Walter teaches safety stops in midwater.
Are very slow final ascents better?
- Thread starter BigTuna
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NetDoc:
liberato
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Multiple mini-stops allow virtualizing ascent rate.
"...come up 10 feet and wait until the time catches up. In other words, it should take 30 seconds to come up 10 feet. If it takes the diver 3 seconds to come up 10 feet, the diver then waits another 27 seconds before going up another 10 feet. The ascent rate between the different 10 foot "mini stops" isn't overly critical as long as one does not ascend more than 10 feet at one time and does not "rocket" to the stop. But a more uniform ascent is desired."
Ascending From A Dive (Ascent Rates, Deep Safety/Deco Stops & Time Limits) by Brian R. Morris
A couple of interesting extracts from DAN (1st, where the original 60ft/min. came from):
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.
And... the math that makes mid-water and standard LONG safety stops even more important than ascent rates:
At 30 feet per minute (which is the ascent rate more commonly used today with a five-minute safety stop at 20 feet), the time to surface from 100 feet will be some eight minutes. This is better, but still a lot shorter than the 12.5-minute halftime of the spinal cord (not considering that gas elimination is slower than uptake). A plausible alternative might therefore be to ascend at 30 feet per minute but to add an additional "Haldanian" stop at about half the depth (remember, the depth is 100 feet / 15 meters) at 50 feet for five minutes. This gives 13.3 minutes of total ascent time2.
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.
And... if you need the complete text (interesting bathroom reading):
http://www.diversalertnetwork.org/news/article.asp?newsid=514
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.
And... the math that makes mid-water and standard LONG safety stops even more important than ascent rates:
At 30 feet per minute (which is the ascent rate more commonly used today with a five-minute safety stop at 20 feet), the time to surface from 100 feet will be some eight minutes. This is better, but still a lot shorter than the 12.5-minute halftime of the spinal cord (not considering that gas elimination is slower than uptake). A plausible alternative might therefore be to ascend at 30 feet per minute but to add an additional "Haldanian" stop at about half the depth (remember, the depth is 100 feet / 15 meters) at 50 feet for five minutes. This gives 13.3 minutes of total ascent time2.
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.
And... if you need the complete text (interesting bathroom reading):
http://www.diversalertnetwork.org/news/article.asp?newsid=514
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