FRESH water Physics

Physics — Topics

Pressure Depth Calcs Density Partial Pressure Buoyancy Lift Bags Fresh Water General Knowledge
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Fresh water pressure calculations — practical exam method

Will Welbourn explains the practical exam method for fresh water pressure questions — the approach that works 9 times out of 10.

Why Fresh Water Pressure Is Different

Pressure at depth comes from the weight of water above you. The heavier the water, the higher the pressure. Salt water and fresh water weigh different amounts — and that single fact is the source of every fresh water question in the PADI physics exam.

The core difference 1 litre of salt water weighs 1.03 kg. 1 litre of fresh water weighs exactly 1.00 kg. Fresh water is approximately 3% less dense than salt water. At any given depth, the column of fresh water pressing down on you weighs slightly less — so the pressure is slightly less.

The practical consequence is that fresh water pressure values are not round numbers. At 10 m in salt water, absolute pressure is a clean 2 atm. At 10 m in fresh water, it is 1.97 atm. These non-round values make it harder to work with fresh water numbers directly — which is exactly why the practical exam method below exists.

Salt Water vs Fresh Water — Pressure Reference Table

Use this table as a reference for both the practical method questions and the exact calculation questions. Salt water pressures are the round numbers you already know. Fresh water pressures are approximately 3% lower at every depth.

Depth SW Absolute (atm) FW Absolute (atm) SW Gauge (atm) FW Gauge (atm)
Surface 1.0 1.0 0 0
10 m 2.0 1.97 1.0 0.97
20 m 3.0 2.94 2.0 1.94
30 m 4.0 3.91 3.0 2.91
40 m 5.0 4.88 4.0 3.88
What the table tells you Salt water: 1 litre = 1.03 kg. Fresh water: 1 litre = 1.00 kg. Fresh water pressures are approximately 3% lower at any given depth. At the surface, atmospheric pressure is identical in both water types — both start at 1.0 atm absolute, 0 atm gauge.

The table is a reference — you do not need to memorise the values in it. What you need to be able to do is calculate them. Here is how two of those table values were found, and how to find the same answer for any depth, including the non-round depths that come up in exam questions.

How the table values are calculated — two examples
  1. 10 m: SW absolute = 2.0 atm. Subtract 1 = 1.0. Divide by 1.03 = 0.97. Add 1 back = 1.97 atm absolute in fresh water
  2. 30 m: SW absolute = 4.0 atm. Subtract 1 = 3.0. Divide by 1.03 = 2.91. Add 1 back = 3.91 atm absolute in fresh water
Every value in the table was found the same way. The method, not the table, is what to learn.

Non-Round Depths

Exam questions often use depths that do not fall on a clean 10 m interval. The method is identical — the only extra step is reading off the salt water absolute pressure from between two chart values.

Worked Example — Absolute pressure at 15 m fresh water
  1. Find SW absolute: 15 ÷ 10 = 1.5 + 1 = 2.5 atm
  2. Subtract 1: 2.5 − 1 = 1.5
  3. Divide by 1.03: 1.5 ÷ 1.03 = 1.456
  4. Add 1 back: 1.456 + 1 = 2.46 atm absolute in fresh water
Fresh water at 15 m: 2.46 atm absolute. Gauge = 2.46 − 1 = 1.46 atm.
Worked Example — Absolute pressure at 25 m fresh water
  1. Find SW absolute: 25 ÷ 10 = 2.5 + 1 = 3.5 atm
  2. Subtract 1: 3.5 − 1 = 2.5
  3. Divide by 1.03: 2.5 ÷ 1.03 = 2.427
  4. Add 1 back: 2.427 + 1 = 3.43 atm absolute in fresh water
Fresh water at 25 m: 3.43 atm absolute. Gauge = 3.43 − 1 = 2.43 atm.

Exact Method vs the Quick and Dirty — Side by Side

The quick and dirty — taking the salt water absolute pressure and dividing the whole thing by 1.03 without isolating the water column first — gives a slightly lower number than the correct answer at every depth. The comparison below shows what you get from each method, and how far apart they are.

Depth SW Absolute Exact FW Absolute Quick and Dirty (÷ 1.03 only) Difference
10 m 2.0 atm 1.97 atm 1.94 atm 0.03 atm
15 m 2.5 atm 2.46 atm 2.43 atm 0.03 atm
25 m 3.5 atm 3.43 atm 3.40 atm 0.03 atm
30 m 4.0 atm 3.91 atm 3.88 atm 0.03 atm
Why the quick and dirty is always 0.03 atm low The quick and dirty divides the entire salt water absolute pressure by 1.03 — including the 1 atm of atmospheric pressure, which is the same in both water types and should never be adjusted. Dividing 1 by 1.03 gives 0.97 instead of 1, so the quick and dirty always underestimates by exactly 0.03 atm, at any depth. In a multiple-choice exam this 0.03 atm gap is small enough that both the exact answer and the quick and dirty answer will point you to the same option.

The Practical Exam Method

If a question is in fresh water and involves a calculation, your best option is usually the quick and dirty method: take the salt water absolute pressure and divide by 1.03, then use that number in your calculation. As shown above, this gives a result that is only 0.03 atm off the correct answer — close enough to identify the right option in any multiple-choice question.

This approach is a last resort — useful for students who find the multi-step arithmetic genuinely difficult in exam conditions. If you are comfortable with the exact formula or the quick and dirty method described above, use those instead. The practical method removes all fresh water arithmetic entirely: ignore the water type, solve using clean salt water numbers, then adjust your answer.

