PADI IE Instructor Exam Answers - Or at least how to find them!

Pressure Fundamentals

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge
Practice Questions ← Physics Hub ← Theory Hub
Refraction, visual reversal and properties of water — PADI IDC physics
Refraction, Visual Reversal & Water Properties — PADI IDC / DM Physics

Refraction and Visual Reversal

Refraction and visual reversal are two of the most frequently tested topics in the PADI physics exam — and the easiest to mix up. Knowing the exact difference, and the one word PADI uses to signal visual reversal, is enough to get both questions right every time.

Refraction Light bends every time it passes from one medium into another of different density. As a diver, light from the sun passes through air → water → mask glass → mask air pocket before reaching your eyes, bending at each boundary. The result: objects underwater appear larger or closer than they really are.
Visual Reversal Visual reversal is the opposite of refraction. Instead of appearing larger or closer, objects appear smaller or more distant. It occurs in turbid water (water with suspended particles that scatter light). PADI exams describe visual reversal as a phenomenon.
Exam trap — the word “phenomenon” When you see the word phenomenon in a physics question, it is referring to visual reversal. A phenomenon is something unusual or strange — visual reversal is the unusual case where things appear smaller or more distant, the opposite of what refraction normally causes.

Water Properties

Water properties questions appear in every PADI IDC and Divemaster physics exam. There are three key facts to know: how water absorbs colour, how much faster it conducts heat away from your body than air does, and how much faster sound travels through it.

Colour absorption Water ABSORBS light starting at the red end of the spectrum. As you descend, red disappears first, then orange, yellow, and green. Blue light penetrates deepest. Colours look faded at depth — the closer a colour is to the red end of the spectrum, the shallower it will appear black. This is also why underwater photos appear very blue without artificial lighting.
Heat loss in water Water conducts heat away from your body 20 times faster than air. The three mechanisms of heat loss for a diver, in order of significance, are:
  1. Conduction — direct contact between your body and the surrounding water molecules (greatest effect)
  2. Convection — warmed water rises away from your body and is replaced by cooler water (second greatest effect)
  3. Radiation — emission of electromagnetic waves (least significant for divers; radiation is the biggest heat loss mechanism in air, but water overwhelms it)
Sound in water Sound travels 4 times faster in water than in air, because water is denser and more elastic than air. The reduced interval between the sound striking each ear means divers cannot accurately judge the direction a sound is coming from, and will often perceive it as coming from above.
Refraction explained — PADI IDC physics
Refraction Explained — PADI IDC / DM Physics
Pressure types and the depth-pressure table — PADI IDC physics
Pressure Types & The Depth-Pressure Table — PADI IDC / DM Physics

The Depth-Pressure Table

Every pressure calculation question in the PADI IDC and Divemaster physics exam depends on knowing the pressure at a given depth. This table is the foundation. Draw it on scrap paper at the very start of your exam — before you read a single question.

Depth — SALT WATER ONLY Pressure (ATA / atmospheres)
Surface (0 m)1 ATA
10 m2 ATA
20 m3 ATA
30 m4 ATA
40 m5 ATA
50 m6 ATA
Exam trap — SALT water vs FRESH water This table is for salt water only. Before selecting your answer, always reread the question and check whether the words fresh water are hiding anywhere in it. Fresh water is less dense than salt water, so the pressure values are slightly different. If the question is in fresh water, use your salt water answer as a guide: if you multiplied to get your answer, the fresh water answer will be a little less. If you divided, the fresh water answer will be a little more. This rule of thumb works for the vast majority of questions. For a full explanation of fresh water pressure calculations, see the Fresh Water Physics page.
Exam trap — the single most common mistake The most frequent error in pressure calculation questions is dividing the depth by 10 and using that as the pressure. At 40 m, depth ÷ 10 = 4 — but the pressure at 40 m is 5 ATA, not 4. Always use the table. The pressure is always one more than depth ÷ 10, because you must include the 1 ATA of surface atmospheric pressure.
Non-round depths For any non-round depth: divide by 10, then add 1.

Depth ÷ 10 + 1 = Pressure
17 m1.7+ 12.7 ATA
43 m4.3+ 15.3 ATA
26 m2.6+ 13.6 ATA
Pressure types PADI exams use four terms for pressure. You may be asked to identify each one.
  • Absolute pressure (ambient pressure) — the total pressure at a given depth. This is the value from the table above. At 30 m: 4 ATA.
  • Gauge pressure — absolute pressure minus 1 ATA (it ignores surface atmospheric pressure). At 30 m: 3 ATA. See the gauge pressure section below.
Exam trap — barometric pressure Barometric pressure is the atmospheric pressure at the surface, which varies slightly with weather systems. That variation is so small it is insignificant to divers. When you see barometric pressure as an answer option, it is almost always a red herring. Do not confuse it with absolute or gauge pressure — they are not the same thing.

The video below has no educational value, but it does prove why you need to do physics using the metric system! There are some VERY IMPORTANT videos and content after it though!

Comedy interlude — PADI IDC physics
The 3-step method for pressure calculations — PADI IDC physics
The 3-Step Method for Pressure Calculations — PADI IDC / DM Physics

The 3-Step Method

The 3-step method works for every single pressure calculation question in the PADI physics exam — air consumption, balloon volume, BAR, PSI, litres. Learn these three steps and you have a reliable process for all of them.

