Confined Space Training Certificate Gas Testing & Monitoring 2

Question 1

What effect does an oxygen-deficient atmosphere have on the accuracy of a catalytic bead LEL sensor?

Accepted Answer

It causes the sensor to under-read flammable gas concentrations, potentially showing safe readings when explosive levels are present

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

Volatile organic compounds (VOCs) at very low concentrations, to detect solvent or hydrocarbon vapours not detected by LEL sensors

Answer

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?

Accepted Answer

20 ppm as an 8-hour time-weighted average (TWA)

Answer

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?

Accepted Answer

To remove any residual gas from the previous test so the sample drawn represents current conditions in the space

Answer

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.

Confined Space Practice Test

Confined Space Practice Test

Confined Space Training Certificate Gas Testing & Monitoring 2