scubamoo
Guest
So long as you and your buddy are comfortable, is a fast descent safe from a physiological standpoint? I rather like the flight down.
Descent speed -- does it matter?
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Some deco software is designed to calculate your bottom time based on a certain descent rate.scubamoo:
There is some speculation out there that rapid descents are a contributing factor to a phenomenon known as deep water blackout
miketsp
Contributor
Most tables are calculated at a given descent rate. If you go down faster you need to compensate your bottom time as the average depth will be different and the effect will be more significant the deeper you go. A computer of course will compensate automatically.
The US Navy recommends a maximum descent rate of 75 feet per minute. You'll notice the effects of narcosis much more if you make a rapid descent.
DennisW
Contributor
I've always been able to descend as fast as I could kick down. However, I have recently began to make slower decents. Shoot, I'm getting to be an old fart and taking it slow seems to make more sense. Besides, as Walter pointed out, slower decents are recommended.
There are several factors to look at when choosing a descent rate, which include the gas being breathed and the depth of the dive. When on a high helium/low oxygen blend, a fast descent rate may be more beneficial from a DCS point of view. This is because a slow descent rate allows for the helium to get into nuclei ... so it is better to get pressure on the diver fast to keep the nucei smaller. Erik Maiken has written on this and it is known that more pressure on a nuclei is good. Another way to view this is trying to crush nuclei by pressure so they can't add to DCS later ... but true crushing in a diver is probably not possible. However, there is the competing factor that faster descent rates could add to a narcosis issue. I have found this to be the case for me ... but it could also just be a comfort level/mental issue too. When diving to about 250' plus, I use slower descent rates since I am more comfortable. But if I am diving in the 200' or less range, then I will descend at about 100'/minute which is a comfortable rate for me. On nice EAN mixes in middle and shallower ranges (i.e. 70' or less), then I would not expect the descent rate to affect much of anything.
If you are getting narcosis due to the fast descent, change your mix! I only used to have an issue with this when I was diving deep air. Now, I no longer engage in this foolishness thanks to helium based mixes.
I set my software (V-Planner) for 200 feet per minute on the descent so that I can calculate my gas consumption rate and my deco profile more conservatively than I actually dive. I probably descend more like 70 to 100 feet per minute in actual practice but this builds in a safety margin for calculation purposes.
There are times where the faster descent is important like when there is strong current and we need to get down to and inside the shelter of the wreck quickly. I have seen people go full speed down on scooters to over 300 feet. Sometimes, this is the only way to get any kind of meaningful bottom time on these dives. I can't really see where the fast descent has been a problem.
I set my software (V-Planner) for 200 feet per minute on the descent so that I can calculate my gas consumption rate and my deco profile more conservatively than I actually dive. I probably descend more like 70 to 100 feet per minute in actual practice but this builds in a safety margin for calculation purposes.
There are times where the faster descent is important like when there is strong current and we need to get down to and inside the shelter of the wreck quickly. I have seen people go full speed down on scooters to over 300 feet. Sometimes, this is the only way to get any kind of meaningful bottom time on these dives. I can't really see where the fast descent has been a problem.
Walter:
COMEX divers use a descent rate of 80 meters per minute under many operations.
Apart from the fact that a rapid descent kind of increases the narcosis hit a little, there is a factor that I have found with rapid long descents (greater than 150')
it is called diabatic osteo arthuralgia
It is basically joint pain, caused I believe by an osmotic effect of rapidly increasing nitrogen differentials "sucking" the lubricating fluid out of the joints such as hips, knees and shoulders.
I noticed it for years before I found it documented and it described in the nss cave diving manual. It never really bothered me as it was on descent not ascent, and it goes away after a few minutes at depth. I dont know if it causes any long term effects and havent noticed any.
it is called diabatic osteo arthuralgia
It is basically joint pain, caused I believe by an osmotic effect of rapidly increasing nitrogen differentials "sucking" the lubricating fluid out of the joints such as hips, knees and shoulders.
I noticed it for years before I found it documented and it described in the nss cave diving manual. It never really bothered me as it was on descent not ascent, and it goes away after a few minutes at depth. I dont know if it causes any long term effects and havent noticed any.
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