Gas Testing & Monitoring

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Overview

Many lethal gases give no warning — gas testing finds the hazard first.

Test in order — oxygen, flammable, then toxic — by an authorised gas tester.

Test order & acceptable limits

Oxygen first — acceptable 19.5–23.5%.
Flammable next — below 10% LEL.
Toxic last — H₂S below 10 ppm; CO below 25–50 ppm.
Other toxics judged against occupational exposure limits.

Step-by-step procedure

Confirm the detector suits the expected hazards.
Bump-test before use and check calibration date.
Test in order — oxygen, flammable, then toxic.
Test top, middle and bottom — gases stratify.
The authorised gas tester records results on the permit.
Allow entry only when every reading is within limits.
Keep continuous monitoring with personal monitors during work.
Evacuate and re-test if any alarm sounds.

Detector calibration & bump testing

Bump test — confirm sensors and alarms respond before use.
Calibration — adjust against a certified gas at set intervals.
Continuous monitoring — personal monitors worn during work.

Roles & responsibilities

Role	Responsibility
Authorised Gas Tester	Calibrates, tests in order, records on the permit.
Permit Issuer	Confirms readings are within limits before authorising.
Entrant / Standby	Wears a personal monitor and evacuates on alarm.

Common mistakes to avoid

Using an uncalibrated or un-bump-tested detector.
Testing only at the entry point, not the bottom.
Testing once on entry and stopping continuous monitoring.

Legal requirements (Thailand)

Occupational Safety, Health and Environment Act B.E. 2554 (2011) — the governing workplace-safety law in Thailand.
Ministerial Regulation on OSH for work in confined spaces B.E. 2562 (2019).
Ministerial Regulation on OSH for fire prevention and protection B.E. 2555 (2012).

Frequently asked questions

What is Gas Testing & Monitoring?

You cannot see, smell or feel many of the gases that kill workers on site. A confined space can be short of oxygen, full of flammable vapour, or contaminated with a toxic gas such as hydrogen sulphide — often with no warning to the senses. Gas testing measures the atmosphere with a calibrated detector so that a hazard is found before anyone is exposed to it.

Who is responsible?

Authorised Gas Tester: Calibrates the detector, tests in order and records the results on the permit.; Permit Issuer: Confirms the readings are within limits before authorising the work.; Entrant / Standby: Wears a personal monitor, watches the alarm and evacuates if it sounds.

What are the most common mistakes to avoid?

Using a detector that has not been bump-tested or is out of calibration date. Testing only at the entry point and not at the bottom where heavy gases collect. Doing one test on entry and then stopping continuous monitoring during the work.

References — Detector calibration standards; confined space and hot work permits; Thai OSH Act B.E. 2554 and the confined-space regulation.