Swimming to Surface Question

[TSandM]

There's no exact point. Once you have required decompression stops, it's much more dangerous to skip stops, but it's possible to do so without getting DCS, but your chances of serious injury or death increase with the more decompression obligation you skip.

Walter nailed it. There is no black and white line about decompression obligations. In fact, if you compare the output from a variety of decompression algorithms, they can vary quite a bit (as much as 10 minutes, if I recall correctly, at some depths) in what they consider the "No deco" time to be. You are not "safe" for ten minutes at 120 feet, and "unsafe" beyond that. Instead, there is a gradual increase in the likelihood of symptomatic DCS as you exceed your no-deco time. That risk can be controlled by controlling the ascent (making stops, as Walter describes). Although we use the word "stop" for this, in fact, swimming gently along at 20 feet is a 20 foot "stop". (For example, in cave diving, the profile of the cave will often give us a "stop" at a given depth, although we are progressing toward the exit throughout the time.)

At some point, the amount of nitrogen you have absorbed makes it essentially certain that, if you ascend without stopping, you will end up with symptoms. No one can put a finger on exactly what the combination of depth and time is that will get you there.

The best practice is not to push your no-deco limits, and to do some kind of "stops" to control your ascent rate. But in a lot of dives (as with the one I did yesterday) you can continue to swim and enjoy the terrain and the critters as you come up. Much better than hanging in blue water, in my opinion!

 

[Thalassamania]

Safety stops were "invented" as a way to compensate for the fact that divers were grossly exceeding the then 60 fpm ascent rate. Actual ascent rates are built into the tables you use. If you are using 60 FPM based tables Spenser did show that a 60 fpm ascent following 50 min at 2.8 ata would exhibit bubbles in some cases. But the reality is that with a 30 FPM ascent you're not going to have detectable bubbles anyway. Remember that the original U.S. Navy tables were almost based on 120 FPM to satisfy the needs of Doug Fane's UDT folks.

But ... I believe that the PADI tables were doppler tested and a 60 fpm ascent within the limits of those tables did not show bubbles, making a stop a waste of time. If memory serves (Ray Rogers was my roommate at a conference some time back), Ray told me that there were two connected issues,the lawyers did not want PADI to change the ascent rate for liability reasons and so he used that rate as an input for his calculations and later on, when slower rates had become de regur, it was too late to recalculate the whole thing (not to mention the liability issue of having a revision).

Safety stops were really developed to slow the diver's overall ascent rate and to be sure that that diver was in control during the last critical ascent phase, they are an artifact of a bygone era of 60 fpm ascent rates using U.S. Navy Tables, and no ascent instrumentation. Additionally, everyone should realize that there has always been a problem with slowed ascent rates and the "suspense time" that creates, usually that time has to added to bottom time.

 

[Walter]

would maintaining these depths and times, for an example, while still swimming be considered a 'stop' - although you're not really stopped??

You have stopped. You've stopped ascending. As long as you maintain a constant depth, you've stopped with regard to your ascent.

 

[kouhoutek]

So, in a multi-level dive to deep depths, would maintaining these depths and times, for an example, while still swimming be considered a 'stop' - although you're not really stopped??

That said, every dive I've done we've literally stopped for the final 5 min. @ ~ 15 ft.

Have a nice day!!

That's right. A "stop" refers only to your vertical motion.

Now bear in mind, it is trickier to maintain the right depth on your safety stop if you are moving around. I've thought I was doing it on occasion, only to have my computer tell me different later on.

It was probably not enough to make a difference, but too many "probablies" add up to "maybe", which isn't good enough underwater. I'd just as soon make sure I my stop right, and stay put...or better yet, use a hang bar.

 

[boulderjohn]

DAN published a study on ascent rates a few years ago. They compared 60 FPM, 30 FPM and 10 FPM, with 30 FPM coming out the best. That study found, though, that safety stops were more important than the ascent rate. The very best results were found with 30 FPM and both a deep stop and a conventional stop. That's what I personally aim for.

 

[Thalassamania]

DAN published a study on ascent rates a few years ago. They compared 60 FPM, 30 FPM and 10 FPM, with 30 FPM coming out the best. That study found, though, that safety stops were more important than the ascent rate. The very best results were found with 30 FPM and both a deep stop and a conventional stop. That's what I personally aim for.

From DAN site:

DAN Medical Research

Ascent Rate

Most recreational divers practice No-Stop Decompression diving. The No-Stop Decompression Limits represent the depths and maximum times divers can stay on the bottom and then surface. All recreational dive tables assume that divers will not exceed recommended ascent rates.

Recommended ascent rates vary between 20 and 60 feet per minute. Although it is generally acknowledged that ascent rates may significantly affect dive safety, there is not enough human data to suggest the correct rate.

