Confined Space Training Certificate Gas Testing & Monitoring 2 — Questions and Answers
Question 1: What effect does an oxygen-deficient atmosphere have on the accuracy of a catalytic bead LEL sensor?
- It causes the sensor to under-read flammable gas concentrations, potentially showing safe readings when explosive levels are present (Correct answer)
- It causes the sensor to over-read, triggering false alarms
- It has no effect on LEL readings
- It only affects readings above 50% LEL
Correct answer: It causes the sensor to under-read flammable gas concentrations, potentially showing safe readings when explosive levels are present
Catalytic bead sensors require oxygen to oxidise the gas and produce a reading. In oxygen-deficient atmospheres, the sensor cannot fully react and under-reads the true flammable gas concentration. The atmosphere may actually be at explosive concentrations while the detector shows a safe reading.
Question 2: What is the bump test for a gas detector and when should it be performed?
- A functional test exposing the detector to a known concentration of test gas to verify sensors and alarms respond correctly; performed before each use (Correct answer)
- A physical drop test to check instrument robustness
- A calibration procedure performed annually by the manufacturer
- A test of the detector's waterproofing
Correct answer: A functional test exposing the detector to a known concentration of test gas to verify sensors and alarms respond correctly; performed before each use
A bump test (or functional test) exposes the instrument to a known concentration of test gas and verifies that: the sensor responds, the reading is within acceptable tolerance, and the alarm activates. It should be performed before each use day to confirm the instrument is functioning correctly.
Question 3: What does a photoionisation detector (PID) measure, and why might one be used in a confined space?
- Volatile organic compounds (VOCs) at very low concentrations, to detect solvent or hydrocarbon vapours not detected by LEL sensors (Correct answer)
- High concentrations of flammable gas above the LEL
- Oxygen concentration only
- Carbon monoxide at concentrations below 1 ppm
Correct answer: Volatile organic compounds (VOCs) at very low concentrations, to detect solvent or hydrocarbon vapours not detected by LEL sensors
A PID uses ultraviolet light to ionise gas molecules, measuring volatile organic compounds at very low concentrations (sub-ppm). It is used to detect hydrocarbon vapours, solvents, and other VOCs that may not trigger a standard LEL sensor at the concentrations of concern for health and toxicity.
Question 4: What is the Workplace Exposure Limit (WEL) for carbon monoxide in the UK?
- 20 ppm as an 8-hour time-weighted average (TWA) (Correct answer)
- 100 ppm as an 8-hour TWA
- 5 ppm as an 8-hour TWA
- 50 ppm as a short-term exposure limit only
Correct answer: 20 ppm as an 8-hour time-weighted average (TWA)
The UK Workplace Exposure Limit for carbon monoxide is 20 ppm as an 8-hour time-weighted average (TWA) and 100 ppm as a 15-minute short-term exposure limit (STEL). These are set in EH40 (Workplace Exposure Limits) published by the HSE.
Question 5: Why should gas detector sample lines be purged before taking a reading in a confined space?
- To remove any residual gas from the previous test so the sample drawn represents current conditions in the space (Correct answer)
- To clean the sample line of moisture before calibration
- Purging is not required for accurate readings
- To warm up the sensor at the end of the sample line
Correct answer: To remove any residual gas from the previous test so the sample drawn represents current conditions in the space
Sample lines (extension tubes used to lower detectors into a space) may contain residual gas from previous tests or ambient air. Purging the line by drawing air through it for 30–60 seconds before reading ensures the sample drawn represents current conditions at the measurement point.
Question 6: What personal gas detector alarm response is required for an entrant working alone in a confined space?
- Immediate self-rescue and evacuation before incapacitation occurs (Correct answer)
- Wait for the attendant to instruct evacuation via radio
- Silence the alarm and assess whether conditions have changed
- Check the reading against the permit to work's acceptable range
Correct answer: Immediate self-rescue and evacuation before incapacitation occurs
An entrant in a confined space must act immediately on any gas alarm, initiating self-rescue before the gas can impair their ability to escape. The entrant should not investigate, silence the alarm, or wait for instructions — every second counts in a rapidly deteriorating atmosphere.
Confined Space Training Certificate Gas Testing & Monitoring 2
What effect does an oxygen-deficient atmosphere have on the accuracy of a catalytic bead LEL sensor?