Fast ascend

[DandyDon]

This is off-topic for the oringal post, but I'm going to offer it here just in case anyone hasn't read it...

From DAN: http://www.diversalertnetwork.org/news/article.asp?newsid=514
Yeah, you have to be a member to read it all. Even if you get your insurance elsewhere, you should be a member, IMO. Join now online if you need to.

Exceprting from....

DAN News
Deep Stops: Can Adding Half the Depth of A Safety Stop Build in Another Safety Margin?

Ascent Rates: A Quick History
Historical guidelines as to rates of ascent are pertinent. In the 19th century, for example, the French physiologist Paul Bert in 1878 quoted rates of 3 feet per minute and the English physiologist John Scott Haldane in 1907 recommended ascent rates between 5 and 30 feet (1.5 and 9 meters) per minute. From 1920-1957, rates of 25 feet (7.5 meters) per minute were recommended.

Then in 1958, during the production of the U.S. Navy Diving Manual, the rate of ascent to be proposed came under review. Cdr. Francis Douglas Fane of the U.S. Navy West Coast Underwater Demolition Team wanted rates for his frogmen of 100 feet (30 meters) per minute or faster. The hardhat divers, on the other hand, considered this impractical for the heavily suited divers who were used to coming up a line at 10 feet (3 meters) per minute. Thus, a compromise was reached at 60 feet (18 meters) per minute, which was also a convenient 1 foot per second.

So from 1957 until 1993 the U.S. Navy tables have consistently advocated an ascent rate of 60 feet per minute, based on this purely empirical decision, with many recreational diving tables and even early computers following suit. In recent years this has been slowed to 30 feet per minute with a recommended safety stop for three to five minutes at 15-20 feet (4.5-6 meters). However, this still brings the diver quite rapidly to the surface, often after some 30-60 minutes at depth.

Now these are only previews, see the whole article...

Haldane versus Hill: A Snapshot
In 1906 J.S. Haldane theorized that divers could ascend quickly to a depth that was half the absolute pressure of their deepest descent without getting DCS: the so-called 2:1 decompression stop. This technique became known as stage decompression.

British physiologist Sir Leonard Hill theorized that decompression should be by linear ascent to the surface; he strongly disagreed with Haldane's approach. However, in the end Haldane was able to prove, using goats, that a slow linear ascent was not only ineffective, but unsafe; too much nitrogen remained on surfacing resulting in frequent DCS. The deep stop was needed to dive safely.

 

[String]

Well, I think there's something else going on there. And that's running out of air! Once you reach the point where you are offgassing, the safest course of action from a DCS perspective would be to go slow to each stop, spend the required time there, and then continue slowly to the next stop. Right? I don't see how ascending faster could possibly make you safer, except....

Deco theory isnt a nice simple clear linear model as you describe. Misses out several things such as gradients and so on.

 

[Bob Vincent]

Thank someone above when we do stupid things and don't get hurt! The tables has changed over the years they got allot safer and with puters now its all different. Back in the day you'd have a profile of 60ft for 60 minutes / 70ft for 55 minutes / 80ft for 50 minutes and so on, check theses against newer tables. And we always came up at 60ft/sec. all the way up to the surface with no safety stop. So, don't try this with a computer because it calculates bounce, something we can't as were diving. But this might have come into play for you. But don't think this is acceptable these days when diving with a puter.

 

[Richesb]

Forget all the B/s. Fast ascents are not safe!!!!!!!!!!
Should be avoided and safety stops are just what they say.
You may get away with it 99 times but the 1 time you don't you will be sorry
Nuff said

 

[Gene_Hobbs]

Haldane versus Hill: A Snapshot
In 1906 J.S. Haldane theorized that divers could ascend quickly to a depth that was half the absolute pressure of their deepest descent without getting DCS: the so-called 2:1 decompression stop. This technique became known as stage decompression.

British physiologist Sir Leonard Hill theorized that decompression should be by linear ascent to the surface; he strongly disagreed with Haldane's approach. However, in the end Haldane was able to prove, using goats, that a slow linear ascent was not only ineffective, but unsafe; too much nitrogen remained on surfacing resulting in frequent DCS. The deep stop was needed to dive safely.

Sorry but I feel this needs some qualification. Nothing in medicine, science, and/ or diving is an absolute. The statement above, holds true for the case tested AND ONLY the case tested. Slow linear ascents are used operationally as are staged decompressions. This should be taken on a case by case basis and not distributed as THE answer. Commercial divers and hyperbaric medicine would be in trouble if both techniques were not available and proven to work in their applications. Just my 2c...

For the record, this is the problem faced by Haldane and Hill. From: DAN Inert Gas Exchange, Bubbles, and Decompression Theory Seminar

 

[DandyDon]

Thanks Gene!