The goal of the Ascent Rate Study was to provide data to describe the effects of ascent rate.

Methods

Experiments were conducted in the hyper/hypobaric chambers at the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Center. All the dives were to 100 feet of seawater (fsw). The first profile had a bottom time of 15 minutes. When researchers accepted the dive profile, they increased the bottom time by three minutes. The tests involved single dives only.

Researchers tested ascent rates of 10 and 60 feet per minute. The objective of the study was to estimate if there is any difference in No-Decompression Limits, incidence of vascular bubbles, or decompression sickness between the ascent rates. Researchers accepted or rejected dive profiles based upon the number of DCS incidents and total exposures. They chose the acceptance and rejection rules to minimize the total number of DCS incidents, allowing for some mild cases but limiting more serious DCS. The Institutional Review Board of Duke University Medical Center approved these rules.

Subjects were dry and at rest throughout the dives. Subjects must have been certified recreational divers or have hyperbaric experience, be at least 18 years old, and their body composition must no exceed 40% of their ideal weight. Subjects could participate only once in each profile. Subjects were eligible for more than one study as the profile changes and they were encouraged to participate in each ascent rate profile. They under went a medical history review and a physical examination the day of the study by a DAN/Duke dive physician.

During the study, participants activity before, during, and after the 100 fsw dives were controlled and minimized. Except for essential activities such as medical examinations, clothing changes, Doppler monitoring, or trips to the bathroom, subjects remained seated for one hour before the dive and two hours after the dive. Climbing or descending stairs was strictly avoided for two hours before and three hours after diving. Subjects were compensated and participation was voluntary.

Ascent Rate : Results

Results will be made available when data analysis is complete.

 

[BlueDevil]

DAN published a study on ascent rates a few years ago. They compared 60 FPM, 30 FPM and 10 FPM, with 30 FPM coming out the best. That study found, though, that safety stops were more important than the ascent rate. The very best results were found with 30 FPM and both a deep stop and a conventional stop. That's what I personally aim for.

A more recent study by the same authors is indicating that the optimal time for the deep stop is 2.5 min at 15m following the 25m dive profile they used in the study. A 1 min deep stop provided no benefit, and 2 mins was beneficial, but not as good as 2.5min! (I doubt that most timers would allow you to accurately time 2.5 min - my computer just works on whole numbers)

Following the 2.5 min stop at 15m the length of the shallow stop at 6m made little difference (ie there was not a lot of difference between a 1 min stop and a 5 min stop)

 

[dougchartier]

Ascending faster than 30 FPM is not advised under normal circumstances. A 60 FPM can be made safely as well and in fact, used to be the standard ascent rate. There is also the old rule of surface no faster than your smallest bubbles.

Nitrogen buildup is not the only reason to do a slow ascent. As we breathe pressurized air at depth the risk of an embolism or barotrauma is significantly increased with fast ascents.

Ascending without a safety stop is also not advised but not absolutely required if you stay well within the limits. But for your own safety, make a stop, it can not hurt and only should be skipped in an emergency.

I learned the 60 feet/min limit in my open water certification but never learned specifically why it was necessary. From what I've read in the many great responses to the OP's questions, the ascent rate limit is there (1) to help prevent DCS and (2) to help prevent "embolism or barotrauma." I'm not clear exactly how the ascent rate limit helps this.

Could someone explain (1) why, if you've dived within the NDL, you need to limit your ascent rate, and (2) what are the "embolism or barotrauma" risks (does that include lung overexpansion injury?) of a fast ascent?

Thanks in advance.

 

[LeadTurn_SD]

I learned the 60 feet/min limit in my open water certification but never learned specifically why it was necessary. From what I've read in the many great responses to the OP's questions, the ascent rate limit is there (1) to help prevent DCS and (2) to help prevent "embolism or barotrauma." I'm not clear exactly how the ascent rate limit helps this.

Could someone explain (1) why, if you've dived within the NDL, you need to limit your ascent rate, and (2) what are the "embolism or barotrauma" risks (does that include lung overexpansion injury?) of a fast ascent?

Thanks in advance.

1.) Ascending too quickly, even within NDL's, increases risk of DCS because you increase the rate at which nitrogen is released from the tissues into the blood stream. Even within NDL limits, if you "zoom" up to the surface in an uncontrolled ascent you risk a DCS hit. The closer you are to an NDL limit, the greater the risk of an overly fast ascent. This is a simplification, but that is the "basic idea".

2.) The faster you ascend the faster the air in your lungs expands. You have a higher risk of not venting it fast enough to prevent an embolism. This is especially true in the final 15 feet or so.

 

[Thalassamania]

The ascent rate is an integral part of the model from which the table are calculated. Change the ascent rate, you change the M-values and you change the No-D limits. Slower is okay, as long as its not too slow, but faster breaks assumptions required by the model.