BRW
Contributor
All, 
Got some emails about optimal switch strategies in modern
deco frameworks, and impacts of isobaric countertransport.
So for those who asked me, here's my read using dual
phase dynamics (RGBM)), what tec sectors dive and report,
some earlier experiments and observation by Chris Lamberstsen,
field tests, plus building on earlier posts.
1) -- light-to-heavy switches are the ONLY safe switches,
and once started, should never be reversed back
the other way;
2) -- good way to deco overall is riding He to 20 fsw zone
and switching pure O2 or 80/20 if you can (you will
most likely feel better on He deco, be in better position
to repet, and have roughly equal deco time as with some
switch to nitrox earlier);
3) -- the lighter the He mix, the less benefits of switching
to nitrox;
4) -- heavy nitrox switches (depths beyond 70 fsw) are not
an optimal (nor good) strategy for short or long
exposures;
5) -- EAN50 at 70 fsw is usually N2 light enough for longer
exposures
What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.
So, it goes like this.
Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. The
He to O2 changes look a little like an RB, and the lower N2
nitrox switches avoid gradient slams and further ingassing of
N2.
Practically, this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.
Cheers, and safe diving always,
Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr

Got some emails about optimal switch strategies in modern
deco frameworks, and impacts of isobaric countertransport.
So for those who asked me, here's my read using dual
phase dynamics (RGBM)), what tec sectors dive and report,
some earlier experiments and observation by Chris Lamberstsen,
field tests, plus building on earlier posts.
1) -- light-to-heavy switches are the ONLY safe switches,
and once started, should never be reversed back
the other way;
2) -- good way to deco overall is riding He to 20 fsw zone
and switching pure O2 or 80/20 if you can (you will
most likely feel better on He deco, be in better position
to repet, and have roughly equal deco time as with some
switch to nitrox earlier);
3) -- the lighter the He mix, the less benefits of switching
to nitrox;
4) -- heavy nitrox switches (depths beyond 70 fsw) are not
an optimal (nor good) strategy for short or long
exposures;
5) -- EAN50 at 70 fsw is usually N2 light enough for longer
exposures
What this all adds up to is the following isobaric switch
prescription, which is, of course, outside of most dive
logistics. But useful on first principles -- and some places
DO have the support in place for just such logistics.
So, it goes like this.
Best deco strategy is to increase O2 on way up in same proportion
as He is reduced, while keeping N2 relatively constant. Switch to
a nitrox mix with less N2, and thus outgas both He and N2. The
He to O2 changes look a little like an RB, and the lower N2
nitrox switches avoid gradient slams and further ingassing of
N2.
Practically, this means all the above plus EAN50 at the 70 fsw
level for long exposures, and/or He to the surface for shallow
exposures, plus O2 in the 20 fsw zone either way.
Cheers, and safe diving always,

Bruce Wienke
Program Manager Computational Physics
C & C Dive Team Ldr