Confined Space Training Certificate Gas Testing & Monitoring 3

Question 1

What is the Workplace Exposure Limit (WEL) for hydrogen sulphide (H2S) in UK workplaces?

Accepted Answer

1 ppm (8-hour TWA) and 5 ppm (15-minute STEL)

Answer

The UK WEL for H2S is 1 ppm as an 8-hour TWA and 5 ppm as a 15-minute STEL (EH40). These are very low values reflecting the extreme toxicity of H2S. At 100 ppm, rapid unconsciousness occurs; at higher levels, single-breath death is possible.

Question 2

A gas detector alarm setpoint for LEL is set at 20%. What does this mean in practice?

Accepted Answer

The alarm activates when flammable gas concentration reaches 20% of the Lower Explosive Limit, well before the explosive range

Answer

An alarm at 20% LEL means the instrument alarms when the gas concentration reaches 20% of the minimum concentration needed for explosion. This provides a substantial safety margin before the actual explosive range. Work should be stopped and the space evacuated at this alarm level.

Question 3

What type of atmospheric test should be performed after isolating an inlet to a confined space?

Accepted Answer

A comprehensive multi-gas test including oxygen, LEL, CO, and H2S to confirm isolation has prevented gas ingress

Answer

After isolation, a full atmospheric test should confirm that no hazardous gases are entering the space. The test should cover oxygen levels, flammable gas concentration, and toxic gas levels appropriate to the space. If hazardous readings are detected after isolation, the isolation may be incomplete.

Question 4

What is the risk of using a gas detector that was calibrated at a different altitude or temperature?

Accepted Answer

Readings may be inaccurate, as gas detector calibration is affected by atmospheric pressure and temperature

Answer

Gas detector readings can be affected by atmospheric pressure (altitude) and temperature. Instruments calibrated at sea level may give inaccurate readings at altitude. Temperature affects sensor response rates. Calibration should be performed under conditions representative of the intended use environment.

Question 5

What precaution should be taken when using a gas detector near a source of radio frequency (RF) interference?

Accepted Answer

Check the detector's specification for RF immunity and use an RF-immune detector; readings may be unreliable near RF sources

Answer

Some older or less-robust gas detectors can be affected by strong electromagnetic/RF fields (from radio transmitters, welding equipment), producing false readings or false alarms. Detectors should be selected and tested for RF immunity to IEC standards. This is particularly relevant in confined spaces with active communications equipment.

Confined Space Practice Test

Confined Space Practice Test

Confined Space Training Certificate Gas Testing & Monitoring 3