How do you select gas detectors for different applications?

Gas detector selection: Catalytic bead (pellistor) - detects combustible gases by oxidation, requires oxygen, affected by poisons, good for general hydrocarbon detection; Infrared (IR) point and open-path - specific hydrocarbon detection, fail-safe (no false negatives), immune to poisons, works in inert atmospheres; Electrochemical - toxic gas detection (H2S, CO), good selectivity, limited life; Photoionization (PID) - VOC detection, broad response; Semiconductor - general detection, less selective. Selection factors: target gas, environmental conditions, response time requirements, false alarm tolerance, and maintenance capability. Consider coverage analysis for detector placement.