gas detection for selected combustible, toxic, or air-quality applications. | Change in electrical resistance of a heated semiconductor material. | Selected hydrocarbons, carbon monoxide, hydrogen, and other gases depending on the sensing material. | Gas adsorption changes the electrical conductivity of the heated sensing surface, producing a measurable resistance change. | Threshold alarm, relative concentration indication, or process-monitoring signal. | Often has limited selectivity and can be affected by humidity, temperature, background gases, and sensor aging. It is usually less suitable for precise gas-specific measurement. |
| Sampling System | Brings air from a remote or difficult-to-access location to the sensor. | Diffusion, aspirated sampling, pump-assisted sampling, or tubing networks. | Depends on the installed sensor and monitored area. | Diffusion allows gas to reach the sensor naturally, while an aspirated system actively draws air through filters, tubing, and the sensor chamber. | Concentration reading, flow failure alarm, blocked-line alarm, and gas alarm. | Tube length, flow rate, leaks, condensation, filters, and transport time can influence the measurement and alarm response. |
| Signal Processor | Interprets the sensor signal and determines gas concentration and alarm state. | Analog-to-digital conversion and programmed signal processing. | All gases supported by the connected sensor. | The processor applies calibration data, temperature compensation, filtering, alarm thresholds, and diagnostic checks to the raw sensor signal. | Normal, warning, high alarm, over-range, sensor fault, calibration due, or communication fault. | Alarm logic should account for response time, sensor recovery time, time delays, latching requirements, and applicable safety procedures. |
| Alarm Indicators | Warn personnel when gas concentration reaches a configured limit or when the instrument develops a fault. | Visual, audible, and sometimes vibrating indicators. | Any gas monitored by the alarm system. | The system compares the measured concentration with preset thresholds and activates indicators when a threshold is exceeded. | Flashing light, audible sounder, text message, vibration, beacon, or event notification. | Alarm levels should be selected using the gas hazard, occupational exposure limits, flammability limits, site risk assessment, and emergency procedures. |
| Relay and Control Interface | Connects the gas alarm to ventilation, process shutdown, emergency systems, or building controls. | Relay contacts, analog current output, digital communication, or industrial network connection. | All gases monitored by the system. | The alarm controller sends a signal when a gas threshold, fault condition, or maintenance state occurs. | Fan activation, valve closure, equipment shutdown, remote alarm, or control-room notification. | Fail-safe design, power-loss behavior, wiring supervision, isolation, and periodic functional testing are important for safety-related actions. |
| Power Supply | Provides continuous electrical power to sensors, processing circuits, indicators, and communication equipment. | AC power, DC power, battery backup, or a combination. | Applies to the complete gas alarm system. | The power system converts and distributes electrical energy while monitoring for undervoltage, battery failure, or loss of supply. | Power-on status, low-battery alarm, power-failure relay, or backup operation. | Critical installations may require emergency power, backup batteries, surge protection, and a clearly defined safe state after power loss. |
| Calibration and Test Function | Verifies that the sensor and alarm system respond correctly to a known test gas. | Zero adjustment, span calibration, bump test, and alarm-function test. | Target gas specified for the installed sensor. | A known concentration is applied to confirm sensor response, display accuracy, alarm activation, and signal transmission. | Calibration result, pass or fail status, maintenance reminder, and recorded test event. | Testing frequency depends on the sensor type, application risk, manufacturer instructions, environmental exposure, and site safety program. |
| Environmental Compensation | Reduces measurement errors caused by changing site conditions. | Temperature, pressure, humidity, and sensor-diagnostic compensation. | All gases, with effects varying by sensor technology. | Additional measurements or correction algorithms adjust the sensor signal or identify conditions outside the operating range. | Corrected concentration, environmental warning, sensor fault, or out-of-range indication. | Extreme temperature, pressure changes, high humidity, dust, vibration, and corrosive atmospheres can reduce reliability or shorten sensor life. |
| Data Logging and Communication | Records gas concentrations, alarms, faults, calibration events, and maintenance information. | Local memory, wired communication, wireless communication, or supervisory control integration. | All monitored gases and system events. | The controller stores time-stamped measurements and transmits selected information to a display, control room, or monitoring platform. | Trend charts, event history, remote alarm, reports, and maintenance records. | Communication failure must not prevent local alarms from operating. Data integrity, access control, and reliable time synchronization should be considered. |
| Protective Enclosure | Protects internal components from dust, moisture, impact, and hazardous-area conditions. | Industrial enclosure with application-specific ingress and hazardous-location protection. | Applies to the detector, controller, or junction box. | The enclosure and cable entries limit environmental ingress and may prevent ignition of a surrounding flammable atmosphere when correctly certified and installed. | Enclosure status, tamper indication, or no direct output. | Protection rating, material compatibility, grounding, cable glands, installation location, and hazardous-area requirements must match the site conditions. |