The rule Solve the question as though it is in salt water, using your standard pressure chart. Then, because the question is fresh water, pick the answer option that is slightly less than your calculated salt water result. This works approximately 9 times out of 10 in the multiple-choice format.

Worked Example 1 — Gauge Pressure

Worked Example 1 — What is gauge pressure at 20 m fresh water?
  1. Solve for salt water: gauge at 20 m = 2 atm
  2. Question says fresh water → quick and dirty: 2 ÷ 1.03 = 1.94 atm
Answer: 1.94 atm.

Worked Example 2 — Density Question

Worked Example 2 — How many times denser is the air at 30 m fresh water compared to the surface?
  1. Solve for salt water: absolute pressure at 30 m = 4 atm, so air is 4× denser
  2. Question says fresh water → quick and dirty: 4 ÷ 1.03 = 3.88
Answer: 3.91. The quick and dirty gives 3.88 — the correct answer is 3.91, but both values point to the same option in the multiple-choice exam.

Worked Example 3 — Container Fill at Depth

Worked Example 3 — How much air must be pumped from the surface to fill a 20 L container at 40 m fresh water?
  1. 40 m in salt water = 5 atm. Quick and dirty FW pressure: 5 ÷ 1.03 = 4.85 atm
  2. Air compresses as it descends, so you need to pump more. Multiply: 4.85 × 20 = 97 L
Answer: 98 L. The quick and dirty gives 97 L — the closest option in the exam is 98 L.

Worked Example 4 — Tank Duration

Worked Example 4 — If a diver takes 120 minutes to breathe through a tank at the surface, how long will the same tank last at 20 m fresh water?
  1. 20 m in salt water = 3 atm. Quick and dirty FW pressure: 3 ÷ 1.03 = 2.91 atm
  2. At depth you breathe faster, so the tank lasts less time. Divide: 120 ÷ 2.91 = 41.2 minutes
Answer: 41 minutes.
Note — if using the last-resort method on this type of question Tank duration questions are the one case where the last-resort method requires extra thought. Solving in salt water gives 120 ÷ 3 = 40 minutes, but you divided to get there — so you pick slightly more than 40 (not slightly less). The quick and dirty avoids this entirely: use 2.91 atm as your fresh water pressure and divide normally. You get 41.2, which immediately points you to 41 minutes with no guesswork about direction.
Summary of the practical method 1. Solve using salt water numbers. 2. If you multiplied → pick slightly less. 3. If you divided → pick slightly more. Reserve this for situations where the arithmetic is not working for you at all. The quick and dirty method above is simpler and more reliable.

The Exact Calculation Method

Exact calculation method for fresh water pressure

Will Welbourn works through the exact formula for absolute and gauge pressure at depth in fresh water, with a full worked example at 33 m.

The four-step formula Step 1: Find the absolute (ambient) pressure at that depth in salt water. Step 2: Subtract 1 — this isolates the water column pressure (the 1 atm from the atmosphere above is the same in all water types). Step 3: Divide by 1.03 — this adjusts the water column for fresh water density. Step 4: Add 1 back — restore the atmospheric pressure component. The result is absolute pressure in fresh water.

To find gauge pressure in fresh water: subtract 1 from the fresh water absolute pressure, just as you would in salt water.

Worked Example — Absolute and gauge pressure at 33 m fresh water
  1. Find SW absolute at 33 m: 33 ÷ 10 = 3.3 + 1 = 4.3 atm
  2. Subtract 1: 4.3 − 1 = 3.3 atm (water column pressure only)
  3. Divide by 1.03: 3.3 ÷ 1.03 = 3.2 atm (fresh water column)
  4. Add 1 back: 3.2 + 1 = 4.2 atm absolute
  5. Gauge pressure: 4.2 − 1 = 3.2 atm gauge
Fresh water at 33 m: 4.2 atm absolute, 3.2 atm gauge.

The most common mistake is dividing the entire salt water pressure by 1.03 without isolating the water column first — at 33 m: 4.3 ÷ 1.03 = 4.17 atm instead of 4.2. As the comparison table above shows, this is exactly what the quick and dirty formalises: technically wrong, always 0.03 atm low, but close enough to identify the right option in any PADI multiple-choice question.

Where Fresh Water Appears Across the Physics Exam

Fresh water is not a single topic in the PADI physics exam — it appears as a modifier across multiple question types. Understanding how it affects each area lets you spot fresh water questions quickly and apply the right approach.

Topic area How fresh water applies Method
Pressure questions Absolute and gauge pressure are approximately 3% lower at any given depth Exact formula or quick and dirty method (this page)
Density questions Fresh water density = 1.00 kg/L vs salt water 1.03 kg/L See the density page
Buoyancy questions Objects neutrally buoyant in salt water become negatively buoyant in fresh water — same volume displaces less weight See the buoyancy page
Lift bag questions In fresh water, 1 litre of water displaced = 1 kg of upward force exactly — no division by 1.03 needed See lift bag calculations
Depth-to-depth calculations Practical method applies — use salt water pressures to solve, then adjust the answer See the depth calculations page
Temperature and tank pressure Not water-type-dependent — Charles's Law applies equally in salt or fresh water See general knowledge
The single most important habit Whenever you finish calculating an answer in the physics exam, ask yourself: what type of water does the question say? If it says fresh water and you used salt water numbers to solve it, check whether your answer needs adjusting before you mark it. This check takes three seconds and catches the most common error on the paper.

For more practice on all fresh water question types, work through the physics practice questions.

Fresh Water Physics — Practice Questions

Physics — Topics

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