Two recent articles on Nitrox

[Charlie99]

I think what charlie is saying is:
80' for 30 on air, and 100' for 30 on 32% should be exactly the same in terms of deco owed for the current dive and repetitive dives.

100' for 20 on air would not be the same as 100' for 30 on 32% but some deeper dive on 32% (assuming PPO2 is not too high)

Right?

100' on EAN32 has an EAD of 80'. That simply means that the ppN2 on EAN32 at 100' is the same as the ppN2 of air at 80'. Although not clearly taught in all nitrox courses, that's how EAD is defined.

Same ppN2 leads to the same exact deco. Comparing 30 min EAN32 dive at 100' to a 20 min air dive at 100' is kind of like comparing an air dive of 70' 40 minutes with an air dive of 100' 20 minutes. Both are to NDL, but the compartment loadings are different.

of course, in practice I generally ignore all this. I take a 60 or 90 min SI and use the same NDL time for repetitive dives. So for my diving it's kind of irrelevant somewhat but still I do like to (try to) understand what the heck is actually going on under covers

I assume that you are extending your shallow stops on repetitive dives. Even then, you will still have signficant loading in the slow compartments when you start your repetitive dives.

An interesting parallel is that USN tables are based upon 120 minute compartment. The slow offgassing of this compartment makes repetitive dives calculated on the USN tables slow compared to ones calculated on PADI RDP, even though compartment limits of USN/Workmann model are more agressive. The PADI RDP is based upon 60 minute compartment. In testing and modeling, though, they found that in very long, shallow dives, the controlling compartment is slower than 60 minutes and the tables would allow the model to be violated. That was solved by 1) reducing slight the 35' and 40' NDLs, and 2) adding some extra rules that require SIs to be extended to 1 hour minimum, or in some cases 3 HOURS MINIMUM when you are doing 3 or more dives in one day.

I'd be very cautious in ignoring previous dives, particularly when you are doing long, shallow dives. Unless you are using very significantly reduced NDL values for shallow dives, you could probably come up with dives that met your rule set, but which allowed you to surface with what most deco programs would consider a deco obligation.

The extra "W,X, Y, Z" rule of PADI table has the effect of limiting the average (air)depth, including SI time to less than 25'. That is a simple check that you could apply to profiles that were extraordinarily long/shallow.

 

[limeyx]

I assume that you are extending your shallow stops on repetitive dives. Even then, you will still have signficant loading in the slow compartments when you start your repetitive dives.
...

I'd be very cautious in ignoring previous dives, particularly when you are doing long, shallow dives. Unless you are using very significantly reduced NDL values for shallow dives, you could probably come up with dives that met your rule set, but which allowed you to surface with what most deco programs would consider a deco obligation.
...

for rec dives: less than 90 min SI = double the shallow stops. 90 min SI, do the same stops as the first dive. Do not reduce the NDL. Dive as much as possible on 32% nitrox.

Tec dives: same 60/90 min rule applies, but not more than two tech dives per day.

This has been used up to 7 rec dives in a single day, with 13 or so over a 3 day period.