Partial Pressure

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Partial pressure concepts — Dalton's Law, contaminated tanks, altitude — PADI physics exam

Partial pressure questions are among the most straightforward calculations in the PADI physics exam — once you know the single rule that applies to all of them. Unlike Boyle's Law questions, where you have to stop and decide whether to multiply or divide, partial pressure questions have no decision. Like density, the operation is fixed every time. With density you always divide. With partial pressure you always multiply. Understanding why makes the rule impossible to forget.

Dalton's Law

Dalton's Law In a mixture of gases, each gas exerts pressure independently — as if the other gases were not there. The pressure exerted by any single gas in the mixture is called its partial pressure. The partial pressures of all gases in the mixture add up to the total pressure.

At sea level, the total pressure is 1 ata. Air is made up of approximately 21% oxygen and 79% nitrogen. So the partial pressure of oxygen in air at sea level is 0.21 ata, and the partial pressure of nitrogen is 0.79 ata. They add up to 1 ata — the total pressure.

As a diver goes deeper, total pressure increases. Because each gas still makes up the same percentage of the mix, each gas's partial pressure increases in direct proportion to the total pressure. At 10m (2 ata), the partial pressure of oxygen in air is 0.42 ata. At 20m (3 ata), it is 0.63 ata. The percentage stays fixed; the total pressure drives the partial pressure up.

Gas percentages to know

PADI exam questions always use the same assumed gas compositions. You must know these without looking them up.

Gas Blend Percentage Decimal (Step 1)
Oxygen Air 21% 0.21
Nitrogen Air 79% 0.79
Oxygen Nitrox 32 32% 0.32
Oxygen Nitrox 36 36% 0.36
Nitrox shortcut The number after the word "nitrox" is the oxygen percentage. Nitrox 32 = 32% oxygen. Nitrox 36 = 36% oxygen. The remaining percentage is nitrogen. You do not need to memorise nitrox compositions — the name tells you.

The 3-step method

Every partial pressure calculation uses the same three steps — no exceptions, no variations.

3-step method — partial pressure Step 1 — Write down the percentage of the target gas as a decimal.
Step 2 — Always multiply. No decision needed.
Step 3 — Write down the total pressure at the depth given. If the question gives a pressure directly, use that number. If it gives a depth, convert it to ata first.

Partial pressure is determined by two things: the fraction of the gas in the mix, and the total pressure that mix is under. Multiplying them together gives the share of the total pressure that belongs to that gas. This is why you always multiply — it is not a rule to memorise, it is the definition of what partial pressure means.

Concept topics from the introductory video

The video above covers several concept questions that appear in the PADI physics exam alongside the calculation questions. These do not require a calculator — they test whether you understand how partial pressure behaves in specific situations.

Contaminated tanks

If a compressed gas tank contains a contaminant such as carbon monoxide, its partial pressure increases with depth just as any other gas does. A contamination level within acceptable limits at the surface can become dangerously toxic at depth. This is why diving gas purity standards are stricter than for surface use, and why tanks must be filled from a clean, properly maintained compressor.

Contaminated tank questions are among the most frequently misread questions in the PADI physics exam. Read the three questions below carefully. They describe exactly the same scenario — a cylinder containing 0.5% carbon monoxide taken to 40m — but each asks for something different. Before reading on, think about what each question is actually asking for. The explanation follows.

Question A A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What percentage of carbon monoxide is in the cylinder at this depth?

a) 0.5%    b) 1.0%    c) 2.0%    d) 2.5%
Question B A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. If a diver breathes from it at this depth, it would have the same effect as breathing ______% carbon monoxide at the surface.

a) 0.5%    b) 2.0%    c) 2.5%    d) 0.1%
Question C A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What is the partial pressure of the carbon monoxide the diver is breathing at this depth?

a) 0.025 ata    b) 0.020 ata    c) 0.05 ata    d) 0.25 ata
Question D A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What is the surface equivalent percentage of carbon monoxide the diver is breathing at this depth?

a) 0.5%    b) 2.0%    c) 2.5%    d) 0.1%

Four questions, same scenario — yet the answers split three ways and the methods split two ways. The only thing that changes is a single phrase in each question. Here is how to decode them.

