I do also wonder if both the setpoint and ppo2 of dil is effected by mccr, hccr vs eccr.
Setpoint during fast decent, what do you use?
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Not for me. OC I go up to 1.6 as long as we are not talking about cave diving and/or long dives. If my small wreck is at 66m I will not dive every minute at 66m and I will not be there for a long time. 1.6pO2 for a short time at target depth is totaly OK for me. It is OK on 6m deco as well. And the hotter mix (21% at 66m) reduces deco time at any depth. But that's the way I do it, everbody is free to choose any other mix.
Very well explained, thank you.
A low pO2 does not come quickly. A high pO2 may come very quick, solenoid stuck open. But what is high? 1.6 is not high, at least not high=dangerous. 2.0 is high but you have very good chances to survive. I guess a dil flush by 1.3 or by 1.1 brings 2.0 down quite fast. Never tried but I guess.
Yes, choose your priorities.....
If you do the math (which I won't go into here, but it's much as @Wibble outlined), the rate of change of PO2 during descent is remarkably simple and intuitive. The PO2 rate is dependent on the descent rate (Rate_atm in atm/min), the diluent O2 fraction (Fd), and ratio of metabolism to loop volume (M):
Rate_PO2 = Fd * Rate_atm + (Fd - 1) * M
Note the two opposing factors of descent rate (positive term) and metabolism (the term involving M is negative), each scaled by a dil-dependent term. Simply beautiful. As @tbone1004 said, it's all about a balance.
For example, for Wibble's 1.5 L/min O2 metabolism and 5 L loop (M = 0.3), using 15/55 for dil (Fd = 0.15), taking 3 minutes to hit 60 m (Rate_atm = 2 atm/min), and starting at PO2=0.7. We would expect the PO2 to change by +0.045 atm/min. After 3 minutes, we arrive at the bottom with a PO2 of 0.84. (Wibble, this differs from your final result because you neglected to refill the loop after metabolism.)
At that 100 fpm (3 atm/min) rate some of you use, you would arrive with a PO2 of 1.1. (Faster descent means less time to metabolize.)
Rate_PO2 = Fd * Rate_atm + (Fd - 1) * M
Note the two opposing factors of descent rate (positive term) and metabolism (the term involving M is negative), each scaled by a dil-dependent term. Simply beautiful. As @tbone1004 said, it's all about a balance.
For example, for Wibble's 1.5 L/min O2 metabolism and 5 L loop (M = 0.3), using 15/55 for dil (Fd = 0.15), taking 3 minutes to hit 60 m (Rate_atm = 2 atm/min), and starting at PO2=0.7. We would expect the PO2 to change by +0.045 atm/min. After 3 minutes, we arrive at the bottom with a PO2 of 0.84. (Wibble, this differs from your final result because you neglected to refill the loop after metabolism.)
At that 100 fpm (3 atm/min) rate some of you use, you would arrive with a PO2 of 1.1. (Faster descent means less time to metabolize.)
I have no idea why the OP is bumping up the ppO2 shallow then adding a crazy hot dil with such a low fraction of helium. Then ending up with a ppO2 in the 1.4 to 1.5 range at depth. This whole approach to diving CCR is like trying to do a 1990s era best mix dive, but the wreck is just a wee bit deeper than you thought. It's 2023, there are better ways to approach this and calculating your "exact" planned arrive ppO2 on the bottom isnt part of that.
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