CMC Atmospheric Hazard Testing 3 — Questions and Answers
Question 1: Under NFPA 306, a 'Safe for Hot Work' certificate for a vessel tank is valid for a maximum of how long after issuance?
- 4 hours
- 8 hours
- 12 hours
- 24 hours (Correct answer)
Correct answer: 24 hours
NFPA 306 specifies that a marine chemist's certificate is valid for no more than 24 hours, after which re-testing and re-certification are required.
Question 2: When a photoionization detector (PID) is used to assess toxic vapor levels in a cargo tank, which factor most affects its accuracy?
- Ambient temperature variation
- The ionization potential (IP) of the target compound versus the lamp eV rating (Correct answer)
- Humidity above 50% RH
- Battery voltage fluctuation
Correct answer: The ionization potential (IP) of the target compound versus the lamp eV rating
A PID lamp can only ionize compounds whose ionization potential is below the lamp's photon energy (eV); compounds with IP above the lamp rating give zero or false readings.
Question 3: Carbon monoxide (CO) is particularly hazardous in marine spaces because it:
- Is heavier than air and accumulates in bilges
- Is colorless and odorless, providing no sensory warning (Correct answer)
- Reacts explosively with seawater
- Displaces oxygen at concentrations above 10%
Correct answer: Is colorless and odorless, providing no sensory warning
CO is colorless and odorless, making it impossible to detect without instruments, and it binds to hemoglobin with ~240 times the affinity of oxygen.
Question 4: Which gas is most likely to accumulate at the top of a poorly ventilated cargo hold rather than the bottom?
- Hydrogen sulfide (H₂S)
- Carbon dioxide (CO₂)
- Hydrogen (H₂) (Correct answer)
- Propane (C₃H₈)
Correct answer: Hydrogen (H₂)
Hydrogen has a molecular weight of ~2 g/mol, far lighter than air (~29 g/mol), so it rises and accumulates at the highest points of an enclosed space.
Question 5: A marine chemist must re-test an inerted cargo tank before it can be certified for hot work. The FIRST step upon re-opening the tank is to:
- Immediately insert the sampling probe and begin reading
- Allow the tank to ventilate naturally for 30 minutes first
- Test the atmosphere at the entry point before entering or inserting personnel (Correct answer)
- Purge the tank with compressed air from the bottom
Correct answer: Test the atmosphere at the entry point before entering or inserting personnel
The atmosphere at the access point must be confirmed non-hazardous before any person or unprotected sampling equipment is deployed into the space.
Question 6: The Threshold Limit Value–Ceiling (TLV-C) for a substance differs from TLV-TWA in that it:
- Is averaged over an 8-hour workday
- Must never be exceeded even instantaneously (Correct answer)
- Applies only during the first 15 minutes of exposure
- Is set 10 times higher than the TWA for short-duration work
Correct answer: Must never be exceeded even instantaneously
TLV-C is a concentration that must never be exceeded at any time during the workday, unlike the TWA which is averaged over 8 hours.
Question 7: When calibrating a combustible gas indicator (CGI) for use in a tank that previously held gasoline, the technician should calibrate using:
- Methane as the reference gas and apply a correction factor for gasoline (Correct answer)
- Pure hydrogen gas to simulate worst-case flammability
- Carbon monoxide at known concentration
- Instrument air to set a zero baseline only
Correct answer: Methane as the reference gas and apply a correction factor for gasoline
CGIs are typically calibrated with methane or pentane as reference gases, and published correction factors are applied when the target gas is a different hydrocarbon such as gasoline.
CMC Atmospheric Hazard Testing 3
Under NFPA 306, a 'Safe for Hot Work' certificate for a vessel tank is valid for a maximum of how long after issuance?