aue-mike
Contributor
Hi Tom-
As far as O2 content, yes, hypoxia begins to develop at .16 after *sustained* breathing. Around .13 and lower is when it gets a bit more interesting. It should also be remembered, that for this PO2 ballpark, one breath doesn't mean instant blackout/death as everyone might think. However, tempting fate by breathing a hypoxic mix for any duration is not really a prudent exercise. Also, as with O2 toxicity, this varies from person to person and some individuals may be more sensitive than others.
The point I was trying to make is, you are not (or should not) breathing your backgas at the surface, especially not for extended periods of time. If, as is typical in the NE, you must jump in off the stern and work your way to the bow to descend down the anchor line, you can use your deco gas if choppy and/or a current; if not, take your time and fin/pull along the suface on your back, etc. However, many boats use a hang bar under the midships/stern, so you can jump off expecting to hit 10fsw and then pull up on the granny line leading from the hang bar to the anchor line submerged (i.e., at a great ambient pressure than the surface).
If doing a freedrop or using a shotline (as in Florida), a travel gas is entirely pointless. Put it this way, we have done several +/- 300fsw dives and our O2 may be 13% or less, and we do not worry about "travel gas" or other silliness. You have your reg half in your mouth (and breathing regular air or diesel fumes out the side of your mouth) and when it is time to dive, you insert (wait to breath) and drop. In a couple nanoseconds, you are 10-20fsw below the surface (where the PO2 is not a concern anymore) and *rapidly* descending so the gas is plenty safe to breath.
The only time we have *ever* had a premeditated gas switch was for a dive to 420fsw, where we had a third deco gas anyway and the PO2 was pretty darn low.
For the majority of tech/trimix diving, we are probably talking about depths less than 300fsw, with +/- 250fsw perhaps a good median depth. In these depths, any travel gas is really unecessary and adds to task loading, extraneous gear and potential failure points, etc. It is basically pointless and should be dropped from agency teaching, in lieu of efficiency, common sense, and logic.
Just my thoughts,
Mike
As far as O2 content, yes, hypoxia begins to develop at .16 after *sustained* breathing. Around .13 and lower is when it gets a bit more interesting. It should also be remembered, that for this PO2 ballpark, one breath doesn't mean instant blackout/death as everyone might think. However, tempting fate by breathing a hypoxic mix for any duration is not really a prudent exercise. Also, as with O2 toxicity, this varies from person to person and some individuals may be more sensitive than others.
The point I was trying to make is, you are not (or should not) breathing your backgas at the surface, especially not for extended periods of time. If, as is typical in the NE, you must jump in off the stern and work your way to the bow to descend down the anchor line, you can use your deco gas if choppy and/or a current; if not, take your time and fin/pull along the suface on your back, etc. However, many boats use a hang bar under the midships/stern, so you can jump off expecting to hit 10fsw and then pull up on the granny line leading from the hang bar to the anchor line submerged (i.e., at a great ambient pressure than the surface).
If doing a freedrop or using a shotline (as in Florida), a travel gas is entirely pointless. Put it this way, we have done several +/- 300fsw dives and our O2 may be 13% or less, and we do not worry about "travel gas" or other silliness. You have your reg half in your mouth (and breathing regular air or diesel fumes out the side of your mouth) and when it is time to dive, you insert (wait to breath) and drop. In a couple nanoseconds, you are 10-20fsw below the surface (where the PO2 is not a concern anymore) and *rapidly* descending so the gas is plenty safe to breath.
The only time we have *ever* had a premeditated gas switch was for a dive to 420fsw, where we had a third deco gas anyway and the PO2 was pretty darn low.
For the majority of tech/trimix diving, we are probably talking about depths less than 300fsw, with +/- 250fsw perhaps a good median depth. In these depths, any travel gas is really unecessary and adds to task loading, extraneous gear and potential failure points, etc. It is basically pointless and should be dropped from agency teaching, in lieu of efficiency, common sense, and logic.
Just my thoughts,
Mike
