CMC CMC Toxic Gas Detection & Monitoring 1 — Questions and Answers
Question 1: What is the IDLH (Immediately Dangerous to Life or Health) concentration for hydrogen sulfide (H2S)?
- 50 ppm
- 100 ppm (Correct answer)
- 200 ppm
- 500 ppm
Correct answer: 100 ppm
NIOSH established the IDLH for hydrogen sulfide at 100 ppm, above which escape without respiratory protection becomes impossible.
Question 2: Which instrument is most commonly used to detect oxygen-deficient atmospheres in marine confined spaces?
- Colorimetric tube detector
- Catalytic bead combustible gas indicator
- Electrochemical oxygen sensor (Correct answer)
- Photoionization detector
Correct answer: Electrochemical oxygen sensor
Electrochemical oxygen sensors are the standard instrument for detecting oxygen-deficient atmospheres, providing continuous real-time O2 readings.
Question 3: What minimum oxygen concentration must a marine confined space atmosphere maintain for safe entry without SCBA?
- 16.0%
- 18.0%
- 19.5% (Correct answer)
- 21.0%
Correct answer: 19.5%
OSHA and NFPA 306 require a minimum of 19.5% oxygen by volume for an atmosphere to be considered safe for entry without supplied-air respirators.
Question 4: A photoionization detector (PID) is best suited for detecting which class of compounds in marine spaces?
- Carbon monoxide
- Methane
- Volatile organic compounds (VOCs) (Correct answer)
- Hydrogen sulfide
Correct answer: Volatile organic compounds (VOCs)
PIDs use UV light to ionize molecules and are highly sensitive to volatile organic compounds such as benzene, toluene, and styrene.
Question 5: What does a Lower Explosive Limit (LEL) reading of 10% on a combustible gas indicator actually represent?
- 10% of the atmosphere is flammable gas
- The gas concentration is 10% of the LEL value (Correct answer)
- The space is 10% as dangerous as the UEL
- 10% of all molecules are combustible
Correct answer: The gas concentration is 10% of the LEL value
A 10% LEL reading means the combustible gas concentration is 10% of the lower explosive limit — not 10% of the total atmosphere.
Question 6: Which condition requires immediate evacuation of a marine confined space during monitoring?
- Oxygen at 20.5%
- Combustible gas at 5% LEL (Correct answer)
- Carbon monoxide at 35 ppm
- H2S at 1 ppm
Correct answer: Combustible gas at 5% LEL
NFPA 306 requires evacuation when combustible gas readings reach or exceed 10% LEL; a reading of 5% LEL warrants heightened caution but 10% triggers mandatory evacuation.
CMC CMC Toxic Gas Detection & Monitoring 1
What is the IDLH (Immediately Dangerous to Life or Health) concentration for hydrogen sulfide (H2S)?