ID brings up a interesting question, which has me wondering...
I'm an engineer (don't tell joewr) and I have a very, very narrow question to ask; let's see if I can do it in such a way that I can be clear on what I'm really asking.
First we'll simplify. A pure O2 rebreather diver (so we don't have to worry about dilutent) goes to 10 feet and sits there breathing their cylinder dry. Let's say we stop the test the moment the O2 cylinder reaches the point where it cannot inject any O2 into the loop, so the loop is *exactly* the same volume as when we started the test.
In other words I'm trying to remove loop gas volume from the equation completely.
The question is simple: Does the divers buoyancy change and if so, in what way? Note: I realize were probably talking very, very small changes here at best, but the interest is purely theoretical, given the much greater variable in the real world is the loop volume
On one hand were emptying the cylinder, and because the cylinder does not change volume, the system (diver, rebreather and cylinder) is becoming more buoyant.
But on the other hand, the divers body is metabolizing O2 and turning it into CO2, which is being chemically removed by the scrubber. So for every O2 molecule removed from the cylinder, were depositing a CO2 molecule in the scrubber, which also does not change in volume, so the scrubber is becoming more negative. Not only is it becoming more negative, but also its becoming more negative at a faster rate than the cylinder because for every O2 molecule removed from the cylinder, a CO2 molecule (an extra carbon atom) is being deposited in the scrubber.
So now we have the conclusion that a rebreather diver becomes more negative over the dive.
But where does that carbon atom come from? From the diver. Since a diver is a liquid for all intents and purposes and the removal of matter *will* cause a liquid to decrease in volume, we have in addition to the decreasing buoyancy of the external system (cylinder and scrubber/loop), we have the displacement of the diver decreasing as well, as well as their mass. So the divers buoyancy change may be a wash.
So if the divers buoyancy change over the dive is a wash, along with the buoyancy loss of the cylinder/scrubber system, it seems that during a dive a rebreather diver actually loses buoyancy.
Is this correct, or am I full of it?
Roak