Industrial gas detection relies on two primary physical principles to transform molecular presence into measurable signals. Achieving high Selectivity and **Sensitivity** is the engineering goal.

• Catalytic Oxidation (Pellistor): Uses a platinum coil embedded in a ceramic bead to combust flammable gas. This exothermic reaction increases resistance. Critical Warning: Pellistors are vulnerable to Sensor Poisoning from silicones, lead, and sulfur compounds which "vitiate" the catalyst.
• Electrochemical Cells: Toxic gases (H₂S, CO, NH₃) diffuse into a sensor containing electrolyte, initiating a redox reaction. The resulting current is linearly proportional to the ppm concentration.
• Infrared (IR) Absorption: Uses the "Beer-Lambert Law" to measure light attenuation at specific wavelengths. IR sensors are immune to poisons but cannot detect Hydrogen (which has no IR absorption band).

Detector height is not standardized; it is dictated by the Buoyancy Parameter ($B$) and vapor density relative to ambient air. Improper placement creates "blind zones" in the safety system.

Standard design practice mandates:

• Lighter Gases (H₂, CH₄): Mount within 0.3m of highest ceiling points or roof peaks.
• Heavier Gases (LPG, H₂S): Mount 0.15m to 0.3m above the lowest grade level.
• Breathing Zone: For oxygen deficiency or CO monitoring, mount at 1.5m to 1.8m height.

Safety engineers must distinguish between LEL/UEL (Fire Hazard) and **PPM Thresholds** (Toxicity Hazard). A gas may be toxic at levels 1000x lower than its explosive range.

• LEL (Lower Explosive Limit): Concentration below which gas won't ignite. Standard alarms at 20% and 50% LEL.
• IDLH (Immediately Dangerous to Life or Health): Concentration that causes irreversible health effects or death within 30 minutes.
• TWA (Time Weighted Average): Permissible 8-hour daily exposure limit.

Modern GDS design has moved from "Prescriptive" (rule-of-thumb) to **"Performance-Based Mapping."** This involves 3D modeling of the facility to identify detection gaps.

• Geographic Coverage: Percentage of the facility volume monitored by at least one sensor.
• Scenario Coverage: Probability that a specific leakage scenario will trigger an alarm.
• CFD Analysis: "Computational Fluid Dynamics" simulates how wind and piping drag dissipate gas plumes.

Safety architectures balance Safety (Trip when gas is present) against **Availability (Don't trip if sensor fails)**. Voting logic is the solution.

• 1ooN (1-out-of-N): Maximizes safety. Any single sensor triggers alarm. High risk of false shutdowns.
• 2ooN (2-out-of-N): The Industrial Gold Standard. Requires two sensors to alarm. High reliability against nuisance trips.