IDC Guide

Dive Theory –

More Physics of Diving

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PADI IDC Theory: Physics – Pressures Underwater

The Basics

Calculating pressures in sea water is essential. Without this knowledge you’ll really struggle with some PADI Physics exam questions, ones like where they ask you what size a balloon will be when you take it to 30 metres / 99 feet.

The good news is that it’s easy!

How it’s done

For every 10 metres or 33 feet, the water (or gauge) pressure changes by 1 atm.
The total pressure at depth includes both gauge and atmospheric pressure.
So, pressure at 10 metres or 33 feet = 1 atm for the gauge pressure 1 atm = 2 atm.

This might help

For every one metre in sea water, pressure changes by 0.1 atm. Example: at 23 metres pressure is: 23 X 0.1 = 2.3 atm for gauge, +1 = 3.3 for absolute.

For every foot of sea water, pressure changes by 0.0303 atm. Example at 76 feet pressure is 76 X 0.0303 = 2.3 atm for gauge + 1 = 3.3 for absolute

Fresh Water
In fresh water, it’s much the same, but for metric use 0.097 for every metre, and in imperial use 0.0294 for every foot.

PADI IDC Theory: Physics – Effects of Pressure

The Basics

These questions usually make up around 40% to 50% of all PADI IDC Physics exams. Anything to do with changes at depth: Balloons, Breathing rate, Density, Partial Pressure of gasses, How much air needs to be pumped from the surface etc.

How it’s done
As you get deeper, some things get smaller (e.g volumes) and some things get larger (e.g.density, breathing rates)

First, work out whether the answer will be larger or smaller. That will tell you whether you’ll multiply or divide.

Please don’t over complicate Physics. It’s easier than you think.

PADI IDC Physics – Displacement

The Basics

This subject is much simpler than some instructors make it out to be.

How it’s done

In fresh water, it’s often as simple as taking one number (displaced water) from another (weight).

In salt water, first divide the weight by 1.03, and then go ahead as with fresh.

Multiply the cubic feet of displaced water by 62.4 in fresh water, or 64 in sea water, then deduct that number from the weight of the object. Divide that number by either 62.4 or 64 depending on fresh or sea to get the answer in cubic feet.

You multiply (x) the volume of the air the diver uses when the diver descends (goes down deeper) You divide (/) the volume of the air the diver uses when the diver ascends (comes up shallower)

More Dive theory that I will get categorised shortly.

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