Diver heat balance: will I get cold?
How much heat you lose during your dive, what cools you most, and the exposure protection that would change everything.
Your dive
The bottom phase, then the stop where you barely move and cool down fastest.
At depth
Your activity at depth
At the stop
Your exposure protection
It nearly always decides how comfortable you are.
Thickness
Your gas and you
Your gas consumption and build set the losses through breathing and skin.
Breathing gas
Cold tolerance
Fine settings
Water
Inhaled gas is assumed at water temperature. Choosing an activity resets heat production and consumption at depth to their typical values.
How it is calculated
The principle
Balance = body heat production − breathing losses − skin losses
Phase by phase, at steady state. A positive balance is released by the body (it is not stored for the stop). A negative balance comes out of your heat reserve: about 3.47 kJ per kg and per °C. Hence the drop in mean body temperature (core and skin combined).
Breathing
Warming the gas. Each litre inhaled at water temperature leaves at exhaled temperature. The number of molecules warmed grows with depth:
n = P × V ÷ (R × T); loss = n × Cp × (T exhaled − T gas)
Cp = 29.1 J/(mol·K) for O₂ and N₂, 20.8 for helium.
Humidifying it. Cylinder gas is dry and leaves saturated with water. The amount evaporated depends only on the volume breathed and the exhaled temperature, not on depth (≈ 30 W for 20 L/min).
Skin
Heat flows from the core (37 °C) to the water through layers in series: tissue under the skin (very insulating at rest, much less under exertion), suit, undersuit, water film.
Loss = Surface × (37 − T water) ÷ Resistance
Surface comes from your weight and height (Du Bois formula). "Feel the cold" or "Cold-resistant" lowers or raises your tissue insulation by 20 to 25 %.
Neoprene gets compressed. Its gas bubbles follow Boyle’s law:
Insulation(P) = surface insulation × (0.15 + 0.85 ÷ P)
About 58 % of the insulation remains at 10 m and 36 % at 30 m. A drysuit undersuit keeps its loft because gas is added on descent. Argon insulates slightly better than air (modest gain, ≈ 10 %).
Verdict thresholds
Less than 1 °C drop: cool but comfortable. From 1 to 2.5 °C: noticeably cold, shivering possible. Beyond: severely cold, risk of hypothermia. These are orders of magnitude, not a standard.
Limits
An educational estimate. It ignores shivering (which raises heat production), the condition of the suit, fatigue and wind at the surface. It replaces neither experience nor what you feel: at the first shiver, cut the dive short.
Sources for orders of magnitude: Du Bois (1916); Wolff et al. (1985) on neoprene conductivity; Hoke et al. (1976) on heliox breathing losses; Toner & McArdle on tissue insulation.
Mean body temperature at the end of the dive
−2.1°C
Noticeably cold
You will come out chilled, shivering is possible. Warmer protection or a shorter dive would help.
40 min · balance −546 kJ (−131 kcal) · Wetsuit 7 mm
At 30 m, your suit keeps only 36 % of its surface insulation: the neoprene is compressed.
Balance by phase
Production − losses.
What if you changed your protection?
Tap a row to try it.
The details
| Power (W) | Bottom | Stop | Total |
|---|---|---|---|
| Body heat production | 200 | 110 | |
| Gas warmed | −29 | −8 | |
| Humidification | −27 | −21 | |
| Skin, body | −337 | −200 | |
| Head and hands | −46 | −27 | |
| Breathing losses | −57 | −29 | |
| Skin losses | −383 | −228 | |
| Balance | −239 | −146 |
| Energy | Bottom | Stop | Total |
|---|---|---|---|
| Expenditure (kJ) | 420 | 33 | 453 |
| Expenditure (kcal) | 100 | 8 | 108 |
| Heat balance (kJ) | −502 | −44 | −546 |
| Heat balance (kcal) | −120 | −11 | −131 |
| Reference values | Bottom | Stop | Total |
|---|---|---|---|
| Absolute pressure (bar) | 4.03 | 1.52 | |
| Suit insulation (m²·K/W) | 0.040 | 0.080 | |
| Total conductance (W/°C) | 16.6 | 10.8 | |
| Estimated skin (°C) | 22.9 | 26.2 |
Body surface 1.90 m² · core 37 °C · exhaled gas 34 °C. Energy expenditure counts all the heat produced; the balance is what is missing (or left over).
Understanding cold in diving
Why divers get cold
Water conducts heat about 25 times better than air. The body loses heat through the skin, across the suit, and through breathing: every breath warms and humidifies a cold, dry gas. As long as heat production (exertion) keeps up, all is well; at the stop, barely moving, the balance quickly turns negative.
Neoprene loses insulation at depth
The gas bubbles in neoprene are compressed by pressure, as Boyle’s law predicts: a wetsuit keeps only about 58 % of its insulation at 10 m and 36 % at 30 m. A drysuit keeps its insulation because gas is added on descent. For the pressure at each depth, see the converter.
Choosing protection by water temperature
Orders of magnitude: 3 mm wetsuit above 25 °C, 5 mm from 20 to 25 °C, 7 mm from 15 to 20 °C, semi-dry or drysuit from 10 to 15 °C, drysuit below. Hood and gloves matter a lot: head and hands are about 12 % of the skin. On trimix, breathing takes a much larger share of the heat: see also the trimix blend tool.
Frequently asked questions
Which suit for 14 °C water?
A 7 mm semi-dry or a drysuit. For 35 min at 30 m at an easy pace with a 5 min stop, a 7 mm wetsuit with hood and gloves gives a drop of about 2 °C in mean body temperature: noticeably cold.
Why do you get colder at the safety stop?
You barely move, so you produce less heat (about 110 W at rest versus 200 W on an easy tour), while skin losses go on. Even in slightly warmer water, the balance often turns negative.
Should you inflate your drysuit with argon?
Argon conducts heat 32 % less than air, but the felt gain stays modest (about 10 % more insulation). Never inflate a drysuit with trimix, though: helium conducts heat very well.
How many calories does a dive burn?
It depends on the effort: about 110 W still, 200 W on an easy tour, 320 W finning steadily. A 40 min easy dive burns about 480 kJ, i.e. 115 kcal.