General Knowledge — PADI IDC Physics
Physics — Topics
Charles's Law
Will explains how temperature changes affect the pressure inside a scuba tank, and what this means for your air supply at the start of a dive.
Temperature and pressure are directly linked inside a sealed container. Increase the temperature and the pressure rises. Decrease the temperature and the pressure falls. For a scuba tank, the rate is 0.6 bar for every 1°C change.
Temperature up → pressure up. Temperature down → pressure down.
The most common exam scenario is a tank sitting in the sun on a hot day, then used in cold water. The tank pressure drops before the diver has used a single breath.
- Surface temperature: 30°C. Water temperature: 0°C. Temperature change: 30°C
- 30 × 0.6 = 18 bar expected pressure drop
- On a 200 bar fill, 18 bar is close to 10% of the tank
The practical implication: if a significant temperature drop is expected, consider reducing your turn pressure slightly to account for the loss due to temperature rather than air consumption.
Sound Underwater
Will covers why sound travels faster in water than in air, why divers cannot determine the direction of a sound, and how thermoclines can resist sound transmission.
Sound travels faster through denser, more elastic mediums. Because water is both denser and more elastic than air, sound travels four times faster in water than in air.
Direction Confusion
The brain determines the direction of a sound by detecting the tiny delay between the sound reaching one ear and then the other. In air, this delay is small but measurable. Underwater, sound travels so fast that the delay between ears becomes insignificant — both ears receive the signal almost simultaneously. The brain cannot calculate a direction from this, and divers typically perceive the sound as coming from above.
Thermoclines and Sound Transmission
Warm water is less dense than cold water. Where two layers of different temperature meet — a thermocline — there is an abrupt change in density. This density boundary can resist the transmission of sound, reducing or distorting the signal passing through it.
Pressure Gradients
Will explains what a tissue gas pressure gradient is, why an excessive gradient causes DCS, and how safety stops and decompression stops reduce that gradient.
A gradient is a rate of change — how steep a slope is. In a diving context, the tissue gas pressure gradient is the difference between the pressure of nitrogen dissolved in a diver's tissues and the ambient pressure surrounding the diver.
Consider a diver ascending from 30 metres. At that depth, tissues may have absorbed nitrogen at a pressure equivalent to 4 atm. If the diver ascends at maximum rate directly to the surface, the difference between tissue nitrogen pressure and ambient pressure changes rapidly — a steep gradient. A steep gradient may cause bubble formation and DCS.
How a Safety Stop Reduces the Gradient
A three-minute stop at 5 metres extends the time of the ascent, reducing the rate at which the pressure difference changes. The slope becomes less steep — the gradient is reduced. Decompression stops work on the same principle, holding the diver at depth long enough for excess nitrogen to off-gas at a controlled rate before the ambient pressure drops further.
The decompression planning application of pressure gradients — including M-values, half-times, and dive computer algorithms — is covered in full on the decompression theory hub.
Henry's Law
Will covers Henry's Law and the open container question type — what happens to dissolved gas in a liquid when surrounding pressure increases or decreases.
Increase the pressure on a liquid and more gas dissolves into it. Decrease the pressure and gas comes out of solution. This is Henry's Law, and it underpins everything that happens to nitrogen in a diver's tissues.
Pressure down → gas comes out of solution.
The Open Container Question
The exam presents this as a scenario involving a glass of water, a mug, or any open liquid-filled container placed inside a pressure chamber. The question asks what happens to the dissolved gas when pressure changes.
- Pressure increased: the amount of gas dissolved in the liquid increases
- Pressure decreased: the amount of gas dissolved in the liquid decreases
Henry's Law is also why nitrogen dissolves into tissues at depth and must be managed carefully on ascent. That clinical application is covered on the decompression theory hub.
Altitude After Diving
Will explains why driving to altitude after diving steepens the tissue gas pressure gradient and increases the risk of decompression sickness.
