ADCI Surface-Supplied
Mixed Gas Diver Certification Exam
Questions & Answers with Rationales
About this guide: This study resource is designed to help candidates prepare for a
surface-supplied mixed gas commercial diving certification examination consistent with
the general body of knowledge referenced by the Association of Diving Contractors
International (ADCI) Consensus Standards. It covers diving physics, physiology and diving
medicine, decompression theory, mixed gas (heliox/trimix) theory, surface-supplied diving
equipment, gas management and life support, emergency procedures and diver rescue,
chamber operations and therapeutic recompression, dive planning and supervision, and
industry safety standards.
,1. Diving Physics
Question 1
According to Boyle's Law, as a diver descends and ambient pressure
increases, the volume of a fixed mass of gas at constant temperature
will:
A. Increase proportionally with pressure
B. Decrease inversely with the increase in absolute pressure
C. Remain unchanged regardless of depth
D. Increase only below 100 feet
Answer: B. Decrease inversely with the increase in absolute pressure
Rationale: Boyle's Law states that at constant temperature, the volume of a
gas is inversely proportional to its absolute pressure (P1V1 = P2V2). As a
diver descends and pressure rises, any gas space -- lungs, mask, suit, or
equipment air spaces -- is compressed to a smaller volume, which is why
equalization of air spaces is critical during descent.
Question 2
Dalton's Law of partial pressures is most directly applied in commercial
diving to:
A. Calculate a diver's buoyancy at depth
B. Determine the partial pressure of individual gases (such as oxygen or
nitrogen) in a breathing mixture at a given depth, which governs toxicity
and narcosis risk
C. Predict water temperature at depth
D. Calculate umbilical hose length needed for a dive
Answer: B. Determine the partial pressure of individual gases (such as
oxygen or nitrogen) in a breathing mixture at a given depth, which
governs toxicity and narcosis risk
Rationale: Dalton's Law states that the total pressure of a gas mixture equals
the sum of the partial pressures of its component gases. In diving, this is
used to calculate the partial pressure of oxygen, nitrogen, or helium in a
breathing mix at depth, which directly determines risks such as oxygen
,toxicity, nitrogen narcosis, or the need for a different mix altogether.
Question 3
Henry's Law explains the diving phenomenon of:
A. Increased gas density at depth
B. The absorption and later release of inert gas into and out of body
tissues as a function of the partial pressure of that gas in the breathing
mixture, which underlies decompression sickness
C. The compression of air spaces during descent
D. The refraction of light underwater
Answer: B. The absorption and later release of inert gas into and out of
body tissues as a function of the partial pressure of that gas in the
breathing mixture, which underlies decompression sickness
Rationale: Henry's Law states that the amount of gas that dissolves in a
liquid (such as body tissue) is proportional to the partial pressure of that gas
above the liquid. This governs inert gas uptake during descent/bottom time
and off-gassing during ascent, forming the physical basis of decompression
theory.
Question 4
As depth increases, gas density increases, which primarily affects a
surface-supplied diver by:
A. Making the diver more buoyant
B. Increasing the work of breathing, since denser gas is harder to move
through the airway and breathing apparatus
C. Reducing narcosis risk
D. Increasing the diver's field of vision
Answer: B. Increasing the work of breathing, since denser gas is harder
to move through the airway and breathing apparatus
Rationale: As ambient pressure rises, breathing gas density increases
proportionally, which raises the physical effort (work of breathing) required to
move gas through the regulator, hose, and airway. This is one of the key
reasons deep or heavy-exertion dives may require lower-density mixes such
, as helium-based blends instead of air or nitrox.
Question 5
Underwater, sound travels compared to air:
A. Slower, and direction is easier to determine
B. Faster, but direction is more difficult for a diver to determine due to how
quickly it reaches both ears
C. At exactly the same speed with no change in directionality
D. Not at all, since water blocks sound transmission
Answer: B. Faster, but direction is more difficult for a diver to determine
due to how quickly it reaches both ears
Rationale: Sound travels roughly four times faster in water than in air.
Because it arrives at both ears with very little time difference, divers typically
find it much harder to localize the direction of an underwater sound source
compared to on the surface.
Question 6
Light underwater is affected such that a diver will generally notice:
A. Colors appear more vivid at all depths with no change
B. Progressive loss of colors (starting with red) with increasing depth,
along with reduced visibility due to scattering and absorption
C. No difference in visibility regardless of turbidity
D. Blue light is absorbed first, before any other color
Answer: B. Progressive loss of colors (starting with red) with
increasing depth, along with reduced visibility due to scattering and
absorption
Rationale: Water absorbs light selectively by wavelength; longer
wavelengths (red, then orange, yellow) are absorbed first with increasing
depth, while blue penetrates deepest. Combined with scattering from
suspended particles, this reduces color rendition and visibility as depth and
turbidity increase.
