mempilot:
. . . But I'm an advocate of planning your dive and diving your plan. Using 2 gas mixes for a dive without planning for it is not good SOP.
I completely agree with you. Okay, I see where you are coming from regarding the CNS issue - which I agree always needs to be taken into consideration. However, the 3 minutes of O2 at 20 only adds 7% to each dive and you are not giving yourself credit for the CNS being reduced between dives. There are several ways to view this. One is by using a 90 min. half-time decay (as most tech divers do) and another on the conservative side is a linear 24 hour digression ... among others. My numbers will be slightly different than yours. The way you did your calculations, the problem was not the stop, but the actual CNS buildup from the dive.
90 minute half-life method:
Dive 1
90/25 on EAN36 with 20/3 on O2: 21% CNS
1.5 hour SI now gives 11%
(the CNS was reduced by one-half due to the 90 min SI)
Dive 2
60/55 on EAN36 with 20/3 on O2: 32% CNS
21 hours SI: 0% CNS
(14 half times)
So you can see that the second day of diving would not be a problem.
Using NOAAs 24 hour window (conservative), the following results would be obtained
Dive 1
90/25 on EAN36 with 20/3 on O2: 21% CNS
1.5 hour SI now gives 20%
(6% reduction of the 21%)
Dive 2
60/55 on EAN36 with 20/3 on O2: 49% CNS
21 hours SI: 6% CNS
(87% reduction of the 49%)
Day 2
Dive 1
90/25 on EAN36 with 20/3 on O2: 27% CNS
1.5 hour SI now gives 26%
(6% reduction of the 27%)
Dive 2
60/55 on EAN36 with 20/3 on O2: 55% CNS
and likewise Day 3 is not a problem. No matter how it is calculated, the 21 hour surface interval is significant and will reduce the CNS to virtually nothing. But you are absolutely correct that CNS needs to be accounted for.
The simple solution if one is desired is to not count the 1.5 hour surface interval (so calculations need not be done) and then treat yourself as clean after the 21 hour SI - which is acceptable.