Moving between Depths

IDC Physics part 2

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Depth-to-depth calculations — 3-step-twice method — PADI IDC / DM physics
Depth-to-Depth Calculations — The 3-Step-Twice Method — PADI IDC / DM Physics

Depth-to-Depth Calculations

Depth-to-depth questions are the trickiest calculation type in the PADI physics exam — but only because most students don't realise they can use the exact same 3-step method, just twice. Once you see that, these questions are no harder than the ones you already know how to answer.

Why use the method twice? In Physics Part 1 you learned how to compare a depth to the surface. Depth-to-depth questions give you two depths with no surface involved. The fix is simple: use the surface as a stepping stone.

Step one: go from the first depth up to the surface.
Step two: go from the surface down to the second depth.

Two rounds of the same 3-step process. That's it.

The 3-Step-Twice Method

The method works the same way for any quantity that changes with depth — balloon volume, minutes of air, PSI consumption, density. The logic is always the same: treat the surface as your midpoint.

Round 1 — First depth to surface
  1. Step 1: Write down the starting value given in the question.
  2. Step 2: Multiply or divide? Think about what happens as the object rises to the surface. Balloons get bigger → multiply. Air lasts longer at the surface → multiply. Denser air at depth means you breathe through more PSI per minute → divide.
  3. Step 3: Find the pressure at the first (starting) depth and apply it. This gives you the surface value.
Round 2 — Surface to second depth
  1. Step 1: Write down the surface value you just calculated in Round 1.
  2. Step 2: Multiply or divide? Now think about what happens as the object descends to the second depth. Going deeper is the reverse of Round 1.
  3. Step 3: Find the pressure at the second depth and apply it. This is your final answer.
Exam trap — reading the question carefully Depth-to-depth questions give you two depths. Make sure you identify which is the starting depth and which is the ending depth before you begin. The starting depth is where the object is now. The ending depth is where it is going.
Depth-to-depth worked example — balloon volume — PADI IDC / DM physics
Depth-to-Depth Worked Example 1 — Balloon Volume — PADI IDC / DM Physics

Worked Example 1 — Balloon Volume

This is the first depth-to-depth worked example. The question gives a balloon volume at a starting depth and asks for the volume at a different depth — neither of which is the surface. Use the 3-step method twice.

Example Q1 — A balloon holds 9 litres at 20 m. What is its volume at 10 m? Round 1 — 20 m to the surface
  1. Step 1 — Starting value: The answers are in litres, so find litres in the question. The balloon holds 9 litres at 20 m.
  2. Step 2 — Multiply or divide? The balloon is rising from 20 m to the surface. Balloons get bigger as they ascend — so multiply.
  3. Step 3 — Pressure at the starting depth: Pressure at 20 m = 3 atm.   9 × 3 = 27 litres at the surface.
Round 2 — Surface to 10 m
  1. Step 1 — Starting value: Take the surface result from Round 1. The balloon is now 27 litres at the surface.
  2. Step 2 — Multiply or divide? The balloon is descending from the surface to 10 m. Balloons get smaller as they descend — so divide.
  3. Step 3 — Pressure at the second depth: Pressure at 10 m = 2 atm.   27 ÷ 2 = 13.5 litres.
Answer: 13.5 litres
Depth-to-depth worked example — air consumption in minutes — PADI IDC / DM physics
Depth-to-Depth Worked Example 2 — Air Consumption in Minutes — PADI IDC / DM Physics

Worked Example 2 — Air Consumption in Minutes

This example uses minutes instead of litres, but the method is identical. The key is getting the multiply/divide decision right for each round — and remembering that the logic flips between Round 1 and Round 2.

Example Q2 — A tank lasts 45 minutes at 5 m. How long would it last at 35 m? Round 1 — 5 m to the surface
  1. Step 1 — Starting value: The answers are in minutes, so find minutes in the question. The tank lasts 45 minutes at 5 m.
  2. Step 2 — Multiply or divide? Going from 5 m to the surface. Air lasts longer at the surface than at depth, because the air is less dense there. More minutes means a bigger number — so multiply.
  3. Step 3 — Pressure at the starting depth: Pressure at 5 m = 1.5 atm.   45 × 1.5 = 67.5 minutes at the surface.
Round 2 — Surface to 35 m
  1. Step 1 — Starting value: Take the surface result from Round 1. The tank would last 67.5 minutes at the surface.
  2. Step 2 — Multiply or divide? Going from the surface down to 35 m. The tank lasts less time at depth — fewer minutes means a smaller number — so divide.
  3. Step 3 — Pressure at the second depth: Pressure at 35 m = 4.5 atm.   67.5 ÷ 4.5 = 15 minutes.
Answer: 15 minutes
Exam trap — the multiply/divide logic flips between rounds In Round 1 you multiplied to get more minutes (ascending = longer tank duration). In Round 2 you divided to get fewer minutes (descending = shorter tank duration). The logic is always about what physically happens to the quantity — think it through fresh each time rather than assuming both rounds use the same operation.
Depth-to-depth worked example — PSI air consumption — PADI IDC / DM physics
Depth-to-Depth Worked Example 3 — PSI Air Consumption — PADI IDC / DM Physics

