Karl´s Rebreather Page

Thermal Aspects of Rebreather-Design

When you hear about the advantages of rebreather-diving one of the first things you hear about is the warm, humid breathing-gas.

When you dive OC- instead of Rebreather-SCUBA you inhale a gas that has a humidity close to zero and is quite cold. Dual-hose Regulators deliver a gas that is nearly warmed up to the temperature of the surrounding water, other regulators deliver it quite colder, for example with -10°C (14°F).

So warming up the breathing-gas can be a remarkable factor of "workload" for the diver, because no matter what he inhales he exhales with 100%humidity at about 36°C.

Let's compare the situation for 3 typical dive-situations, all diving in 10°C (50°F) warm water and all breathing 25 liter/minute.

Observation (or decompression) in 6m (~20ft)
Easy dive in 30m (~90ft)
Scientific exploration in 100m (~300ft) (breathing Heliox with 10% Oxygen)

Approximated Heating Power spend by the diver to breath 25 l gas per minute (in order to consume 1bar*l/min oxygen):

	6m	30m	100m
"-10°C"-Regulator	40 W + 44.8W	100W + 44.8W	205W + 44.8W
Dual-Hose (+10°C)	22.5W + 44.8W	56W + 44.8W	116W + 44.8W
Condensation-Type- Rebreather (10°C, 100% humidity)	22.5W + 35.4W	56W + 35.4W	116W + 35.4W
Thermal neutral Rebreather (36°C, 100% humidity)	0 W	0 W	0 W
Rebreather run at 46°C, 100% humidity	The diver receives 8.65W + 31.5W	The diver receives 21.6W + 31.5W	The diver receives 44.5W + 31.5W

Thermal capacity:
Cp( Air )     ~  0.79*Cp(N₂) + 0.21*Cp(O₂) ~ 1.298 Watt*sec/(bar*Liter*Kelvin)
Cp(Heliox10) = 0.9*Cp(He) + 0.1*Cp(O₂) ~ 0.9705 Watt*sec/(bar*Liter*Kelvin)
    6m ~   1.6bar : 1.6bar*Cp(Air)      *    25Liter/60sec ~ 0.8653 Watt/Kelvin
  30m ~   4   bar:   4bar *  Cp(Air)     *    25Liter/60sec ~ 2.163   Watt/Kelvin
100m ~ 11   bar: 11bar *  Cp(Heliox10)*25Liter/60sec ~ 4.448   Watt/Kelvin

Evaporation (100% relative Humidity, Liter here measures the volume of Breathing-Gas):
ppH₂O(+10°C) ~ 0.0123bar this means 0.00998g/l or   22.5 Joule/l (Breathing-Gas)
ppH₂O(+36°C) ~ 0.06    bar this means 0.04772g/l or 107.5 Joule/l
ppH₂O(+42°C) ~ 0.082  bar this means 0.06592g/l or 148.5 Joule/l
ppH₂O(+46°C) ~ 0.1      bar this means 0.08123g/l or 183    Joule/l

1Joule = 1Watt*sec
107.5Wsec/l * 25l / 60sec ~ 44.8W

0.04772g/l*25l/min*60min ~ 72g/h which means 0.072 Liter Water per Hour
This means during an OC-Dive you loose 0.1 Liter Water per 1.4 hours because of Evaporation, when you breath 25 l/min.

Evaporation-balance (breathing 25 l/min for an oxygen-consumption of 1 l/min):

	Dehydration/hour	Condensation/hour
OC-Dive	-0.072 Liter	exhausted
Rebreather, inhalation-temperature 10°C	-0.057 Liter	0.107 Liter
Rebreather, inhalation-temperature 42°C	+0.0273Liter: Hydration	0.023 Liter
Rebreather, inhalation-temperature 46°C	+0.05 Liter: Hydration	Zero

That Calculations assume that the CO2 is absorbed by breathing-lime and the reaction-product water leaves the lime as steam (if the gas is warm enough). Hydration means that the diver does not loose water by breathing, he gets 'water' because he breaths a quite humid warm steam.

So, what do all those figures mean?

They show what

using a proper designed rebreather means a serious way of thermal protection for the diver.
rebreather-divers have fewer problems with Dehydration.
diving a rebreather in water what is colder or deeper than the engine was mainly designed for means additional condensation-water and so can cause the need of shorter dives as well as earlier change of breathing-lime.
(The more cooling by colder Input-Gas or by more thermal Capacity (for example by higher Pressure of the "Cooling-Gas") per CO2 Mol the more Problems with slower Reaction by lower Temperature plus by Condensation-Water drowning the Scrubber arise.)

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