Monitoring and Detection
Monitoring and Detection
Monitoring supports decisions: where to isolate, what PPE to wear, when entry is safe, and when decon is complete. The exam tests instrument selection, limitations, and interpretation.
Core rules
- Define the monitoring objective first: identification, quantification, leak detection, perimeter, or decon verification.
- Calibrate and bump-test instruments before use. Record results.
- A photoionization detector (PID) responds to many volatile organic compounds but does not identify them; response factors and lamp energy affect readings.
- Electrochemical sensors target specific gases, such as carbon monoxide, hydrogen sulfide, chlorine, or oxygen. Cross-sensitivity can mislead.
- Colorimetric tubes can help identify or quantify a known suspect but are not broad-spectrum identifiers.
- Oxygen must be measured in confined spaces. Normal oxygen does not rule out toxic or flammable gas.
- Use action levels and multiple instruments; do not rely on one reading.
Worked example
An unknown solvent is leaking in a utility vault. A four-gas meter shows 20.9% oxygen, 0% LEL, and no H2S or CO. A PID reads several hundred ppm. The reasoning: the atmosphere is not oxygen-deficient and not currently explosive by LEL, but the PID reading shows a volatile organic compound and does not identify it. Entry should not proceed on that alone. Use colorimetric tubes or a field GC/MS if available, monitor the perimeter, and select PPE for the unknown. The absence of an LEL reading does not prove the product is nonflammable.
Traps
- Treating a PID reading as identification.
- Using an LEL meter to evaluate toxic exposure.
- Skipping bump tests or calibration.
- Monitoring only at the entry point and not the perimeter.
- Entering before establishing action levels.