Decode the question first — always Before doing anything else, identify what the question is asking for. There are three possible question types. Each has a trigger phrase. Two of those phrases mean multiply. One means do nothing at all.
Question type Trigger phrase to spot What to do
Composition inside the cylinder in the tank / in the cylinder Nothing — composition does not change with depth. The answer is the same percentage as at the surface.
Surface equivalent percentage surface equivalent percentage / same effect as breathing ___% at the surface 3-step multiply. Express your answer as a percentage.
Partial pressure in ata partial pressure 3-step multiply. Express your answer in ata.
Surface equivalent percentage and "effect" mean the same thing PADI uses both phrasings to ask the same question. "Surface equivalent percentage" is the formal term — it means: what percentage of this gas would a diver need to breathe at the surface to feel the same physiological effect as breathing it at depth? The word effect often appears italicised in the actual exam paper, which is PADI signalling that this is the key word in the question. Both phrasings require the same 3-step multiply and a percentage answer.
IN THE TANK / IN THE CYLINDER — do not multiply A sealed cylinder contains a fixed amount of gas. Depth changes the pressure the cylinder is under, but it does not change what is inside it. In the tank and in the cylinder are the phrases that tell you the question is asking about composition — which is unchanged. No calculation is needed. Students who multiply here are answering a question that was not asked.
Answer A — "In the cylinder" (do nothing) A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What percentage of carbon monoxide is in the cylinder at this depth?
  1. Spot the trigger: in the cylinder — composition inside a sealed container. No calculation needed.
  2. Composition does not change with depth.
Answer: a) 0.5% — the same as at the surface.
Why the wrong answers are wrong — Question A Option b (1.0%) tempts students who doubled, as if 40m = 2 ata. Option c (2.0%) catches those who used 4 ata and multiplied. Option d (2.5%) catches those who correctly used 5 ata for 40m and multiplied — the right method applied to the wrong question. No multiplication is needed here at all.
Answer B — Surface equivalent percentage (multiply, answer in %) A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. If a diver breathes from it at this depth, it would have the same effect as breathing ______% carbon monoxide at the surface.
  1. Spot the trigger: effect — surface equivalent percentage. Multiply, express answer as a percentage.
  2. Step 1 — 0.5% as a decimal = 0.005
  3. Step 2 — always multiply: 0.005 × …
  4. Step 3 — pressure at 40m salt water = 5 ata
    0.005 × 5 = 0.025 → as a percentage = 2.5%
Answer: c) 2.5% — breathing 0.5% CO at 40m has the same physiological effect as breathing 2.5% CO at the surface.
Why the wrong answers are wrong — Question B Option a (0.5%) catches students who treated this as a "do nothing" question — but the trigger phrase is effect, not "in the cylinder." Option b (2.0%) catches those who used 4 ata instead of 5 ata for 40m. Option d (0.1%) catches those who divided instead of multiplied.
Answer C — Partial pressure in ata (multiply, answer in ata) A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What is the partial pressure of the carbon monoxide the diver is breathing at this depth?
  1. Spot the trigger: partial pressure — multiply, express answer in ata.
  2. Step 1 — 0.5% as a decimal = 0.005
  3. Step 2 — always multiply: 0.005 × …
  4. Step 3 — pressure at 40m salt water = 5 ata
    0.005 × 5 = 0.025 ata
Answer: a) 0.025 ata — the partial pressure of carbon monoxide at 40m.
Why the wrong answers are wrong — Question C Option b (0.020 ata) catches those who used 4 ata instead of 5 ata for 40m. Option c (0.05 ata) catches those who used 0.05 as the decimal for 0.5% — a decimal place error. Option d (0.25 ata) catches those who used 0.5 as the decimal, treating 0.5% as if it were 50%.
Answer D — Surface equivalent percentage (multiply, answer in %) A scuba cylinder containing 0.5% carbon monoxide at the surface is taken to 40m. What is the surface equivalent percentage of carbon monoxide the diver is breathing at this depth?
  1. Spot the trigger: surface equivalent percentage — multiply, express answer as a percentage.
  2. Step 1 — 0.5% as a decimal = 0.005
  3. Step 2 — always multiply: 0.005 × …
  4. Step 3 — pressure at 40m salt water = 5 ata
    0.005 × 5 = 0.025 → as a percentage = 2.5%
Answer: c) 2.5% — the same answer as Question B, because "surface equivalent percentage" and effect are two ways of asking for exactly the same thing.
Why the wrong answers are wrong — Question D The distractors are identical to Question B, because the question is identical in everything except phrasing. Option a (0.5%) catches students who treated this as a "do nothing" question — but surface equivalent percentage is a multiply trigger, not an "in the cylinder" phrase. Option b (2.0%) catches those who used 4 ata instead of 5 ata for 40m. Option d (0.1%) catches those who divided instead of multiplied.

Open liquid-filled containers

Gases dissolve into liquids under pressure — the greater the pressure, the more gas goes into solution (Henry's Law). When pressure decreases, dissolved gas comes back out of solution. An open liquid-filled container brought up from depth will release dissolved gas as the surrounding pressure drops. This same principle underlies decompression sickness: nitrogen dissolves into body tissues under pressure and must be allowed to come out slowly during ascent.

