I was reading Kevin Gurr's book on tec diving last night, and in particular I was reading about "air breaks". The book was stressing that if you take your "air breaks" on a helium rich bottom mix then it doesn't really affect your overall decompression time because by the time you are at your 30' stop, all the helium has already been off-gassed, so you are only concerned about eliminating remaining nitrogen, and a helium rich bottom gas will be exerting a very low partial pressure of nitrogen so you will still be off-gassing efficiently.
This got me thinking: if that is right, then why do the final decompression stop on pure O2 at all? Why not do it on a blend of (say) 75% oxygen and 25% helium? It would be just as efficient at off-gassing nitrogen, but with a lower risk of oxygen toxicity.
Then I extrapolated the thought some more: helium is expensive, but why could you not use another gas for your 25% filler, such as argon? There would never be any dissolved argon in your tissues, so the same logic would apply - you would be breathing a ppN2 of 0.0 and a ppHe of 0.0 - you are off-gassing as efficiently as you possibly can, but with a much reduced oxygen toxicity risk. And argon is dirt cheap. The knock on argon has always been that it is highly narcotic, but if you were breathing a 25% argon mix at 30 feet, then you still only have an equivalent narcotic depth of 45 feet.
If you wanted to push the envelope, you could even make up a mix of 60% oxygen, 40% argon and start breathing a maximum efficiency off-gassing mix at about 50 feet, and your equivalent narcotic depth is still only around 90 feet.
I can't be the first person to have thought of off-gassing with other gases not already dissolved in the tissues. So does anyone know why this isn't this done?
This got me thinking: if that is right, then why do the final decompression stop on pure O2 at all? Why not do it on a blend of (say) 75% oxygen and 25% helium? It would be just as efficient at off-gassing nitrogen, but with a lower risk of oxygen toxicity.
Then I extrapolated the thought some more: helium is expensive, but why could you not use another gas for your 25% filler, such as argon? There would never be any dissolved argon in your tissues, so the same logic would apply - you would be breathing a ppN2 of 0.0 and a ppHe of 0.0 - you are off-gassing as efficiently as you possibly can, but with a much reduced oxygen toxicity risk. And argon is dirt cheap. The knock on argon has always been that it is highly narcotic, but if you were breathing a 25% argon mix at 30 feet, then you still only have an equivalent narcotic depth of 45 feet.
If you wanted to push the envelope, you could even make up a mix of 60% oxygen, 40% argon and start breathing a maximum efficiency off-gassing mix at about 50 feet, and your equivalent narcotic depth is still only around 90 feet.
I can't be the first person to have thought of off-gassing with other gases not already dissolved in the tissues. So does anyone know why this isn't this done?