CMC CMC Toxic Gas Detection & Monitoring 2 — Questions and Answers
Question 1: What is the TWA (Time-Weighted Average) PEL for carbon monoxide established by OSHA?
- 25 ppm
- 35 ppm
- 50 ppm (Correct answer)
- 100 ppm
Correct answer: 50 ppm
OSHA's 8-hour TWA PEL for carbon monoxide is 50 ppm; exposure above this level requires respiratory protection.
Question 2: Which marine cargo residue is most likely to produce benzene vapor requiring PID monitoring?
- Crude oil and petroleum distillates (Correct answer)
- Grain dust
- Fertilizer (urea)
- Coal
Correct answer: Crude oil and petroleum distillates
Crude oil and petroleum distillate residues are primary sources of benzene vapor, a known carcinogen requiring PID or specific benzene tube monitoring.
Question 3: When calibrating a multi-gas detector for marine use, what gas mixture is typically used for LEL calibration?
- Pure propane
- 50% LEL methane in air (Correct answer)
- Pure hydrogen
- 10% LEL pentane in nitrogen
Correct answer: 50% LEL methane in air
Standard practice uses a certified calibration gas of 50% LEL methane in air (approximately 2.5% methane) to set the LEL sensor response.
Question 4: A Marine Chemist finds H2S readings of 3 ppm in a cargo tank. What should the certificate state?
- Safe for workers — no restrictions needed
- Safe for workers with H2S monitoring in place (Correct answer)
- Not safe — H2S must be zero before entry
- Restricted — SCBA required at all times
Correct answer: Safe for workers with H2S monitoring in place
Low-level H2S (below 1 ppm ACGIH TLV) may permit entry with monitoring requirements noted on the Marine Chemist certificate.
Question 5: What type of detector is required when evaluating a tank that previously held leaded gasoline?
- Standard catalytic bead CGI only
- Electrochemical H2S sensor
- PID with an appropriate lamp plus lead-specific monitoring (Correct answer)
- Infrared gas analyzer
Correct answer: PID with an appropriate lamp plus lead-specific monitoring
Leaded gasoline residue presents both LEL and benzene/VOC hazards, requiring a PID in addition to standard combustible gas testing for complete hazard assessment.
Question 6: Why must catalytic bead sensors NOT be used in oxygen-deficient atmospheres?
- They give falsely high LEL readings
- Combustion on the bead requires sufficient oxygen to function accurately (Correct answer)
- They are damaged by nitrogen
- They read in %vol, not % LEL
Correct answer: Combustion on the bead requires sufficient oxygen to function accurately
Catalytic bead sensors require ambient oxygen to oxidize the gas on the bead; oxygen deficiency suppresses the reaction, causing falsely low or zero LEL readings.
CMC CMC Toxic Gas Detection & Monitoring 2
What is the TWA (Time-Weighted Average) PEL for carbon monoxide established by OSHA?