Confined Space Training Certificate Gas Testing & Monitoring 1 — Questions and Answers
Question 1: In what order should a multi-gas detector test for atmospheric hazards in a confined space?
- Oxygen, flammable gas (LEL), carbon monoxide, hydrogen sulphide (Correct answer)
- Carbon monoxide first as it is most toxic, then oxygen
- Hydrogen sulphide first as it is the most lethal gas
- Order does not matter as all readings are simultaneous
Correct answer: Oxygen, flammable gas (LEL), carbon monoxide, hydrogen sulphide
Best practice is to test for oxygen first (deficiency or enrichment affects flammability readings), then flammable gases (%LEL), then toxic gases such as CO and H2S. Many instruments test simultaneously, but interpreting results correctly requires understanding oxygen context first.
Question 2: What is the standard alarm setpoint for oxygen deficiency on a confined space multi-gas detector?
- 19.5% O2 (Correct answer)
- 18% O2
- 20% O2
- 17% O2
Correct answer: 19.5% O2
The standard low-oxygen alarm setpoint is 19.5% oxygen — slightly below normal atmospheric concentration of 20.9%. At 19.5% the alarm alerts workers before oxygen levels reach acutely dangerous levels. Some instruments have two alarm levels: low alarm at 19.5% and high alarm at a higher LEL percentage.
Question 3: How should gas testing be carried out before a confined space entry into a deep chamber?
- By lowering the detector on a rope to test at multiple levels — top, middle, and bottom (Correct answer)
- By holding the detector at arm's length above the entry point
- A single test at the entry point is sufficient for any depth
- By inserting a sample tube and testing only at the breathing zone level
Correct answer: By lowering the detector on a rope to test at multiple levels — top, middle, and bottom
Gas concentrations vary by depth: heavy gases (CO2, H2S) accumulate at the bottom; light gases (methane) accumulate at the top; oxygen depletion may be worse in pockets. The detector must sample at multiple levels before anyone enters to map the full atmospheric profile of the space.
Question 4: What is the correct action if a gas detector alarms during a confined space entry?
- All entrants must immediately evacuate via the shortest safe route without delay (Correct answer)
- Continue working while investigating the cause of the alarm
- Contact the permit issuer by phone before taking any action
- Only evacuate if a second alarm sounds within two minutes
Correct answer: All entrants must immediately evacuate via the shortest safe route without delay
If a gas alarm sounds during entry, the immediate response is always evacuation. Workers must not stop to investigate, collect tools, or complete tasks. They must exit by the shortest safe route. The attendant initiates the emergency procedure. Re-entry is only permitted after the cause is identified and resolved.
Question 5: What is the difference between a catalytic bead (pellistor) sensor and an electrochemical sensor in a gas detector?
- Pellistors detect flammable gas concentration (%LEL); electrochemical sensors detect toxic gases and oxygen (Correct answer)
- Pellistors are more accurate than electrochemical sensors in all conditions
- Electrochemical sensors only work in warm temperatures
- Pellistors detect oxygen; electrochemical sensors detect flammable gases
Correct answer: Pellistors detect flammable gas concentration (%LEL); electrochemical sensors detect toxic gases and oxygen
Pellistor (catalytic bead) sensors measure flammable gas concentration as a percentage of the Lower Explosive Limit. Electrochemical sensors measure specific toxic gases (CO, H2S) and oxygen concentration in parts per million or percentage. Multi-gas instruments combine both sensor types.
Question 6: Why must gas detectors be zeroed in fresh air before use in a confined space?
- To establish a clean baseline so that any deviation from normal atmospheric composition is detected accurately (Correct answer)
- To warm up the sensors — zeroing has no calibration function
- To comply with permit to work paperwork requirements
- To check battery level before entering the space
Correct answer: To establish a clean baseline so that any deviation from normal atmospheric composition is detected accurately
Zeroing in fresh air calibrates the detector to normal atmospheric concentrations (20.9% O2, 0 ppm CO, etc.) as the baseline. If zeroed in contaminated air, the detector treats that contaminated level as normal and will not alarm correctly. Fresh-air zeroing ensures all readings are relative to clean air.
Confined Space Training Certificate Gas Testing & Monitoring 1
In what order should a multi-gas detector test for atmospheric hazards in a confined space?