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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 -10C (14F).

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 36C.

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

  1. Observation (or decompression) in 6m (~20ft)
  2. Easy dive in 30m (~90ft)
  3. 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

"-10C"-Regulator

40   W + 44.8W

100W + 44.8W

205W + 44.8W

Dual-Hose (+10C)

22.5W + 44.8W

  56W + 44.8W

116W + 44.8W

Condensation-Type- Rebreather (10C, 100% humidity)

22.5W + 35.4W

  56W + 35.4W

116W + 35.4W

Thermal neutral Rebreather (36C, 100% humidity)

0 W

0 W

0 W

Rebreather run at 46C, 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(+10C) ~ 0.0123bar this means 0.00998g/l or   22.5 Joule/l (Breathing-Gas)
ppH2O(+36C) ~ 0.06    bar this means 0.04772g/l or 107.5 Joule/l
ppH2O(+42C) ~ 0.082  bar this means 0.06592g/l or 148.5 Joule/l
ppH2O(+46C) ~ 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 10C

-0.057 Liter

0.107 Liter

Rebreather, inhalation-temperature 42C

+0.0273Liter: Hydration

0.023 Liter

Rebreather, inhalation-temperature 46C

+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

 

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http://Rebreather.de/rebreather/thermal_design.html © Karl Kramer, 14.10.1998