Confined Space Training Certificate Gas Testing & Monitoring 4 — Questions and Answers
Question 1: What does 'gas testing' before confined space entry legally require under UK regulations?
- Testing must be performed by a competent person using a suitable, calibrated instrument appropriate to the identified hazards (Correct answer)
- Any person may test using any commercially available detector
- Testing is only required if the risk assessment identifies a toxic atmosphere hazard
- Testing must be witnessed by an HSE inspector for legal validity
Correct answer: Testing must be performed by a competent person using a suitable, calibrated instrument appropriate to the identified hazards
UK regulations require that atmospheric testing is carried out by a competent person using a suitable, appropriately calibrated instrument for the identified hazards. The competent person must understand how to interpret the results and take appropriate action. The results should be recorded on the permit to work.
Question 2: Why might a gas detector fail to detect hydrogen sulphide in high concentrations?
- Some electrochemical H2S sensors become saturated at high concentrations and read zero (sensor 'poisoning') (Correct answer)
- H2S is not detectable by electrochemical sensors
- Modern detectors automatically increase sensitivity at high concentrations
- High concentrations trigger the oxygen sensor, masking the H2S reading
Correct answer: Some electrochemical H2S sensors become saturated at high concentrations and read zero (sensor 'poisoning')
Electrochemical sensors for H2S can become saturated or 'poisoned' when exposed to very high concentrations, causing the reading to drop back toward zero even as concentration continues to rise. This is dangerous as it can give a false indication of safe conditions. This is why bump testing and regular calibration are essential.
Question 3: What should be done with gas detector readings before they are used to issue a confined space permit?
- Readings must be recorded on the permit to work, dated, and signed by the competent person who performed the test (Correct answer)
- Readings need only be verbally confirmed to the permit issuer
- Readings should be averaged over a 5-minute period before recording
- Readings should be reported to the HSE electronically
Correct answer: Readings must be recorded on the permit to work, dated, and signed by the competent person who performed the test
Atmospheric test results form part of the permit to work documentation. They must be recorded (space location, date, time, gases tested, readings, instrument ID and calibration date), and signed by the competent person. This creates a legal record and ensures the permit issuer has accurate information.
Question 4: What is the significance of the 'response time' of a gas detector?
- The time taken for the sensor to display an accurate reading after exposure; entrants must not move into the space faster than the detector can respond (Correct answer)
- The time allowed between alarm and evacuation
- The time the detector can operate before battery replacement
- The speed at which the audible alarm activates
Correct answer: The time taken for the sensor to display an accurate reading after exposure; entrants must not move into the space faster than the detector can respond
Gas detector response time (T90 — time to reach 90% of the actual reading) can range from seconds to over a minute depending on sensor type and sample tube length. If the detector is moved through a space faster than its response time, it may pass through a hazardous pocket without registering a full alarm.
Question 5: What does 'time-weighted average' (TWA) mean in the context of workplace exposure limits?
- The average concentration over an 8-hour working day; accounts for periods of high and low exposure (Correct answer)
- The highest reading recorded during a shift
- The average of the minimum and maximum readings
- The concentration at which symptoms first appear
Correct answer: The average concentration over an 8-hour working day; accounts for periods of high and low exposure
TWA is the average airborne concentration of a substance over an 8-hour working day. It accounts for variations in exposure level during the shift. In confined spaces, short-duration high exposures can exceed the STEL even if the 8-hour TWA remains below the WEL, so both limits must be monitored.
Question 6: What type of gas detector is most appropriate for a confined space with an unknown atmosphere?
- A calibrated multi-gas detector measuring oxygen, LEL, carbon monoxide, and hydrogen sulphide as a minimum (Correct answer)
- A single-gas CO detector as CO is the most common confined space hazard
- A natural gas detector only for spaces in buildings with gas supply
- A photoionisation detector only for spaces with known VOC hazards
Correct answer: A calibrated multi-gas detector measuring oxygen, LEL, carbon monoxide, and hydrogen sulphide as a minimum
When the atmosphere is unknown, a multi-gas detector covering oxygen, flammable gases (%LEL), and the most common toxic gases (CO and H2S) provides the broadest baseline assessment. Additional sensors can be added based on the specific history and use of the space. A single-gas instrument is inadequate for unknown conditions.
Confined Space Training Certificate Gas Testing & Monitoring 4
What does 'gas testing' before confined space entry legally require under UK regulations?