| Oxygen (O₂) | Electrochemical | 0–25% volume | Confined-space entry, inerting areas, tank and vessel inspection | Choose alarms appropriate to the site’s oxygen-deficiency and oxygen-enrichment limits. Sensor response and service life can be affected by environmental conditions. |
| Combustible gases and vapors | Catalytic bead | 0–100% LEL | General flammable-gas monitoring in industrial facilities, utilities, and maintenance work | Requires sufficient oxygen to operate. Some substances, including silicones and sulfur compounds, can inhibit or poison the sensor. Confirm calibration gas and correction factors for the target gas. |
| Methane and other infrared-absorbing hydrocarbons | Infrared (IR) | Commonly 0–100% LEL; some instruments offer volume-percent ranges | Oil and gas facilities, pipelines, and areas where catalytic sensors may be unsuitable | IR sensors do not require oxygen and are generally resistant to catalytic-sensor poisoning. Standard hydrocarbon IR sensors do not detect hydrogen. |
| Carbon monoxide (CO) | Electrochemical | Commonly 0–500 ppm; higher ranges may be available | Combustion-related work, industrial sites, garages, and emergency response | Check the expected concentration and potential cross-sensitivities, including interference from other gases, before selecting the range. |
| Hydrogen sulfide (H₂S) | Electrochemical | Commonly 0–100 ppm; some versions extend to 0–1,000 ppm | Wastewater treatment, oil and gas operations, sewers, and confined spaces | Use a range suited to both routine exposure monitoring and credible peak concentrations. Verify alarm settings against applicable site rules and regulations. |
| Ammonia (NH₃) | Electrochemical | Common ranges include 0–100 ppm or 0–1,000 ppm | Refrigeration plants, chemical handling, and fertilizer operations | Sensor range and materials must suit the expected concentration and exposure conditions. Confirm response characteristics and cross-sensitivity for the specific sensor. |
| Volatile organic compounds (VOCs) | Photoionization detector (PID) | Often 0–2,000 ppm, depending on instrument configuration | Solvent use, hazardous-materials assessment, site remediation, and leak surveys | Readings are commonly reported as an equivalent to the calibration gas, often isobutylene. Response varies by compound and correction factor. A standard PID does not detect methane or many other gases with ionization energies above the lamp energy. |
| Sulfur dioxide (SO₂) | Electrochemical | Commonly 0–20 ppm or 0–100 ppm | Combustion processes, chemical plants, and industrial emission-area monitoring | Select a range that covers expected peaks while retaining suitable resolution near the required alarm level. Check for cross-sensitivity to other acidic gases. |