Worked Example 3 — PSI Air Consumption

PSI questions follow exactly the same pattern. The only difference from minutes is that the multiply/divide logic in Round 1 is reversed — because PSI per minute measures how fast you breathe through air, not how long it lasts.

Example Q3 — A diver consumes 10 PSI per minute at 50 m. How much do they consume at 20 m? Round 1 — 50 m to the surface
  1. Step 1 — Starting value: The answers are in PSI, so find PSI in the question. The diver breathes through 10 PSI per minute at 50 m.
  2. Step 2 — Multiply or divide? Going from 50 m to the surface. At the surface, air is less dense, so the diver breathes through less PSI per minute — a smaller number — so divide.
  3. Step 3 — Pressure at the starting depth: Pressure at 50 m = 6 atm.   10 ÷ 6 = 1.6̅ PSI per minute at the surface.
Round 2 — Surface to 20 m
  1. Step 1 — Starting value: Take the surface result from Round 1. The diver consumes 1.6̅ PSI per minute at the surface.
  2. Step 2 — Multiply or divide? Going from the surface down to 20 m. At depth, air is denser, so the diver breathes through more PSI per minute — a bigger number — so multiply.
  3. Step 3 — Pressure at the second depth: Pressure at 20 m = 3 atm.   1.6̅ × 3 = 5 PSI per minute.
Answer: 5 PSI per minute
Exam trap — PSI vs minutes logic is opposite With minutes, ascending = multiply (more time). With PSI per minute, ascending = divide (less consumption rate). This is because PSI per minute is a rate — the faster you breathe through air, the higher the number. Always ask yourself: does the quantity go up or down as I ascend? Then decide accordingly.
You may not need this section. If the 3-step-twice method shown above makes sense to you and maths is not your favourite thing, feel free to skip straight to Physics Part 3. The alternative method below covers the same ground — it is simply a different route to the same answer. Some candidates find it faster; others find it an unnecessary complication. There is no wrong choice.

Skip to Physics Part 3 →

First depth / second depth method introduction — PADI IDC / DM physics
First Depth / Second Depth Method — Introduction — PADI IDC / DM Physics

Introduction to the first depth / second depth method — an alternative approach to depth-to-depth calculation questions.

Alternative Method — First Depth / Second Depth

The 3-step-twice method works for every depth-to-depth question. But there is a faster shortcut that some candidates prefer, especially when the answers include fractions. It gives the same correct answer — it is simply a different route to get there.

The first depth / second depth method Instead of using the surface as a stepping stone, you calculate a change factor directly:

Change factor = pressure at first (starting) depth ÷ pressure at second (ending) depth

Then apply that change factor to the starting value. Ask yourself: does the quantity get bigger or smaller as it moves from the first depth to the second depth? Multiply if it gets bigger. Divide if it gets smaller.

The division is always the same — first depth pressure over second depth pressure. The only judgement call is what to do with the result.
Example 1 — Balloon volume (same question as Worked Example 1 above)

A balloon holds 9 litres at 20 m. What is its volume at 10 m in salt water?

  1. Find the change factor: Pressure at 20 m (first depth) = 3 atm. Pressure at 10 m (second depth) = 2 atm. Change factor = 3 ÷ 2 = 1.5.
  2. Apply to starting value: Starting volume = 9 litres. The balloon is ascending (20 m → 10 m), so it gets bigger — multiply.   9 × 1.5 = 13.5 litres.
Answer: 13.5 litres — identical to the 3-step-twice result.
First depth / second depth method — minutes example and change factor trap — PADI IDC / DM physics
First Depth / Second Depth Method — Example 2 — PADI IDC / DM Physics

First depth / second depth method applied to a minutes question — including the important trap when the change factor is less than 1.

Example 2 — Air Consumption in Minutes

The change factor is straightforward to calculate, but what you do with it depends on whether it is greater or less than 1. This example introduces that trap clearly.

