Why 15 ft for a safety stop?

[murdrcycle]

And you'd be wrong. This is bad advice and full of too many assumptions.

Then tell me why I am wrong - don't just say it.

 

[murdrcycle]

The only depth at which one will not load up with inert gas is 0 feet.

OK - got it - so the assumption that anything 33 ft or less doesn't cause nitrogen to over-absorb is incorrect? My computer says 20 ft, my class (and many divers here) said 33 ft, but the truth is that any time we breath underwater regardless of depth we are taking on excess nitrogen?

 

[Steve SDI/TDI HQ]

Then tell me why I am wrong - don't just say it.

PM sent.

 

[nereas]

OK - got it - so the assumption that anything 33 ft or less doesn't cause nitrogen to over-absorb is incorrect? My computer says 20 ft, my class (and many divers here) said 33 ft, but the truth is that any time we breath underwater regardless of depth we are taking on excess nitrogen?

Anytime we breathe, we are taking on inert gas (nitrogen, argon, neon, helium etc). The amount taken on underwater is in excess of on the surface, and on the surface is in excess of on a mountaintop. For depths shallower than 33 ft, DCS does not appear to occur (an observation, not a theory). For anything deeper than 33 ft, we need to do something to avoid DCS. Staying within NDL time limits and stopping at 15 ft for 3 to 5 minutes seem to work, for avoiding DCS when diving deeper than 33 ft.

DCS is a sickness or injury occuring with "extreme" decompression. It occurs whenever you decompress too quickly. When diving in the range of -0- to 33 ft, you are still compressing, just not enough to cause DCS when you suddenly decompress.

 

[offthewall1]

Recent studies indicate safety stops are not required for recreational divers who stay within recreational limits and dive within the bounds of what their computers tell them.

Modern dive computers are extremely conservative - some more so than others, but all conservative. Because of this, the need for a safety stop of 3-5 minutes is not realistic. With that said, a safety stop is still exactly what its name implies... it is an extra measure of safety... but it is not required. All agencies teach students to do it - as an extra measure of safety.

More important than a 3 - 5 minute safety stop is staying within the bounds of your computer and doing a proper ascent of no more than 30 feet per minute. Generally speaking all the time spent above 30ft is an extended safety stop... so if you're on a wall, ascend to the 10 - 30 ft range and hangout for a while... then start a slow ascent to the surface... if you want to hang out at 10, 15 or 20 ft for 3-5 minutes... it's not a bad idea. Enjoy yourself... take in the sights... Most importantly... ascend those final 10, 15 or 20 feet to the surface very slowly - about 15 ft per minute...and remember to breath... many injuries can occur in those final 15 -20 ft to the surface.

 

[nereas]

...Generally speaking all the time spent above 30ft is an extended safety stop... so if you're on a wall, ascend to the 10 - 30 ft range and hangout for a while... then start a slow ascent to the surface... if you want to hang out at 10, 15 or 20 ft for 3-5 minutes... it's not a bad idea. Enjoy yourself... take in the sights... Most importantly... ascend those final 10, 15 or 20 feet to the surface very slowly - about 15 ft per minute...and remember to breathe... many injuries can occur in those final 15 -20 ft to the surface.

Excellent advice.

Recent studies indicate safety stops are not required for recreational divers who stay within recreational limits and dive within the bounds of what their computers tell them.

Modern dive computers are extremely conservative - some more so than others, but all conservative. Because of this, the need for a safety stop of 3-5 minutes is not realistic. With that said, a safety stop is still exactly what its name implies... it is an extra measure of safety... but it is not required. All agencies teach students to do it - as an extra measure of safety....

I sure would like to read those "recent studies" myself. Until I do, I prefer to encourage others to keep to their 15 ft 3 min stops. I will admit to blowing off safety stops when I could not find the anchor line and I was in a significant current away from the boat. Otherwise, I would always try to keep them. In other words, your safety stop should not make matters worse, such as blowing you down-current and away from the boat out at sea. It should make matters better, such as giving you an additional 3 to 5 mins of decompression time ("off-gassing") in the shallow range of your egression around 20 to 15 ft while on the anchor line.

 

[Charlie99]

Recent studies indicate safety stops are not required for recreational divers who stay within recreational limits and dive within the bounds of what their computers tell them.

Really ?? !!

Do you have any info such as who did the study, when, where published, etc?

One of the problems about recreational decompression is that DCS rates are so low that it is difficult to make any definitive statments. In practice, what is done is to perform tests that are very agressive profiles, and then extrapolate backwards towards more conservative profiles.

The USN tables allowed rather agressive dives for the first dive of the day, with a 60fpm ascent directly to the surface. Several studies have shown that it has a higher risk of DCS than do other tables such as PADI RDP. Since a risk of DCS 3 or 4 times higher is acceptable to many people, one could indeed say that safety stops are "not required". Is this the "recent study" that you are referring to?

