DaleC
Contributor
Huh?
I dive nitrox as air all the time, particularly if I am doing more edgy profiles. I don't know about DAN but the Chief medical officer at the chamber I visited last year definately supported EAN use as one way to mitigate DCS risk. That, and having a redundant air source so one is not predisposed to commit to rapid ascents (which is a whole other issue).
I might suggest that DAN is more worried about divers exceeding the MOD for EAN (which can be done within sport diving limits) while the MOD for air is beyond those same limits. They may be balancing the potential risk of high PO2 OxTox with the risk of DCS and worrying more about the OxTox event. They may also believe (rightly or wrongly) that the average sport diver is too stupid to protect themselves from that risk so they will do it for them by minimizing the advantages of EAN instead of presenting both risk factors and letting the diver decide.
DCS risk increases with older, less healthy divers and/or repetitive diving. The actual low DCS figures may also be due to the fact that many resorts restrict the exposure of their clients to the margins of NDL diving and that many older, less healthy divers may naturally limit themselves from DCS risk (choosing shallower, less strenuous profiles).
If you are doing a week of resort repetitive diving and flying afterwards consider the following: Are you a young, fit diver with the appropriate self discipline to rest, hydrate etc... and plenty of time before your flight out or; are you an older diver, not in the best physical shape, doing a little drinking and late night dancing, flying soon afterwards etc...
How you assess and mitigate risk is ultimately up to you, there are some things you can avoid but others are just a fact of life. You should understand those risks and the tools you have available.
Most people believe EAN doesn't reduce DCS risk because they think the risk is already so low for sport diving that any further reduction won't matter. But, divers still take hits while diving within the NDL.
Partly, this is because NDL tables are actually deco tables based on a prescribed rate of ascent. The "non obligation" is contingent on a set rate of ascent. Don't violate the ascent rate and you don't need a decompression stop. Violate it and you do. Many people surface way to fast and court a DCS hit because of it.
Secondly, while it is a NDL dive there still is N loading occurring. It's not an all or nothing event. Note what happens when you transition during a square profile at 100' from 19 minutes and 59 seconds to just past 20 minutes. Where did that deco obligation come from? If you had ascended directly at 19:59 would you be notably safer from a DCS hit than a direct ascent at 20:01? Actually, you would be if you were breathing EAN 32%.
It's really just simple gas physiology. Excluding biological/environmental contributors, DCS risk is a product of three interrelated factors:
Depth
Exposure time
Gas Mix
Manipulating those three variables will increase or decrease the DCS risk. Saying one of those variables isn't important just doesn't make sense. Why ignore part of your toolbox and maintain one variable as a constant?
Consider this (using PADI tables):
For the same N load/DCS risk I can do each of the following dive profiles at 80fsw:
1. 30 minutes
2. 41 minutes
3. 46 minutes
The minimum time for all profiles to return to "A" PG status: 2:35.
The difference is mix:
1. air
2. EAN32%
3. EAN36%
Now look at what happens when you dive EAN 36% as air at 80fsw:
80' EAD of 60' (rounded up from 59') for 29 minutes. K PG. Minimum time to return to A PG status: 2:05
So, at that depth you can either dive 16 minutes longer on EAN 36% or be 7 PG's less N loaded if diving 36% as air.
Now let's push the NDL's at that depth
1. 30 minutes
2. 45 minutes
3. 55minutes
At 30 minutes, is a diver on air at more or less DCS risk than a diver on EAN 36%? Would you rather do that same profile (30 minutes at 80fsw) repetively on air or EAN 36%? Which mix will manifest SCDCS faster (with the resultant fatigue like symptoms). Again, perhaps DCS is a rare event because SCDCS also acts to self limit the triggering event (diver feels fatigued and takes a dive day off).
I dive nitrox as air all the time, particularly if I am doing more edgy profiles. I don't know about DAN but the Chief medical officer at the chamber I visited last year definately supported EAN use as one way to mitigate DCS risk. That, and having a redundant air source so one is not predisposed to commit to rapid ascents (which is a whole other issue).
I might suggest that DAN is more worried about divers exceeding the MOD for EAN (which can be done within sport diving limits) while the MOD for air is beyond those same limits. They may be balancing the potential risk of high PO2 OxTox with the risk of DCS and worrying more about the OxTox event. They may also believe (rightly or wrongly) that the average sport diver is too stupid to protect themselves from that risk so they will do it for them by minimizing the advantages of EAN instead of presenting both risk factors and letting the diver decide.
DCS risk increases with older, less healthy divers and/or repetitive diving. The actual low DCS figures may also be due to the fact that many resorts restrict the exposure of their clients to the margins of NDL diving and that many older, less healthy divers may naturally limit themselves from DCS risk (choosing shallower, less strenuous profiles).
If you are doing a week of resort repetitive diving and flying afterwards consider the following: Are you a young, fit diver with the appropriate self discipline to rest, hydrate etc... and plenty of time before your flight out or; are you an older diver, not in the best physical shape, doing a little drinking and late night dancing, flying soon afterwards etc...
How you assess and mitigate risk is ultimately up to you, there are some things you can avoid but others are just a fact of life. You should understand those risks and the tools you have available.
Most people believe EAN doesn't reduce DCS risk because they think the risk is already so low for sport diving that any further reduction won't matter. But, divers still take hits while diving within the NDL.
Partly, this is because NDL tables are actually deco tables based on a prescribed rate of ascent. The "non obligation" is contingent on a set rate of ascent. Don't violate the ascent rate and you don't need a decompression stop. Violate it and you do. Many people surface way to fast and court a DCS hit because of it.
Secondly, while it is a NDL dive there still is N loading occurring. It's not an all or nothing event. Note what happens when you transition during a square profile at 100' from 19 minutes and 59 seconds to just past 20 minutes. Where did that deco obligation come from? If you had ascended directly at 19:59 would you be notably safer from a DCS hit than a direct ascent at 20:01? Actually, you would be if you were breathing EAN 32%.
It's really just simple gas physiology. Excluding biological/environmental contributors, DCS risk is a product of three interrelated factors:
Depth
Exposure time
Gas Mix
Manipulating those three variables will increase or decrease the DCS risk. Saying one of those variables isn't important just doesn't make sense. Why ignore part of your toolbox and maintain one variable as a constant?
Consider this (using PADI tables):
For the same N load/DCS risk I can do each of the following dive profiles at 80fsw:
1. 30 minutes
2. 41 minutes
3. 46 minutes
The minimum time for all profiles to return to "A" PG status: 2:35.
The difference is mix:
1. air
2. EAN32%
3. EAN36%
Now look at what happens when you dive EAN 36% as air at 80fsw:
80' EAD of 60' (rounded up from 59') for 29 minutes. K PG. Minimum time to return to A PG status: 2:05
So, at that depth you can either dive 16 minutes longer on EAN 36% or be 7 PG's less N loaded if diving 36% as air.
Now let's push the NDL's at that depth
1. 30 minutes
2. 45 minutes
3. 55minutes
At 30 minutes, is a diver on air at more or less DCS risk than a diver on EAN 36%? Would you rather do that same profile (30 minutes at 80fsw) repetively on air or EAN 36%? Which mix will manifest SCDCS faster (with the resultant fatigue like symptoms). Again, perhaps DCS is a rare event because SCDCS also acts to self limit the triggering event (diver feels fatigued and takes a dive day off).