Atmospheric pressure decreases with altitude. At sea level it is 1 atm. At 10,000 ft (approximately 3,000 m) it falls to 0.8 atm. This reduction in surrounding pressure is the physics behind why altitude is dangerous after diving.
10,000 ft / 3,000 m: 0.8 atm
After a dive, tissues may hold dissolved nitrogen at a pressure above ambient — for example, 1.4 atm. At sea level, the gradient between tissue nitrogen (1.4 atm) and surrounding pressure (1 atm) is 0.4 atm. Driving to altitude where surrounding pressure is only 0.8 atm steepens that gradient to 0.6 atm. The steeper the difference between tissue gas pressure and ambient pressure, the greater the risk of bubble formation and DCS.
- After dive: tissue nitrogen pressure = 1.4 atm
- At sea level: ambient = 1.0 atm. Gradient = 0.4 atm
- Driving to 10,000 ft: ambient = 0.8 atm. Gradient = 0.6 atm
This page covers the physics of the pressure change. PADI procedures for flying and altitude diving — including minimum surface intervals — are covered on the decompression theory hub.
Heat Loss and Thermal Properties
Water conducts heat away from the body approximately 20 times faster than air. Two distinct thermal properties explain why — thermal conductivity and heat capacity — and both are tested in the physics exam.
Mechanisms of Heat Loss
There are three mechanisms by which a diver loses heat:
- Conduction — direct transfer of heat through contact between the diver's skin and the surrounding water. This is the primary mechanism of heat loss for a diver.
- Convection — moving water carries the warmed layer away from the body and replaces it with cooler water, maintaining the temperature gradient and accelerating loss.
- Radiation — electromagnetic emission of heat energy. The least significant mechanism in water.
Thermal Conductivity vs Heat Capacity
Thermal conductivity is how quickly heat transfers through a substance. Water's high conductivity is why it pulls heat away from the body so fast.
Heat capacity is how much heat a substance can absorb before its own temperature rises. Water has one of the highest heat capacities of any naturally occurring substance. The water surrounding a diver can absorb an enormous amount of body heat without becoming noticeably warmer — the temperature gradient between skin and water stays steep, which sustains the rate of heat loss.
High heat capacity: water absorbs that heat without warming up, so the gradient stays steep and conduction continues at a high rate.
Argon as a Drysuit Inflation Gas
Inside a drysuit, the gas layer between skin and suit acts as a thermal insulator. The effectiveness of that insulation depends on the thermal conductivity of the gas — the lower the conductivity, the slower heat escapes through it.
| Gas | Thermal Conductivity | Effect on Diver |
|---|---|---|
| Argon | Low (~0.016 W/m·K) | Excellent insulator — retains body heat effectively |
| Air / Nitrogen | Moderate (~0.026 W/m·K) | Adequate insulation for most recreational diving |
| Helium | High (~0.150 W/m·K) | Very poor insulator — transfers heat away rapidly |
Argon's thermal conductivity is approximately 38% lower than air, making it a significantly better insulator. Technical divers breathing trimix (which contains helium) cannot use their back gas to inflate a drysuit — helium's high conductivity causes rapid heat loss. Argon is the standard alternative.
Nitrox is also unsuitable as a drysuit inflation gas for trimix divers because of isobaric counterdiffusion, where the nitrogen diffuses into tissues at a different rate than helium diffuses out, potentially causing subcutaneous bubbles.
The full treatment of heat loss mechanisms and thermal protection equipment is on the pressure fundamentals page.
Colour Absorption
Water absorbs light selectively. The red end of the spectrum is absorbed first, at relatively shallow depths. As depth increases, progressively more colours are lost until only blue and blue-green wavelengths remain.
Colours that contain red — including orange, purple, and brown — lose their red component at depth and shift toward grey or blue. A red object at depth appears grey or black because the red wavelengths that define it have already been absorbed by the water above.
The full treatment of colour absorption, along with refraction and visual distortion underwater, is on the pressure fundamentals page.
Physics — Topics