  1. Find your starting number — look at the answers to identify the unit of measurement (litres, minutes, BAR, PSI, etc.), then find that unit in the question and take that number as your starting point.
  2. Decide: multiply or divide — use your diving knowledge to think through what should happen. Do not memorise “down = divide”. Think about the specific question. A balloon going down gets smaller → divide. A diver breathing more air per breath at depth → multiply.
  3. Find the pressure at the given depth — use the table above. Every 10 m adds 1 ATA. For non-round depths, divide by 10 and add 1: 17 m = 2.7 ATA.
Do not memorise “down = divide, up = multiply” This shortcut will lead you wrong. Going down can mean divide (balloon gets smaller) or multiply (amount of air you breathe through increases). Always think about what the question is actually asking and use your dive knowledge to decide.
Air consumption in minutes — PADI IDC physics worked example
Worked Example: Air Consumption in Minutes — PADI IDC / DM Physics

Example Question 1 — Air consumption in minutes

Air consumption in minutes is one of the most common question types in the PADI IDC and Divemaster physics exam. The key decision is whether the tank lasts longer or shorter at the given depth compared to the starting point.

Worked Example — Air in minutes (surface to depth) A tank lasts 55 minutes at the surface. How long will it last at 40 m in salt water?
  1. Find the unit → answers are in minutes → take 55 from the question.
  2. Will the tank last longer or shorter at depth? Shorter → we need a smaller number → divide.
  3. Pressure at 40 m = 5 ATA. Calculate: 55 ÷ 5 = 11 minutes.
Answer: 11 minutes.
Balloon volume at depth — PADI IDC physics worked example
Worked Example: Balloon Volume at Depth — PADI IDC / DM Physics

Example Question 2 — Balloon volume

Balloon volume questions test Boyle’s Law directly. The balloon is a good mental image — picture it getting squeezed smaller as it descends, or expanding as it rises.

Worked Example — Balloon volume (surface to depth) A balloon has a volume of 8 litres at the surface. What is its volume at 30 m in salt water?
  1. Find the unit → answers are in litres → take 8 from the question.
  2. Balloons get smaller as you descend (pressure squeezes them) → divide.
  3. Pressure at 30 m = 4 ATA. Calculate: 8 ÷ 4 = 2 litres.
Answer: 2 litres.
Air consumption in BAR — PADI IDC physics worked example
Worked Example: Air Consumption in BAR — PADI IDC / DM Physics

Example Question 3 — Air in BAR

Air consumption questions can be asked in BAR or PSI — the unit does not change the method. This example catches out students who try to memorise “going down means divide” without thinking about what the question is actually asking.

Worked Example — Air in BAR (surface to depth) A diver breathes through 50 BAR of air in a given time at the surface. How much will the same diver breathe through in the same time at 20 m in salt water?
  1. Find the unit → answers are in BAR → take 50 from the question.
  2. At depth you breathe through more air with every breath (denser air) → multiply.
  3. Pressure at 20 m = 3 ATA. Calculate: 50 × 3 = 150 BAR.
Answer: 150 BAR.
Pumping air from the surface — PADI IDC physics worked example
Worked Example: Pumping Air from the Surface — PADI IDC / DM Physics

Example Question 4 — Pumping air from the surface

Pumping air from the surface to fill a container at depth is a question type that trips up many students because the logic feels backwards at first. Think about what happens to the air as it travels down and it becomes straightforward.

Worked Example — Pumping air from the surface to depth How many litres of air must be pumped from the surface to fill a 100-litre container sitting at 40 m in salt water?
  1. Find the unit → answers are in litres → take 100 from the question.
  2. Air pumped from the surface gets denser as it descends and its volume decreases. To end up with 100 litres at 40 m, you need to pump down more than 100 litres from the surface → multiply.
  3. Pressure at 40 m = 5 ATA. Calculate: 100 × 5 = 500 litres.
Answer: 500 litres.
Gauge pressure explained — PADI IDC physics
Gauge Pressure Explained — PADI IDC / DM Physics

Gauge Pressure

Gauge pressure questions are straightforward once you understand why gauge and absolute readings differ. The single most important thing to know: your depth gauge is designed to read zero at the surface, so it always shows one atmosphere less than the true absolute pressure.

Gauge pressure The depth-pressure table above gives you the absolute (ambient) pressure at each depth. Gauge pressure is always one atmosphere less than absolute or ambient. Depth gauges are designed to show zero at the surface, so the 1 ATA of surface atmospheric pressure has been removed. Gauge pressure = absolute pressure − 1 ATA.
Depth — SALT WATER ONLY Absolute / ambient pressure Gauge pressure
Surface (0 m)1 ATA0 ATA
10 m2 ATA1 ATA
20 m3 ATA2 ATA
30 m4 ATA3 ATA
40 m5 ATA4 ATA
50 m6 ATA5 ATA
Exam trap — SALT water vs FRESH water These values are for salt water only. Reread the question before selecting your answer and check whether fresh water is mentioned. If it is, your salt water answer will be slightly off — if you multiplied, pick the answer that is a little less; if you divided, pick the answer that is a little more. This works for most questions, but not all. For a full explanation, see the Fresh Water Physics page.
Absolute vs gauge — read the question carefully The question will specify whether it wants absolute (ambient) pressure or gauge pressure. They differ by exactly 1 ATA. Absolute = table value. Gauge = table value minus 1.

Physics Quick Quiz 1

Boyle’s Law • Gas density • Air consumption at depth

Physics — Topics

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