(Wanted to go through the numbers on this one as an exercise so have shared it here).
Normaly I switch from 0.7 to 1.3 during descent at about 20m. I had a discussion: Dil 21/35, 20m, switch to 1.3, stay for 1 min, then very fast descent to 60m. A lot of dil is added, which brings the mix close to 21/35. But you still have the mix which was in the loop at 20m, this is a hotter mix.
Firstly STOP. Your 21% diluent is way too hot!
The MOD of the diluent should be
less than 1.1 at the target depth of 60m/7ATA.
The PPO2 for 21% at 60m = pressure (7ATA) x mix (0.21) =
1.47 way way too hot for a rebreather (and too hot even for OC where 21% has a mod of 57m/190ft).
The optimum mix for 60m = PPO2 (1.1) / pressure (7) = 0.157, i.e. 15%.
Therefore, the correct diluent would contain 15% oxygen. Am only considering the oxygen here and not interested in the helium/nitrogen mixes as they are both inert gases.
Comparing the pressures: surface=1ATA, 20m/60ft=3ATA, 60m/200ft=7ATA.
Assume a total
5 litre minimum loop volume -- meaning all the fixed spaces: scrubber, hoses, etc. You normally run minimum loop, certainly during a descent.
Jump in with a setpoint of 0.7 x 5 litres = 3.5 litres of oxygen + 1.5 litres of inert diluent gases
As you're descending, the loop collapses and you add ONLY diluent (15%), probably through the ADV (auto diluent valve) or manual diluent injection.
Descend to 20m where the loop volume is now 3ATA x 5 litres = 15 litres as in you've added 10 litres of diluent to be able to breathe. This gives you the original 3.5 litres of oxygen from the surface, plus the rest which is your diluent, 15 - 3.5 = 11.5 litres of 15% diluent (11.5 litres x 0.15% = 1.725 litres of oxygen), total oxygen in the loop is now the original 3.5 litres of oxygen plus the 1.725 litres from the diluent which is about 5.2 litres.
5.2 litres of oxygen in a loop containing 15 litres of gas is about = 35% oxygen and 65% inert gas. For the PPO2, this is multiplied by the absolute pressure; 35% x 3 = 1.05, with 65% x 3 = 1.95 (i.e. 1.05 bar of oxygen, 1.95 bar of inert gas, total = 3 bar)
Continuing the descent through to 60m/7ATA, only adding diluent containing 15% oxygen as the loop collapses...
The loop now contains 5 litres (the surface volume) x 7ATA water pressure = 5 x 7 = 35 litres. This consists of the original 3.5 litres oxygen injected on the surface, plus 35 - 3.5 = 31.5 litres of diluent containing 15% oxygen = 31.5 * 0.15 = 4.725 litres of oxygen from the diluent, total of 3.5 + 4.7 = 8.2 litres of oxygen.
The inert gas is the rest, so 35 litres - 8.2 (oxygen) = 26.8 litres of inert gas, i.e. 0.23% of oxygen and 76% inert gas.
For the PPO2, the percentage (23%) is multiplied by the absolute pressure (7ATA) =
0.23 x 7 =
PPO2=1.61, with 76% * 7 = 5.32 (i.e. 1.61 bar of oxygen, 5.32 bar of inert gas, total = 7 bar)
Hang on... that's a lot of oxygen in the loop as we've not considerd the human who's breathing and metabolising the oxygen.
Jumping in and descending isn't the most relaxing phase of any dive; a bit of stress and exertion. Assume 1.5 litres of oxygen would be consumed per minute and assume it takes 3 mins to descend to the target depth of 60m/200ft (which is pretty quick -- check out your dive logs, most of my 60+m dives take longer to reach the bottom).
This means we can subtract 1.5 litres/min consumed x 3 minutes = 4.5 litres of oxygen from the loop.
Re-work the numbers:
3.5 litres of oxygen on the surface plus 4.7 litres of oxygen from the 15% diluent
minus the 4.5 litres of oxygen metabolised
plus the (4.5 litres of diluent at 15% = 0.675 litres) which replaces the metabolised oxygen = 4.375 litres of oxygen in the loop. 4.375/35 = 0.125 = 12.5%.
PPO2 is 0.125 percent x 7 ATA pressure =
PPO2=0.875.
Once on the bottom you can sort out the setpoint high (1.3 in your case) as part of your arrival checks. Probably you'd have injected some additional oxygen as you descended to bring the PPO2 up a bit -- experience, etc.
Need to restate that the actual bottom PPO2 would depend on a load of factors and this exercise has assumed a fixed 5 litre loop which may be on the low side as the lungs need gas too. Your metabolic rate will vary, as will the time it took to descend. Most of my deeper dives have ended up with around PPO2 of 1.0 to 1.1 at the target depth as my Revo has a "leaky valve" / orifice. Tend to manually inject oxygen and run the unit in parachute mode (e.g. setpoint of 1.2 and run it manually at 1.3; the hiss of the solenoid is a kick in the backside meaning that I've not monitored/predicted the PPO2).
Edit:
(Wibble, this differs from your final result because you neglected to refill the loop after metabolism.)
Many thanks for spotting that. Have updated the lower PPO2.