Question 7
Mixed Gas Diver Certification Exam
Questions & Answers with Rationales
About this guide: This study resource is designed to help candidates prepare for a
surface-supplied mixed gas commercial diving certification examination consistent with
the general body of knowledge referenced by the Association of Diving Contractors
International (ADCI) Consensus Standards. It covers diving physics, physiology and diving
medicine, decompression theory, mixed gas (heliox/trimix) theory, surface-supplied diving
equipment, gas management and life support, emergency procedures and diver rescue,
chamber operations and therapeutic recompression, dive planning and supervision, and
industry safety standards.
,1. Diving Physics
Question 1
According to Boyle's Law, as a diver descends and ambient pressure
increases, the volume of a fixed mass of gas at constant temperature
will:
A. Increase proportionally with pressure
B. Decrease inversely with the increase in absolute pressure
C. Remain unchanged regardless of depth
D. Increase only below 100 feet
Answer: B. Decrease inversely with the increase in absolute pressure
Rationale: Boyle's Law states that at constant temperature, the volume of a
gas is inversely proportional to its absolute pressure (P1V1 = P2V2). As a
diver descends and pressure rises, any gas space -- lungs, mask, suit, or
equipment air spaces -- is compressed to a smaller volume, which is why
equalization of air spaces is critical during descent.
Question 2
Dalton's Law of partial pressures is most directly applied in commercial
diving to:
A. Calculate a diver's buoyancy at depth
B. Determine the partial pressure of individual gases (such as oxygen or
nitrogen) in a breathing mixture at a given depth, which governs toxicity
and narcosis risk
C. Predict water temperature at depth
D. Calculate umbilical hose length needed for a dive
Answer: B. Determine the partial pressure of individual gases (such as
oxygen or nitrogen) in a breathing mixture at a given depth, which
governs toxicity and narcosis risk
Rationale: Dalton's Law states that the total pressure of a gas mixture equals
the sum of the partial pressures of its component gases. In diving, this is
used to calculate the partial pressure of oxygen, nitrogen, or helium in a
breathing mix at depth, which directly determines risks such as oxygen
,toxicity, nitrogen narcosis, or the need for a different mix altogether.
Question 3
Henry's Law explains the diving phenomenon of:
A. Increased gas density at depth
B. The absorption and later release of inert gas into and out of body
tissues as a function of the partial pressure of that gas in the breathing
mixture, which underlies decompression sickness
C. The compression of air spaces during descent
D. The refraction of light underwater
Answer: B. The absorption and later release of inert gas into and out of
body tissues as a function of the partial pressure of that gas in the
breathing mixture, which underlies decompression sickness
Rationale: Henry's Law states that the amount of gas that dissolves in a
liquid (such as body tissue) is proportional to the partial pressure of that gas
above the liquid. This governs inert gas uptake during descent/bottom time
and off-gassing during ascent, forming the physical basis of decompression
theory.
Question 4
As depth increases, gas density increases, which primarily affects a
surface-supplied diver by:
A. Making the diver more buoyant
B. Increasing the work of breathing, since denser gas is harder to move
through the airway and breathing apparatus
C. Reducing narcosis risk
D. Increasing the diver's field of vision
Answer: B. Increasing the work of breathing, since denser gas is harder
to move through the airway and breathing apparatus
Rationale: As ambient pressure rises, breathing gas density increases
proportionally, which raises the physical effort (work of breathing) required to
move gas through the regulator, hose, and airway. This is one of the key
reasons deep or heavy-exertion dives may require lower-density mixes such
, as helium-based blends instead of air or nitrox.
Question 5
Underwater, sound travels compared to air:
A. Slower, and direction is easier to determine
B. Faster, but direction is more difficult for a diver to determine due to how
quickly it reaches both ears
C. At exactly the same speed with no change in directionality
D. Not at all, since water blocks sound transmission
Answer: B. Faster, but direction is more difficult for a diver to determine
due to how quickly it reaches both ears
Rationale: Sound travels roughly four times faster in water than in air.
Because it arrives at both ears with very little time difference, divers typically
find it much harder to localize the direction of an underwater sound source
compared to on the surface.
Question 6
Light underwater is affected such that a diver will generally notice:
A. Colors appear more vivid at all depths with no change
B. Progressive loss of colors (starting with red) with increasing depth,
along with reduced visibility due to scattering and absorption
C. No difference in visibility regardless of turbidity
D. Blue light is absorbed first, before any other color
Answer: B. Progressive loss of colors (starting with red) with
increasing depth, along with reduced visibility due to scattering and
absorption
Rationale: Water absorbs light selectively by wavelength; longer
wavelengths (red, then orange, yellow) are absorbed first with increasing
depth, while blue penetrates deepest. Combined with scattering from
suspended particles, this reduces color rendition and visibility as depth and
turbidity increase.
Question 7