Gas containers at depth

A sealed, rigid gas container — such as a scuba cylinder — does not change the amount of gas it holds as depth increases. The total amount of gas is fixed; only the surrounding water pressure changes. What does change is the pressure differential between the gas inside and the water outside, which is why cylinders are rated to specific working pressures.

Altitude

At altitude, atmospheric pressure at the surface is lower than 1 ata. This means the partial pressures of all gases in the air are lower than at sea level. A diver breathing air at altitude is breathing air with a lower partial pressure of oxygen — not because the percentage has changed (it is still 21%), but because the total pressure is lower. This affects decompression calculations and is the basis of altitude diving procedures.

Worked examples

Partial pressure questions in the PADI exam take one of two forms: a depth is given and you convert it to pressure, or a pressure is given directly and you use it straight away. The four examples below cover both forms and introduce nitrox, so every question type you will see on the paper is represented.

Partial pressure example 1 — PPO2 in air at 20m — PADI physics exam

Will Welbourn works through partial pressure example question 1 — the partial pressure of oxygen in air at a depth of 20 m — PADI Divemaster / IDC physics exam study notes.

Worked example 1 — PPO2 in air at 20m What is the partial pressure of oxygen in the air a diver breathes at a depth of 20m?
  1. Target gas: oxygen in air → write down 0.21
  2. Partial pressure question → always multiply: 0.21 × …
  3. Depth: 20m in salt water → pressure = 3 ata
    0.21 × 3 = 0.63 ata
Answer: 0.63 ata — the partial pressure of oxygen in air at 20m.
Partial pressure example 2 — PPN2 in air at 15m — PADI physics exam

Will Welbourn works through partial pressure example question 2 — the partial pressure of nitrogen in air at a depth of 15 m — PADI Divemaster / IDC physics exam study notes.

Worked example 2 — PPN2 in air at 15m What is the partial pressure of nitrogen in the air a diver breathes at a depth of 15m, salt water?
  1. Target gas: nitrogen in air → write down 0.79
  2. Partial pressure question → always multiply: 0.79 × …
  3. Depth: 15m in salt water → pressure = 2.5 ata
    0.79 × 2.5 = 1.975 ata
Answer: 1.975 ata — the partial pressure of nitrogen in air at 15m.
Pressure at 15m 15m sits halfway between 10m (2 ata) and 20m (3 ata), so the pressure is 2.5 ata. Every 10m adds 1 ata in salt water, so every 5m adds 0.5 ata. This halfway rule applies at any intermediate depth: 5m = 1.5 ata, 15m = 2.5 ata, 25m = 3.5 ata, 35m = 4.5 ata.
Partial pressure example 3 — PPO2 in Nitrox 32 at 33m — PADI physics exam

Will Welbourn works through partial pressure example question 3 — the partial pressure of oxygen when breathing Nitrox 32 at a depth of 33 m — PADI Divemaster / IDC physics exam study notes.

Worked example 3 — PPO2 in Nitrox 32 at 33m What is the partial pressure of oxygen a diver breathes in Nitrox 32 at a depth of 33m, salt water?
  1. Target gas: oxygen in Nitrox 32 → write down 0.32
  2. Partial pressure question → always multiply: 0.32 × …
  3. Depth: 33m in salt water → pressure = 4.3 ata
    0.32 × 4.3 = 1.376 ata
Answer: 1.376 ata — very close to the 1.4 ata maximum PPO2 for nitrox diving.
The 1.4 ata limit PADI sets the maximum partial pressure of oxygen for recreational nitrox diving at 1.4 ata. Beyond this level, oxygen toxicity becomes a serious risk. The answer to example 3 — 1.376 ata — is close enough to this limit that 33m is effectively the maximum operating depth for Nitrox 32. This is why nitrox blends have a maximum depth, and why that depth is shallower for blends with more oxygen.
Partial pressure example 4 — PPO2 at altitude with pressure given directly — PADI physics exam

Will Welbourn works through partial pressure example question 4 — the partial pressure of oxygen at altitude where the ambient pressure is given directly rather than derived from a depth — PADI Divemaster / IDC physics exam study notes.

Worked example 4 — PPO2 at altitude (pressure given directly) A diver diving in a mountain lake where the ambient pressure is 0.8 ata is breathing air at the surface. What is the partial pressure of the oxygen they are breathing?
  1. Target gas: oxygen in air → write down 0.21
  2. Partial pressure question → always multiply: 0.21 × …
  3. Pressure is given directly: 0.8 ata — no depth conversion needed
    0.21 × 0.8 = 0.168 ata
Answer: 0.168 ata — the partial pressure of oxygen in air at the surface of a mountain lake.

