ADCI SURFACE SUPPLIED MIXED GAS DIVER
CERTIFICATION EXAM 250 Practice MCQs for Exam
Preparation 2026
Summary of Topics Covered:
Physics of Diving — Qs 1–10
Mixed Gas Principles — Qs 11–20
Decompression Theory — Qs 21–30
Oxygen Toxicity — Qs 31–38
Decompression Sickness — Qs 39–45
Surface Supplied Equipment — Qs 46–55
Gas Analysis and Mixing — Qs 56–62
Diving Medicine and Physiology — Qs 63–70
Emergency Procedures — Qs 71–75
Saturation Diving — Qs 76–80
Regulations and Standards — Qs 81–86
Gas Planning and Calculations — Qs 87–92
Environmental Considerations — Qs 93–97
Mixed Gas Procedures — Qs 98–102
Equipment Maintenance — Qs 103–108
Operations Planning — Qs 109–112
Advanced Topics — Qs 113–118
Incident Analysis — Qs 119–122
Calculations Review — Qs 123–127
Comprehensive Final Review — Qs 128–250
SECTION 1: PHYSICS OF DIVING
1. At a depth of 99 fsw (feet of seawater), the absolute pressure is:
A) 2 ATA B) 3 ATA C) 4 ATA D) 5 ATA
C) 4 ATA (correct answer) Rationale: Absolute pressure in seawater = (depth ÷ 33) + 1
ATA. At 99 fsw: (99 ÷ 33) + 1 = 3 + 1 = 4 ATA. This calculation is foundational to all mixed
gas dive planning, as partial pressures of all gas components are directly proportional to
absolute pressure.
,2. Boyle's Law states that at constant temperature, the pressure and volume of a gas are:
A) Directly proportional B) Inversely proportional C) Independent of each other D)
Exponentially related
B) Inversely proportional (correct answer) Rationale: Boyle's Law (P₁V₁ = P₂V₂) states
that as pressure increases, volume decreases proportionally at constant temperature. This
explains why a diver's lungs would compress on descent and is critical to understanding
gas consumption rates, buoyancy changes, and equipment behavior at depth.
3. Dalton's Law of Partial Pressures states that:
A) Gas volume decreases as temperature increases B) The total pressure of a gas mixture equals
the sum of the partial pressures of each component gas C) Gas solubility increases as pressure
decreases D) All gases behave identically under pressure
B) The total pressure of a gas mixture equals the sum of the partial pressures of each
component gas (correct answer) Rationale: Dalton's Law (Ptotal = PP₁ + PP₂ + PP₃...) is
the foundational law for mixed gas diving. It allows divers to calculate the partial pressure
of oxygen, nitrogen, and helium at any depth, which is essential for determining oxygen
toxicity risk, narcosis potential, and decompression obligations.
4. Henry's Law relates to diving primarily because it explains:
A) Why gas volume decreases at depth B) Why gases dissolve into body tissues in proportion to
their partial pressure C) Why gas temperature changes with depth D) Why buoyancy changes
with depth
B) Why gases dissolve into body tissues in proportion to their partial pressure (correct
answer) Rationale: Henry's Law states that the amount of gas dissolved in a liquid is
proportional to the partial pressure of that gas above the liquid. In diving, this explains
how nitrogen and helium dissolve into body tissues at depth and must be eliminated during
ascent — the basis of decompression theory.
5. The partial pressure of oxygen in a 32% oxygen Nitrox mixture at 4 ATA is:
A) 0.32 bar B) 0.64 bar C) 1.28 bar/ATA D) 1.28 ATA
C) 1.28 bar/ATA (correct answer) Rationale: Partial pressure of oxygen = FO₂ × Absolute
pressure = 0.32 × 4 ATA = 1.28 ATA (or bar). This value approaches the maximum
,recommended PPO₂ of 1.4–1.6 ATA for working dives, demonstrating why oxygen partial
pressure monitoring is critical when using enriched air mixtures at depth.
6. Charles's Law states that at constant pressure, gas volume is:
A) Inversely proportional to temperature B) Independent of temperature C) Directly proportional
to absolute temperature D) Equal to pressure times temperature
C) Directly proportional to absolute temperature (correct answer) Rationale: Charles's
Law (V₁/T₁ = V₂/T₂) states that gas volume increases with increasing absolute temperature
(Kelvin). This is important when filling diving cylinders — gas compressed in warm
conditions will have lower pressure when cooled to ambient temperature, affecting gas
planning for cold water dives.
7. At what depth in seawater does the absolute pressure equal 5 ATA?
A) 99 fsw B) 132 fsw C) 165 fsw D) 198 fsw
B) 132 fsw (correct answer) Rationale: Absolute pressure = (depth ÷ 33) + 1 ATA. Solving
for depth: 5 ATA = (depth ÷ 33) + 1, so depth = (5-1) × 33 = 132 fsw. This calculation is
essential for mixed gas divers planning dives below 130 fsw where the physics of high
pressure gas mixtures become critical.
