Scuba Diving Physics: How Pressure Changes Underwater and Why It Creates Both Rules and Risks

At 10 metres underwater, pressure doubles — and every additional 10 metres adds another atmosphere

Scuba diving is applied pressure physics. As you descend, the water column above you exerts increasing pressure on your body and on every gas-filled space: your lungs, sinuses, ears, and the compressed air in your tank. Understanding Boyle's Law — the relationship between pressure and gas volume — explains every phenomenon that makes diving both manageable and potentially dangerous.

The pressure increase with depth

At sea level, atmospheric pressure is approximately 1 bar (1 atm, 14.7 PSI). Water is approximately 800 times denser than air. Each additional 10 metres of water adds approximately 1 bar of pressure.

Absolute pressure at depth:

Boyle's Law: at constant temperature, pressure × volume = constant (P₁V₁ = P₂V₂)

A 1-litre air bubble at the surface (1 bar) compressed at 10m depth (2 bar): volume = 0.5 litres. At 30m (4 bar): 0.25 litres.

The same bubble ascending from 30m to surface: it expands 4× in volume.

The squeeze and barotrauma on descent

When diving, any air-filled space in the body must equalise pressure or the pressure differential creates pain and injury ("barotrauma" or "squeeze").

Ears: the middle ear connects to the throat via the Eustachian tube. If you descend without equalising, the external water pressure exceeds the middle ear pressure — pushing the eardrum inward. The solution: "equalising" by gently blowing against a pinched nose (Valsalva manoeuvre) to open the Eustachian tube and admit higher-pressure air into the middle ear.

Sinuses: similar principle — sinus passages must be clear for equalisation. A head cold makes diving inadvisable because congested sinuses can't equalise.

Lungs: the most dangerous squeeze — if a diver holds their breath on a breath-hold dive (freediving), descending compresses the lung volume significantly. At 10m, lung volume halves. Below approximately 30–40m for most people, lung volume approaches residual volume and further compression can cause lung squeeze (pulmonary barotrauma).

Scuba tanks: are pressurised to 200–300 bar and provide air at the ambient pressure on demand, so the diver always breathes air at the surrounding water pressure — the regulator automatically adjusts.

Nitrogen narcosis: depth and partial pressure

At depth, the partial pressure of each gas in the breathing mix increases proportionally:

At 30m (4 bar), breathing air (21% oxygen, 79% nitrogen):

• Partial pressure of oxygen: 0.21 × 4 = 0.84 bar
• Partial pressure of nitrogen: 0.79 × 4 = 3.16 bar

High partial pressure of nitrogen causes nitrogen narcosis — a reversible alteration of consciousness sometimes described as feeling drunk. Typically noticeable below 30m, problematic for some divers below 40m.

Helium (used in Trimix diving for deep dives) does not cause the same narcotic effect, which is why technical deep divers breathe Trimix (oxygen/helium/nitrogen) to manage narcosis at depths below 40m.

Decompression sickness (the "bends")

At depth, nitrogen dissolves into body tissues under elevated partial pressure (Henry's Law: dissolved gas ∝ partial pressure). This dissolved nitrogen is harmless at depth, but must come out of solution slowly on ascent.

The problem: ascending too quickly causes nitrogen to form bubbles in tissues and blood — the same process as opening a carbonated drink bottle quickly. These bubbles cause decompression sickness (DCS) — the "bends."

Symptoms:

• Joint pain ("the bends" proper) — most common
• Skin rash/mottling
• Neurological symptoms — numbness, tingling, paralysis
• Pulmonary DCS — chest pain, breathing difficulty
• Inner ear DCS — vertigo, hearing loss

Prevention:

• Ascent rate limit: typically 9–18 metres per minute maximum
• Safety stop: 3–5 minutes at 5 metres before surfacing
• No-decompression limits (NDLs): maximum bottom times at each depth that allow safe direct ascent without mandatory decompression stops
• Decompression computers calculate real-time nitrogen loading in different tissue compartments

Treatment: recompression in a hyperbaric chamber — pressurising the patient to "re-dissolve" the nitrogen bubbles, then allowing controlled slow decompression.

Recreational vs technical diving limits

Recreational diving:

• Maximum depth: 40 metres (PADI, BSAC standard)
• No mandatory decompression stops (NDL diving only)
• Single gas (air or enriched air nitrox)
• Direct ascent to surface always possible

Technical (tec) diving:

• Depths beyond 40m
• Uses mandatory decompression stops at specific depths
• Requires multiple gas switches (different mixes for descent, bottom, and decompression)
• Trimix for narcosis management
• More equipment, more training, more risk

How to use the Pressure Converter on sadiqbd.com

1. Convert between bar, PSI, atm, kPa — diving equipment specifications use different units by manufacturer region
2. Calculate depth pressure — depth in metres + 1 = pressure in bar
3. Convert tank pressures — SCUBA tanks rated at 200 bar = 2,900 PSI = 20,000 kPa

Frequently Asked Questions

Why do divers fly after diving? Aircraft cabin pressure is lower than sea level (typically equivalent to 1,800–2,400m altitude). If you fly while still off-gassing nitrogen from a dive, the reduced cabin pressure can precipitate bubble formation. Standard guidance: no flying for 12 hours after a single no-decompression dive; 18+ hours after multiple dives or dives requiring decompression stops.

What is enriched air nitrox and why do recreational divers use it? Nitrox contains a higher percentage of oxygen than air (typically 32% or 36% vs air's 21%), with correspondingly less nitrogen. Less nitrogen means slower nitrogen loading at the same depth, extending no-decompression limits — particularly useful for repetitive diving (multiple dives per day). The trade-off: higher oxygen partial pressures at depth, requiring a maximum operating depth to avoid oxygen toxicity.

Is the Pressure Converter free? Yes — completely free, no sign-up required.

Try the Pressure Converter free at sadiqbd.com — convert between PSI, bar, atm, kPa, and mmHg for diving, engineering, and medical contexts.