Subsurface mobile general question

[Robert Helling]

Hi Poltsi,

> On 15. Nov 2021, at 12:18, Paul-Erik Törrönen via subsurface <subsurface at subsurface-divelog.org> wrote:
> 
> So ~21min 30s during which the O2 cyl (2l) pressure dropped 25bar (50l). My metabolic rate for O2 consumption during the bottom time was ~0.6l/min so that means that I vented out about 46l of O2 during the ascend. More realistically, the metabolic rate would be something like 1l/min during ascend due to all the excitement, so ~30l of O2 vented out/lost from the O2-cylinder.

thanks a lot for that explanation. That makes a lot of sense. Would you agree with the following summary:

Summed over the whole dive, the total amount of gas dropped from the loop roughly equals the total amount of diluent used (you added diluent on the descent to maintain loop volume and dropped it on the way up). However, the gas that you drop is _not_ composed as the diluent but rather as the breathing gas (which is determined in terms of the set point). The latter has higher O2 fraction, so you actually also drop O2 from the O2 cylinder. How much? You could compute that if you knew the set point and the depth at which you drop the gas (which we don’t since that is not recorded). But a first guess would be that happens in even amounts during the ascent.

Let’s do an example calculation. Let’s say, you are using air as diluent and constant set point sp.

Then at ambient pressure p, the gas you are breathing contains sp bar O2 and p-sp bar N2. The N2 comes from the air which broad with it 21/79 (p-sp), so the remaining sp - 21/79 (p-sp) = (1-21/79) sp - 21/79 p come from the O2 cylinder. The fraction of gas dropped that came from the O2 cylinder is thus (1-21/79) sp/p - 21/79.

Averaging that over the range pmax (deepest point) to psurface gives the fraction

f = (1-21/79) sp log(pmax/psurface)/(pmax - psurface)

So my guess would be that during the ascent you dropped f times the amount of diluent used during the dive of O2 that was coming from the O2 cylinder. So, to find the metabolic O2 consumption, that should be subtracted from the gas use from the O2 cylinder.

Does that make sense?

Best
Robert
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