8. The density of a gas mixture at depth affects a diver primarily by:
A) Changing the color of the gas B) Increasing the work of breathing, potentially causing CO₂
retention C) Reducing the narcotic effect D) Decreasing oxygen toxicity risk
B) Increasing the work of breathing, potentially causing CO₂ retention (correct answer)
Rationale: As depth increases, gas density increases proportionally with pressure. Denser
gas is harder to breathe through regulators and the respiratory system, increasing the
work of breathing. This can lead to CO₂ retention, which potentiates oxygen toxicity and
narcosis. Helium is added to deep gas mixtures specifically because its low density reduces
this problem.
9. Graham's Law states that the rate of diffusion of a gas is:
, A) Proportional to its molecular weight B) Inversely proportional to the square root of its
molecular weight C) Directly proportional to its partial pressure D) Independent of molecular
weight
B) Inversely proportional to the square root of its molecular weight (correct answer)
Rationale: Graham's Law explains why helium diffuses and off-gasses more rapidly than
nitrogen — helium (molecular weight 4) diffuses approximately 2.65 times faster than
nitrogen (molecular weight 28). This faster diffusion is both an advantage (faster
decompression) and a concern (isobaric counterdiffusion risk in mixed gas diving).
10. The equivalent air depth (EAD) formula is used to:
A) Calculate oxygen toxicity limits B) Determine the nitrogen partial pressure equivalent of a
Nitrox mix for decompression planning C) Calculate maximum operating depth for Heliox D)
Determine CNS clock percentage
B) Determine the nitrogen partial pressure equivalent of a Nitrox mix for decompression
planning (correct answer) Rationale: EAD = [(1 - FO₂) × (depth + 33) ÷ 0.79] - 33. It
converts a Nitrox dive to an equivalent air depth for use with standard air decompression
tables. In mixed gas diving, analogous calculations (END — equivalent narcotic depth) are
used to assess the narcotic effect of Trimix and Heliox mixtures.
SECTION 2: MIXED GAS DIVING PRINCIPLES
11. The primary reason helium is added to a breathing mixture for deep diving is to:
A) Increase the oxygen content B) Reduce nitrogen narcosis and decrease gas density for easier
breathing C) Prevent oxygen toxicity D) Reduce decompression obligation
B) Reduce nitrogen narcosis and decrease gas density for easier breathing (correct answer)
Rationale: Helium is non-narcotic (or minimally so) and has a molecular weight of 4
compared to nitrogen's 28. This dramatically reduces both narcosis at depth and gas
density, improving breathing efficiency. These properties make helium essential for dives
beyond 150 fsw where nitrogen narcosis and breathing work become critical safety
concerns.
12. Trimix is a breathing gas mixture that contains:
CERTIFICATION EXAM 250 Practice MCQs for Exam
Preparation 2026
Summary of Topics Covered:
Physics of Diving — Qs 1–10
Mixed Gas Principles — Qs 11–20
Decompression Theory — Qs 21–30
Oxygen Toxicity — Qs 31–38
Decompression Sickness — Qs 39–45
Surface Supplied Equipment — Qs 46–55
Gas Analysis and Mixing — Qs 56–62
Diving Medicine and Physiology — Qs 63–70
Emergency Procedures — Qs 71–75
Saturation Diving — Qs 76–80
Regulations and Standards — Qs 81–86
Gas Planning and Calculations — Qs 87–92
Environmental Considerations — Qs 93–97
Mixed Gas Procedures — Qs 98–102
Equipment Maintenance — Qs 103–108
Operations Planning — Qs 109–112
Advanced Topics — Qs 113–118
Incident Analysis — Qs 119–122
Calculations Review — Qs 123–127
Comprehensive Final Review — Qs 128–250
SECTION 1: PHYSICS OF DIVING
1. At a depth of 99 fsw (feet of seawater), the absolute pressure is:
A) 2 ATA B) 3 ATA C) 4 ATA D) 5 ATA
C) 4 ATA (correct answer) Rationale: Absolute pressure in seawater = (depth ÷ 33) + 1
ATA. At 99 fsw: (99 ÷ 33) + 1 = 3 + 1 = 4 ATA. This calculation is foundational to all mixed
gas dive planning, as partial pressures of all gas components are directly proportional to
absolute pressure.
,2. Boyle's Law states that at constant temperature, the pressure and volume of a gas are:
A) Directly proportional B) Inversely proportional C) Independent of each other D)
Exponentially related
B) Inversely proportional (correct answer) Rationale: Boyle's Law (P₁V₁ = P₂V₂) states
that as pressure increases, volume decreases proportionally at constant temperature. This
explains why a diver's lungs would compress on descent and is critical to understanding
gas consumption rates, buoyancy changes, and equipment behavior at depth.
3. Dalton's Law of Partial Pressures states that:
A) Gas volume decreases as temperature increases B) The total pressure of a gas mixture equals
the sum of the partial pressures of each component gas C) Gas solubility increases as pressure
decreases D) All gases behave identically under pressure
B) The total pressure of a gas mixture equals the sum of the partial pressures of each
component gas (correct answer) Rationale: Dalton's Law (Ptotal = PP₁ + PP₂ + PP₃...) is
the foundational law for mixed gas diving. It allows divers to calculate the partial pressure
of oxygen, nitrogen, and helium at any depth, which is essential for determining oxygen
toxicity risk, narcosis potential, and decompression obligations.
