Gas Detection and Monitoring: Instruments, Calibration, Sampling and Action Levels
This domain tests whether you can select, verify and use a gas detector correctly, and whether you understand where and when to sample.
Instrument selection
- Sensors matched to the hazards identified: oxygen, LEL, hydrogen sulphide, carbon monoxide, plus specific toxics as required.
- Intrinsically safe certification for the hazardous zone, robust construction, and audible, visual and vibrating alarms.
- Pumped instruments with a sampling probe and tubing for remote testing; datalogging for records.
- Catalytic LEL sensors need oxygen to work and are poisoned by silicones and lead. In inert or oxygen-deficient atmospheres, use a thermal conductivity or infrared sensor instead.
Calibration and bump testing
- Calibration adjusts the response against a certified span gas, at the manufacturer's interval - typically every one to six months - and is recorded.
- Bump test briefly exposes the sensors to span gas to confirm they respond and the alarms trigger; do it before each day's use or before each entry.
- Zero the instrument in clean fresh air and check battery, filter, tubing and alarm setpoints.
Sampling strategy
- Test from outside the space before entry.
- Sample at the top, middle and bottom because gas density varies; allow the tubing to purge - at least three times its volume, or as the manufacturer states.
- Test the point of work, not just the entrance; sample pockets, corners, voids and sludge specifically.
- Monitor continuously during occupancy; re-test after breaks, after ventilation changes and before re-entry.
- Record results, instrument identification, calibration date, time and location.
Action levels
Commonly used values are oxygen 19.5% to 23.5%; flammable gas alarm at 10% LEL and evacuation at 20% LEL; toxics at the occupational exposure limits in the Labour Department's codes of practice, for example H2S 10 ppm and CO 25 ppm. If in doubt, stop and re-assess.
Worked example
A vessel has been purged with nitrogen. The detector reads 0% LEL and the entrant concludes it is safe. The oxygen reading is 8%. Reasoning: the catalytic LEL sensor cannot measure reliably without oxygen, and 8% oxygen is immediately dangerous in any case. Entry is prohibited without BA and full rescue arrangements; the space must be ventilated and re-tested before any normal entry.
Traps that cost marks
- Testing only at the opening.
- Using an instrument past its calibration due date.
- Not allowing the sampling line to purge.
- Ignoring oxygen when interpreting LEL.
- Treating 0% LEL as proof that there is no flammable gas.