Quote: (Originally Posted by
Freef)
Not quite true, in 10m a pO2 of 2.1 equals an fO2 of 105%-something you obviously can't get.
2.1 at 20m = fO2 of 70.0% = 22.05L of O2.
2.1 at 30m = fO2 of 52.5% = 22.05L of O2.
2.1 at 60m = fO2 of 30.0% = 22.05L of O2.
So although you have the same surface volume of O2 in the loop, if your vO2 is 1L/min you still have to get through a lot of O2. To get to a loop pO2 of 1.3 [18.5%/13.65L] would take 8 minutes 24 seconds at a vO2 of 1.
At 20m and looking for a pO2 of 1.3 you will have 43.3% O2 in the loop-13.65L So it will still take 8 minutes 24 seconds at a vO2 of 1 to breathe the loop down from 70%.
Jeez freef, your kinda the SCR-guru here, so I don't want to challenge.... But here goes anyway:
About 10m - goes without saying! But your'e forgetting that you change the ratio of FO2/N2 but the pressure remains constant (remember flow to loop is inhibited).
But lets do simple one in the head first:
Loop-volume 10,5 l. (as in your examples)
PPO = 2.1 @ 11msw ie. FO2 = 100%
How long until you have reduced PPO to 1.3?
Its not the same as reducing the PPO at depth, thats all I wrote.
You forget that the PPO does not scale linearly with "amount" of O2 in a closed loop.
From you examples directly (plus one ekstra to really bring out the difference):
2.1 at 15m = fO2 of 84.0% = 22.05L of O2. + 4,2 l N2
1.3 at 15m = fO2 of 52,0% => Metabolise 17,5 l O2
2.1 at 20m = fO2 of 70.0% = 22.05L of O2 + 9,45l N2.
1.3 at 20m = fO2 of 43,3% => Metabolise 14,8 l O2
2.1 at 30m = fO2 of 52.5% = 22.05L of O2 + 19,95l N2.
1.3 at 30m = fO2 of 32,5% => Metabolise 12,4 l O2
2.1 at 60m = fO2 of 30.0% = 22.05L of O2. + 51,45l N2
1.3 at 60m = fO2 of 18,5% => Metabolise 10,3 l O2
The change in PPO at depth becomes nearly lienar with amount metabolised, but at shallows it does not. What makes it worse at shallows are, that you have to add fresh diluent because you reduces you loop-volume significantly.
Am I wrong?
Quote:
The depth limit for CMF Rebreather's [both SCR and CCR] is determined by the critical ratio of pressures on the upstream [tank] and downstream [loop] side of the jet. Of this ratio is not maintained the flow rate becomes subsonic and will vary with depth. The Dolphin is diveable to 81m while maintaining critical flow, beyond this depth the flow of gas into the loop decreases.
Yes, that I thought was implicit. I was writing that the constant-mass flow concept in theory works well at depth (If you can make a constant flow offcourse), but then can become wastefull when the same flow is used shallow with a new mix. Therefor its not enough to scale the flow only to the deep-part, you also need something to reduce it with for you decomixes.
Regards
Nicolai