Confined Space Training Certificate Gas Testing & Monitoring 3 — Questions and Answers
Question 1: What is the Workplace Exposure Limit (WEL) for hydrogen sulphide (H2S) in UK workplaces?
- 1 ppm (8-hour TWA) and 5 ppm (15-minute STEL) (Correct answer)
- 10 ppm (8-hour TWA) and 15 ppm (STEL)
- 0.1 ppm (8-hour TWA) and 0.5 ppm (STEL)
- 5 ppm (8-hour TWA) and 10 ppm (STEL)
Correct answer: 1 ppm (8-hour TWA) and 5 ppm (15-minute STEL)
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?
- The alarm activates when flammable gas concentration reaches 20% of the Lower Explosive Limit, well before the explosive range (Correct answer)
- The alarm activates when 20% of the gas in the atmosphere is flammable
- The space is 20% of the way to being immediately dangerous to life
- The alarm activates when oxygen drops to 20%
Correct answer: The alarm activates when flammable gas concentration reaches 20% of the Lower Explosive Limit, well before the explosive range
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?
- A comprehensive multi-gas test including oxygen, LEL, CO, and H2S to confirm isolation has prevented gas ingress (Correct answer)
- An oxygen test only to confirm ventilation is adequate
- A visual inspection only if the space was gas-free before isolation
- No test is needed if isolation was performed correctly
Correct answer: A comprehensive multi-gas test including oxygen, LEL, CO, and H2S to confirm isolation has prevented gas ingress
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?
- Readings may be inaccurate, as gas detector calibration is affected by atmospheric pressure and temperature (Correct answer)
- No risk, as modern detectors automatically compensate for all conditions
- The detector will only under-read, so it is conservative and safe
- Only temperature above 40°C affects detector accuracy
Correct answer: Readings may be inaccurate, as gas detector calibration is affected by atmospheric pressure and temperature
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?
- Check the detector's specification for RF immunity and use an RF-immune detector; readings may be unreliable near RF sources (Correct answer)
- RF interference only affects audio alarms, not sensor readings
- Modern detectors are unaffected by RF
- Move the detector at least 1 metre from the RF source
Correct answer: Check the detector's specification for RF immunity and use an RF-immune detector; readings may be unreliable near RF sources
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.
Question 6: How long should natural ventilation be allowed before atmospheric testing in a confined space?
- Sufficient time based on the space's volume and ventilation rate; the atmosphere must then be tested and confirmed safe (Correct answer)
- Exactly 30 minutes for all spaces
- Natural ventilation is never sufficient; mechanical ventilation must always be used
- 10 minutes is always adequate for spaces under 50 cubic metres
Correct answer: Sufficient time based on the space's volume and ventilation rate; the atmosphere must then be tested and confirmed safe
There is no fixed universal time. The ventilation period depends on the space's volume, the ventilation cross-section available, and the hazards present. After ventilation, the atmosphere must be tested to confirm it is safe — timing alone cannot guarantee safety. For many confined spaces, natural ventilation is insufficient and mechanical ventilation is required.
Confined Space Training Certificate Gas Testing & Monitoring 3
What is the Workplace Exposure Limit (WEL) for hydrogen sulphide (H2S) in UK workplaces?