When you dive OC instead of RebreatherSCUBA you inhale a gas that has a humidity close to zero and is quite cold. Dualhose 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 breathinggas 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 divesituations, all diving in 10°C (50°F) warm water and all breathing 25 liter/minute.
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 
DualHose (+10°C) 
22.5W + 44.8W 
56W + 44.8W 
116W + 44.8W 
CondensationType 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 
The diver receives 
The diver receives 
Thermal capacity:
Cp( Air ) ~ 0.79*Cp(N_{2}) + 0.21*Cp(O_{2}) ~ 1.298 Watt*sec/(bar*Liter*Kelvin)
Cp(Heliox10) = 0.9*Cp(He) + 0.1*Cp(O_{2}) ~ 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 BreathingGas):
ppH_{2}O(+10°C) ~ 0.0123bar this means 0.00998g/l or 22.5 Joule/l (BreathingGas)
ppH_{2}O(+36°C) ~ 0.06 bar this means 0.04772g/l or 107.5 Joule/l
ppH_{2}O(+42°C) ~ 0.082 bar this means 0.06592g/l or 148.5 Joule/l
ppH_{2}O(+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 OCDive you loose 0.1 Liter Water per 1.4 hours because of Evaporation, when you breath 25 l/min.
Evaporationbalance (breathing 25 l/min for an oxygenconsumption of 1 l/min):
Dehydration/hour 
Condensation/hour 

OCDive 
0.072 Liter 
exhausted 
Rebreather, inhalationtemperature 10°C 
0.057 Liter 
0.107 Liter 
Rebreather, inhalationtemperature 42°C 
+0.0273Liter: Hydration 
0.023 Liter 
Rebreather, inhalationtemperature 46°C 
+0.05 Liter: Hydration 
Zero 
That Calculations assume that the CO2 is absorbed by breathinglime and the reactionproduct 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.
They show what
http://Rebreather.de/rebreather/thermal_design.html © Karl Kramer, 14.10.1998