Confined Space Training Certificate Gas Testing & Monitoring 4

Question 1

What does 'gas testing' before confined space entry legally require under UK regulations?

Accepted Answer

Testing must be performed by a competent person using a suitable, calibrated instrument appropriate to the identified hazards

Answer

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?

Accepted Answer

Some electrochemical H2S sensors become saturated at high concentrations and read zero (sensor 'poisoning')

Answer

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?

Accepted Answer

Readings must be recorded on the permit to work, dated, and signed by the competent person who performed the test

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

The average concentration over an 8-hour working day; accounts for periods of high and low exposure

Answer

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.

Confined Space Practice Test

Confined Space Practice Test

Confined Space Training Certificate Gas Testing & Monitoring 4