4. Henry's Law relates to diving primarily because it explains:
A) Why gas volume decreases at depth B) Why gases dissolve into body tissues in proportion to
their partial pressure C) Why gas temperature changes with depth D) Why buoyancy changes
with depth
B) Why gases dissolve into body tissues in proportion to their partial pressure (correct
answer) Rationale: Henry's Law states that the amount of gas dissolved in a liquid is
proportional to the partial pressure of that gas above the liquid. In diving, this explains
how nitrogen and helium dissolve into body tissues at depth and must be eliminated during
ascent — the basis of decompression theory.
5. The partial pressure of oxygen in a 32% oxygen Nitrox mixture at 4 ATA is:
A) 0.32 bar B) 0.64 bar C) 1.28 bar/ATA D) 1.28 ATA
C) 1.28 bar/ATA (correct answer) Rationale: Partial pressure of oxygen = FO₂ × Absolute
pressure = 0.32 × 4 ATA = 1.28 ATA (or bar). This value approaches the maximum
,recommended PPO₂ of 1.4–1.6 ATA for working dives, demonstrating why oxygen partial
pressure monitoring is critical when using enriched air mixtures at depth.
6. Charles's Law states that at constant pressure, gas volume is:
A) Inversely proportional to temperature B) Independent of temperature C) Directly proportional
to absolute temperature D) Equal to pressure times temperature
C) Directly proportional to absolute temperature (correct answer) Rationale: Charles's
Law (V₁/T₁ = V₂/T₂) states that gas volume increases with increasing absolute temperature
(Kelvin). This is important when filling diving cylinders — gas compressed in warm
conditions will have lower pressure when cooled to ambient temperature, affecting gas
planning for cold water dives.
7. At what depth in seawater does the absolute pressure equal 5 ATA?
A) 99 fsw B) 132 fsw C) 165 fsw D) 198 fsw
B) 132 fsw (correct answer) Rationale: Absolute pressure = (depth ÷ 33) + 1 ATA. Solving
for depth: 5 ATA = (depth ÷ 33) + 1, so depth = (5-1) × 33 = 132 fsw. This calculation is
essential for mixed gas divers planning dives below 130 fsw where the physics of high
pressure gas mixtures become critical.
8. The density of a gas mixture at depth affects a diver primarily by:
A) Changing the color of the gas B) Increasing the work of breathing, potentially causing CO₂
retention C) Reducing the narcotic effect D) Decreasing oxygen toxicity risk
B) Increasing the work of breathing, potentially causing CO₂ retention (correct answer)
Rationale: As depth increases, gas density increases proportionally with pressure. Denser
gas is harder to breathe through regulators and the respiratory system, increasing the
work of breathing. This can lead to CO₂ retention, which potentiates oxygen toxicity and
narcosis. Helium is added to deep gas mixtures specifically because its low density reduces
this problem.
9. Graham's Law states that the rate of diffusion of a gas is:
, A) Proportional to its molecular weight B) Inversely proportional to the square root of its
molecular weight C) Directly proportional to its partial pressure D) Independent of molecular
weight
B) Inversely proportional to the square root of its molecular weight (correct answer)
Rationale: Graham's Law explains why helium diffuses and off-gasses more rapidly than
nitrogen — helium (molecular weight 4) diffuses approximately 2.65 times faster than
nitrogen (molecular weight 28). This faster diffusion is both an advantage (faster
decompression) and a concern (isobaric counterdiffusion risk in mixed gas diving).
10. The equivalent air depth (EAD) formula is used to:
A) Calculate oxygen toxicity limits B) Determine the nitrogen partial pressure equivalent of a
Nitrox mix for decompression planning C) Calculate maximum operating depth for Heliox D)
Determine CNS clock percentage
B) Determine the nitrogen partial pressure equivalent of a Nitrox mix for decompression
planning (correct answer) Rationale: EAD = [(1 - FO₂) × (depth + 33) ÷ 0.79] - 33. It
converts a Nitrox dive to an equivalent air depth for use with standard air decompression
tables. In mixed gas diving, analogous calculations (END — equivalent narcotic depth) are
used to assess the narcotic effect of Trimix and Heliox mixtures.
SECTION 2: MIXED GAS DIVING PRINCIPLES
11. The primary reason helium is added to a breathing mixture for deep diving is to:
A) Increase the oxygen content B) Reduce nitrogen narcosis and decrease gas density for easier
breathing C) Prevent oxygen toxicity D) Reduce decompression obligation
B) Reduce nitrogen narcosis and decrease gas density for easier breathing (correct answer)
Rationale: Helium is non-narcotic (or minimally so) and has a molecular weight of 4
compared to nitrogen's 28. This dramatically reduces both narcosis at depth and gas
density, improving breathing efficiency. These properties make helium essential for dives
beyond 150 fsw where nitrogen narcosis and breathing work become critical safety
concerns.
12. Trimix is a breathing gas mixture that contains:
