IR sensors work by measuring the absorption of infrared light at specific wavelengths caused by the vibration of molecular bonds (like the C-H bond in hydrocarbons). Hydrogen (H₂) is a diatomic molecule composed of two identical atoms, so it does not possess a dipole moment and does not absorb infrared radiation. Therefore, catalytic bead sensors or specialized electrochemical cells must be used for H₂ detection.

Sensor poisoning occurs when certain chemicals permanently coat or alter the catalytic bead, preventing it from combusting flammable gases. Common poisons include Silicones, Lead compounds, and Phosphates. Other substances like H₂S or Halogens can cause "inhibition," which temporarily reduces sensitivity. This is why bump testing is critical.

Point Detectors: Measure gas at a specific location. Good for enclosed areas or placing directly next to known leak sources (pump seals). Subject to wind blowing the leak away from the sensor.

Open-Path Detectors: Shoot an IR beam across a distance (up to 100m) to a receiver. Measures the total gas concentration across the beam path (in LEL-meters). Excellent for perimeter monitoring and open, windy process areas where a cloud might miss a point detector.

T90 is the time required for a gas detector to reach 90% of its final stable reading after being exposed to a step-change in gas concentration. IEC 60079-29-1 requires rapid T90 times (e.g., < 30 seconds for catalytic sensors) to ensure the SIS has enough time to close valves before a vapor cloud reaches the ignition source.

Gas detectors can occasionally generate false alarms due to transient environmental conditions or maintenance errors. If a single detector directly tripped an Emergency Shutdown (1oo1), it would cause massive production loss. Voting logic (like 2oo3) requires at least two detectors in the same zone to go into High-High alarm before triggering an ESD, vastly improving system availability without compromising safety.

Electrochemical cells are sensitive to Temperature and Humidity. Extreme heat can dry out the liquid electrolyte, leading to premature failure. Extreme cold slows down the chemical reaction, increasing response time. Very low humidity can also dehydrate the sensor, while high humidity can cause condensation and block the gas diffusion membrane.

Bump Test: A brief exposure to a known concentration of test gas to verify that the sensor responds and the alarms activate. It does not adjust the accuracy.

Calibration: Applying a certified standard gas mixture and electronically adjusting the detector's zero and span readings to ensure absolute accuracy. Bump testing is done frequently (e.g., monthly), while calibration is done periodically (e.g., quarterly).

Cross-sensitivity happens when a sensor designed for one specific gas also reacts to the presence of a different gas. For example, a CO electrochemical sensor might give a false positive reading if H₂ or H₂S is present. Engineers must review the manufacturer's cross-sensitivity charts to ensure the sensor won't give false alarms based on the specific background gases in their plant.

Acoustic detectors do not measure gas concentration; they "listen" for the high-frequency ultrasonic noise generated by high-pressure gas escaping from a leak. They respond instantaneously and are unaffected by wind direction, making them ideal for high-pressure outdoor installations (like offshore platforms or compressor stations) where wind would quickly disperse the gas before reaching a traditional point detector.