Example 2 — Minutes (same question as Worked Example 2 above)

A diver takes 45 minutes to breathe through a tank at 5 m. How long will the same tank last at 35 m in salt water?

  1. Find the change factor: Pressure at 5 m (first depth) = 1.5 atm. Pressure at 35 m (second depth) = 4.5 atm. Change factor = 1.5 ÷ 4.5 = 0.33.
  2. Apply to starting value: Starting time = 45 minutes. The diver is descending (5 m → 35 m), so air runs out faster — the answer must be less than 45.
45 × 0.33 = 15 minutes — Answer C.
The change factor less-than-1 trap In this example the change factor is 0.33 — a number less than 1. Your instinct is probably to divide to make 45 smaller, but dividing by 0.33 would give you a number larger than 45. That is the opposite of what you need.

When the change factor is less than 1: multiply to make the value smaller, divide to make it bigger — the reverse of normal intuition.

The safest habit is to check your answer makes sense before moving on. A diver going from 5 m to 35 m must use air faster, so the answer must be less than 45 minutes. If your calculation gives a larger number, you have multiplied when you should have divided, or vice versa.

The 3-step-twice method avoids this trap entirely, because you always end up with pressures greater than 1.
First depth / second depth method — PSI example — PADI IDC / DM physics
First Depth / Second Depth Method — Example 3 — PADI IDC / DM Physics

First depth / second depth method applied to a PSI air consumption question — an example where the change factor is greater than 1 and the standard logic applies.

Example 3 — Air Consumption in PSI

This example uses the same PSI question from Worked Example 3. The change factor comes out greater than 1 here, so the multiply/divide logic is straightforward.

Example 3 — PSI (same question as Worked Example 3 above)

A diver consumes 10 PSI per minute at 50 m. How many PSI per minute will they consume at 20 m in salt water?

  1. Find the change factor: Pressure at 50 m (first depth) = 6 atm. Pressure at 20 m (second depth) = 3 atm. Change factor = 6 ÷ 3 = 2.
  2. Apply to starting value: Starting PSI rate = 10 PSI/min. The diver is ascending (50 m → 20 m), so they breathe through less PSI per minute — the answer must be less than 10. To make 10 smaller using 2, divide.   10 ÷ 2 = 5 PSI/min.
Answer: 5 PSI per minute — Answer B.
Why this example is the cleaner case The change factor here is 2 — a whole number greater than 1. Multiply makes bigger, divide makes smaller, exactly as you would expect. Contrast this with Example 2, where the change factor was 0.33 and the logic reversed. When the first depth is deeper than the second depth, the change factor will always be greater than 1, making the arithmetic intuitive. When the first depth is shallower than the second depth, the change factor will be less than 1 — and that is where the trap lies.
First depth / second depth method — fractional answers shortcut — PADI IDC / DM physics
First Depth / Second Depth Method — Example 4 — PADI IDC / DM Physics

First depth / second depth method — the fractional answers shortcut that makes this method especially useful in the PADI physics exam.

Example 4 — Where This Method Really Earns Its Place

Some exam questions give fractional starting values and fractional answer choices. The 3-step-twice method still works, but the first depth / second depth method can give you the answer before you even finish the calculation.

Example 4 — Fractional balloon volume

A balloon has a volume of 1/10 of a litre at 40 m. What will its volume be at 30 m in salt water?

  1. Find the change factor — write it as a fraction: Pressure at 40 m (first depth) = 5 atm. Pressure at 30 m (second depth) = 4 atm. Change factor = 5/4. Write it as a fraction, not a decimal.
  2. Check the answer choices: The balloon is ascending (40 m → 30 m), so its volume increases. Look at the answers — if you see an option that says the volume increases by a factor of 5/4, that is your answer. No further arithmetic required.
Answer: the volume increases by a factor of 5/4 — i.e. 1/10 × 5/4 = 5/40 = 1/8 of a litre. But in an exam with fraction answer choices, writing 5/4 as your change factor will often match an answer option directly.
The fraction shortcut in practice When an exam question has fractional answers, write the change factor as a fraction (e.g. 5/4, 3/2, 7/5) rather than converting to a decimal. You will often find that fraction staring back at you in the answer choices, combined with the correct direction of change (increase or decrease). This can save significant time under exam pressure, and it eliminates the rounding errors that come from working with recurring decimals.

If none of the answers contains your fraction, convert to a decimal and complete the calculation normally.

Physics Quick Quiz 2

Depth-to-depth calculations • 3-step-twice method

Physics — Topics

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