Harder question types

The following questions require the same 3-step approach but with small additional layers that catch unprepared candidates. Read each question carefully before attempting it.

Enriched air — both percentages given

In diving, the terms "nitrox," "enriched air," and "enriched air nitrox" are used interchangeably to describe any breathing gas with a higher oxygen percentage than air. Enriched air is arguably the more precise term — it describes exactly what it is. For exam purposes, treat them as identical. The question below gives you both gas percentages. Only one is needed.

Question — EANx 30 at 25m A diver and their buddy are each diving an enriched air cylinder containing 30% oxygen and 70% nitrogen. On a wreck dive at 25 metres in the ocean, what is the partial pressure of oxygen?

a) 1.05 ata    b) 0.90 ata    c) 1.20 ata    d) 0.105 ata
Answer — EANx 30 at 25m
  1. The question asks for the partial pressure of oxygen. Oxygen is 30% of the blend → write down 0.30. Ignore the 70% nitrogen figure — it is not needed.
  2. Partial pressure question → always multiply: 0.30 × …
  3. Depth: 25m in salt water → pressure = 3.5 ata
    0.30 × 3.5 = 1.05 ata
Answer: a) 1.05 ata
Why the wrong answers are wrong Option b (0.90 ata) catches those who used 3 ata for 25m, confusing it with 20m. Option c (1.20 ata) catches those who used 4 ata for 25m, confusing it with 30m. Option d (0.105 ata) is a decimal place error — using 0.03 instead of 0.30 as the decimal for 30%. Giving you both gas percentages in the question stem is a deliberate distraction. The question tells you which gas it is asking about — use that percentage only.

Reverse partial pressure — finding the depth

Every question up to this point has given you a depth and asked you to find a partial pressure. This question type works the other way: it gives you a target partial pressure and asks you to find the depth at which it is reached. The method is still 3-step, but Step 2 becomes a division rather than a multiplication — the one exception on this page.

Reverse partial pressure — at what depth does air become potentially toxic — PADI physics exam

Will Welbourn works through the reverse partial pressure question — at what depth would breathing air reach the 1.6 ata CNS oxygen toxicity limit — PADI Divemaster / IDC physics exam study notes.

The one question type on this page where you divide When a question gives you a target partial pressure and asks for a depth, reverse the method. Instead of multiplying the percentage by the pressure to find PPO2, divide the target PPO2 by the gas percentage to find the required pressure — then convert that pressure to a depth.
Worked example — at what depth does air reach the 1.6 ata CNS oxygen toxicity limit? The maximum partial pressure of oxygen a diver should be exposed to is 1.6 ata, beyond which there is a serious risk of CNS oxygen toxicity. At what depth would a diver breathing air reach this limit?
  1. Target gas: oxygen in air → write down 0.21
  2. Target PPO2 is given → divide: 1.6 ÷ 0.21 = 7.619 ata — this is the total pressure at which the limit is reached
  3. Convert pressure to depth: 7.619 ata in salt water → subtract 1 (surface atmosphere), multiply remainder by 10 → (7.619 − 1) × 10 = approximately 66m
Answer: approximately 66m — the theoretical depth at which breathing air reaches the CNS oxygen toxicity limit. In practice, air diving is limited by nitrogen narcosis and other factors long before this depth.

The same reverse method applies regardless of the blend or the target PPO2. Here is a second application using enriched air.

Worked example — at what depth does EANx 33 have the same effect as breathing pure oxygen at the surface? Pure oxygen at the surface has a partial pressure of 1.0 ata. At what depth would a diver breathing EANx 33 experience the same effect as breathing pure oxygen at the surface?
  1. Target gas: oxygen in EANx 33 → write down 0.33
  2. Target PPO2: pure oxygen at the surface = 1.0 ata → divide: 1.0 ÷ 0.33 = 3.03 ata
  3. Convert pressure to depth: 3.03 ata → (3.03 − 1) × 10 = approximately 20m
Answer: approximately 20m — at this depth, the partial pressure of oxygen in EANx 33 equals 1.0 ata, the same as breathing pure oxygen at the surface.
Spotting the reverse question The trigger is that the question gives you an effect or a target and asks for a depth — rather than giving a depth and asking for a PPO2. The word "approximately" in the question stem is also a reliable signal: because the division rarely produces a round number, PADI always uses approximate answer options for this question type. When you see "approximately" alongside a depth question, expect to divide.

Physics Practice — Partial Pressure

8 questions — covers all question types on this page

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge
Practice Questions ← Physics Hub ← Theory Hub