Charlie Allen

 

[japan-diver]

The amount of wrong information is this thread is astounding.

First: You can develop DCS at depths shallower than 33ft- there are hundreds of cases of this having happened.
Second: You can get bent and bent badly even if your computer says you are OK and diving withing the "safe" limits

I know of no study that advocates not doing safety stops or just following your computer- because all decompression tables and computers are based on theory safety stops are alway reccomended because theories are not perfect and any thinking diver should know this.


Modern decompression theory (RGBM, VPM etc..) have us starting decompression stops much deeper than we used to and ascending much slower. These tables seem to work better then what we used to use with less DCS and other problems on extended dives- you will find almost no serious tech divers using the standard Navy Tables for deco anymore must use custom tables or computers based on much more recent work.

The practice of the first safety stop at 50% of depth is adovacated by NAUI and other agencies. Do a search on minimum deco for all dives to see the GUE/DIR take which includes the 10ft increment ascent mentioned earlier. These ascent strageties are advocated for what most divers refer to as "no-decompression diving". Hopefully by now all divers realize that all dives require decompression of some kind- this decompression can be in the form of a controlled ascent or by adding stops at certain depths coupled with a controlled ascent. The depths that decompression starts and where stops would start are controlled by many factors- depth of dive, duration at bottom, gases used etc.

For the average recreational diver diving with no stop limits of tables or computers a controlled ascent rate (no faster than 30ft/min) a 1-2 min stop at 50% of max depth and another stop at 15ft for 3 min is the best ascent strategy to minimize your risk of DCS. There are other more agressive ascent strategies being taught but they will increase your risk - some only slightly others more so. The risk of contracting DCS is small if within your tables/computers no stop times but it is there. I personally like to keep that chance as small as possible and a few extra minutes on ascent is a very easy way to do that. The only way to completely eliminate your risk of DCS is to not dive so everything we do on a dive is to manage the risk of DCS. Even if you do everything right there is still a chance that you will need a ride in the chamber.

 

[offthewall1]

SEE BELOW - "Excerpt from a DAN Divers Alert Network Article"

DAN News
Deep Stops: Can Adding Half the Depth of A Safety Stop Build in Another Safety Margin?
Last Updated: 12/28/2004 10:45:48 AM

By Peter B. Bennett, Ph.D., D.Sc., Alessandro Marroni, M.D., Frans J. Cronjé, M.D., International DAN

Although decompression tables have been significantly modified over the last 20 years, with many now giving much shorter times at depth than the original U.S. Navy tables, the incidence of decompression illness (DCI) has changed very little. Even the recent introduction of dive computers has not made a significant impact on dive injuries.

Decompression illness incidence remains consistent with the distribution for sex, age and training among divers, regardless of the computers or tables they use. The problem, as previously elaborated in Alert Diver1 appears to be a too-short time of ascent; this is the only parameter that has changed very little over the last 40 years and, accordingly, appears to possibly be the real controller of the incidence of DCI.
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).

NOTE: The most important things from this excerpt
1. The incidence of decompression has changed little (this would indicate that ascent rates and safety stops have little to do with your risk of DCI) in the last 20 years. Other factors (physical such as obesity, genetic issues etc..) are likely the more problematic factor. Yes, ppeople can get DCS in 10, 20 even 30 feet of water... but this just means they were pre-disposed to it regardless of depth, ascent rate and safety stops.
2. The conclusion of a "recommended" safety stop surely bears what I said... and that is that it is recommended - not required.

Happy Diving.

 

[Thalassamania]

For depths shallower than 33 ft, DCS does not appear to occur (an observation, not a theory). For anything deeper than 33 ft, we need to do something to avoid DCS.

No, the actual number is shallower, on the order of 24 fsw.

DCS is a sickness or injury occuring with "extreme" decompression. It occurs whenever you decompress too quickly. When diving in the range of -0- to 33 ft, you are still compressing, just not enough to cause DCS when you suddenly decompress.

No, 0 to 24 fsw.

Here are the givens, from the theoretical scientific laws of the physical universe, as least for here on this Earth (the planet Jupiter would have different laws of physics)

Actually the laws of physics are unchanging regardless of the planet you visit.

At pressures greater than 2 ATAs DCS begins to develop given sufficient time (called NDL time).

2 ATA is theoretical, actual is slightly less, about 24 fsw, I believe.

As a result, a safety stop at a depth of 1/2 this critical threshhold depth is agreed upon as a respite point to allow sufficient off-gassing (controlled decompression) before surfacing from a scuba dive.

15 feet has nothing what-so-ever to do with half of this "critical threshold depth," it's only that it's better than 20 and most divers can't handle 10.

Our surrounding universe is merely a collection of observations and theories only. Science is only a theory, even as God is only a theory (for most people, except of course for the prophets themselves).

All of science is not a theory and none of God (the concept being supernatural and thus immune to rational inquiry) is theoretical, God is either mythical or real, there is not other alternative.