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HK Confined Space Certified Worker study materials
HK Confined Space Certified Worker study resources

HK Confined Space Certified Worker: Atmospheric testing and monitoring (oxygen, flammable gas, toxic gas)

HK Confined Space Certified Worker · Work through your exam, one topic at a time.

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Atmospheric testing and monitoring (oxygen, flammable gas, toxic gas)

This domain tests the technique of atmospheric testing, not just the numbers. Expect questions on test order, probe placement, frequency, acceptance criteria and instrument integrity.

Instruments and sequence

Use a calibrated multi-gas meter, bump-test it before use, and check battery, sensors, alarm and calibration date. Test in the order oxygen, flammable gas, toxic gas: the catalytic LEL sensor needs oxygen to work, and oxygen enrichment changes flammability. Test from outside with an extension probe before entry; sample top, middle and bottom because gases stratify, and test at the actual work location. Allow readings to stabilise and record them.

Acceptance criteria and re-testing

The usual entry criteria are oxygen 19.5 to 23.5 percent, flammable gas below 10 percent of the lower explosive limit, and toxic gases below the applicable occupational exposure limits. Hot work needs stricter flammable limits. If a reading fails, ventilate or purge and retest; never enter to investigate a reading. Monitor continuously while the space is occupied, and re-test after a break, shift change, weather change, ventilation stoppage or change in work.

Worked example

A meter shows oxygen 20.9 percent, LEL 12 percent and hydrogen sulphide 8 ppm. Oxygen is acceptable and hydrogen sulphide is below 10 ppm, but LEL exceeds 10 percent, so entry is refused. The team increases ventilation, looks for a source such as disturbed sludge, and retests after stabilisation. If LEL remains above 10 percent, the space is not entered; purging and source control are required. The reasoning is that one failed criterion is enough to stop entry, and the other readings do not cancel it.

Traps that cost marks

  • Testing only at the entrance or only at the top.
  • Testing before isolation and ventilation, then treating the result as final.
  • Using a meter with expired calibration or without a bump test.
  • Confusing 10 percent LEL with 10 ppm or with 0 percent LEL.
  • Failing to record instrument ID, tester, time and location.