Genesis,
The real issue here is the breaks to "back gas" that many do to control O2 exposure.
It may be ideal to reduce the fraction of inert gas as O2 is added to the mix but it isn't the issue. The problem comes in with a heavy to light switch (like switching from EAN50 to trimix).
It doesn't take much of a dive to totally blow away the NOAA max recommended O2 exposure. For many years "AIR" breaks have been recommended when spending prolonged periods on O2 (which we do on most dives). It's certainly worked as far as avoiding whole body/pulmanary symptoms and has more recently been used on exposures that are many many times that of what has conventionaly thought to be possible (or at least wise). So, it seems that it may älso fend off CNS symptoms. Isobaric countertransport becomes a topi when helium is used in the bottom gas and nitrox of any flavor is used for decompression and one wishes to break from a high PPO2 of the decompression gas to the low PPO2 of the bottom gas. The exact flavors of gas don't have anything to do with it.
BRW, dives a rebreather so the diluent stays the same and O2 is added on ascent.
The "dogma" as you put it about standard gasses probably shouldn't get confused with this. One concern is efficient safe decompression. Another is having every one on the same page with their gasses and standardizing therefore becoming very familiar with schedules because the same gasses are always used for the same depths. Some standardization in gasses also simplifies mixing. For instance, if you bank 32% and base all your mixes on topping the the helium with 32%. Now, just as with heliair, if you analyze the O2, you know what your mix is without buying a helium analyzer. That still doesn't give standard mixes but if you take that a step further and decide to use 21/35 to 150, 18/45 to 200, 15/55 to 250 and 10/70 to 400 you have the standard mixes of one group. Mixing is easy because you add the desired amount of helium and top from your 32% bank. Carry one "standard" set of tables and every one is always on the same page as far as decompression and those tables are always in your pocket (though after a while you might not need to pull them out and look at them). It's about simplification. Oh, and since you're banking 32% you just use that for shallow dives.
You could gain many of the same advantages by banking some other flavor of nitrox and stepping up the He in whatever incriments you like.
Also, adding helium to decompression gasses is getting to be SOP even for those who have fallen victim to the "dogma".
The real issue here is the breaks to "back gas" that many do to control O2 exposure.
It may be ideal to reduce the fraction of inert gas as O2 is added to the mix but it isn't the issue. The problem comes in with a heavy to light switch (like switching from EAN50 to trimix).
It doesn't take much of a dive to totally blow away the NOAA max recommended O2 exposure. For many years "AIR" breaks have been recommended when spending prolonged periods on O2 (which we do on most dives). It's certainly worked as far as avoiding whole body/pulmanary symptoms and has more recently been used on exposures that are many many times that of what has conventionaly thought to be possible (or at least wise). So, it seems that it may älso fend off CNS symptoms. Isobaric countertransport becomes a topi when helium is used in the bottom gas and nitrox of any flavor is used for decompression and one wishes to break from a high PPO2 of the decompression gas to the low PPO2 of the bottom gas. The exact flavors of gas don't have anything to do with it.
BRW, dives a rebreather so the diluent stays the same and O2 is added on ascent.
The "dogma" as you put it about standard gasses probably shouldn't get confused with this. One concern is efficient safe decompression. Another is having every one on the same page with their gasses and standardizing therefore becoming very familiar with schedules because the same gasses are always used for the same depths. Some standardization in gasses also simplifies mixing. For instance, if you bank 32% and base all your mixes on topping the the helium with 32%. Now, just as with heliair, if you analyze the O2, you know what your mix is without buying a helium analyzer. That still doesn't give standard mixes but if you take that a step further and decide to use 21/35 to 150, 18/45 to 200, 15/55 to 250 and 10/70 to 400 you have the standard mixes of one group. Mixing is easy because you add the desired amount of helium and top from your 32% bank. Carry one "standard" set of tables and every one is always on the same page as far as decompression and those tables are always in your pocket (though after a while you might not need to pull them out and look at them). It's about simplification. Oh, and since you're banking 32% you just use that for shallow dives.
You could gain many of the same advantages by banking some other flavor of nitrox and stepping up the He in whatever incriments you like.
Also, adding helium to decompression gasses is getting to be SOP even for those who have fallen victim to the "dogma".
