Detection, Monitoring, and Sampling
Monitoring turns an unknown scene into a managed one. Technicians must know the instruments and their limits: combustible gas indicators (read in %LEL, require oxygen and calibration corrections), oxygen meters (normal 20.9%; oxygen-deficient and oxygen-enriched hazards), colorimetric tubes (specific, time-weighted, cross-sensitive), photoionization detectors (broad response, lamp-dependent, poor for some compounds), flame ionization detectors, pH paper, radiation survey meters, and multi-gas meters. Understand calibration versus bump testing, response time, cross-sensitivity, and the difference between direct-reading instruments and laboratory-confirmed sampling. Sampling strategy includes where to sample (low areas for heavy vapors, high areas for light gases, inside confined spaces at multiple levels), how to document, and chain of custody for evidentiary samples.
Traps: treating a PID reading as a specific chemical identification; using a combustible gas indicator in an oxygen-deficient atmosphere without correction; ignoring that some instruments fail in high humidity or with certain chemicals; and forgetting to monitor continuously during the entry, not just at the start.
Rule of thumb: monitor before entry, continuously during entry, and at multiple heights - and never enter a confined space on instrument readings alone without the full permit and backup.
Worked example: A confined space reads 19.5% oxygen and 10% LEL. Oxygen is at the deficiency threshold, so entry is denied regardless of the LEL reading; ventilate and reassess.