Alert diver article on Deep Stops

[TSandM]

The idea of deep stops comes out of the mathematics of bubble mechanics. If you treat all absorbed nitrogen as being and staying in solution, the best way to get rid of it is to push the gradient between what's in your blood and what you're breathing -- which means get shallow quickly, and stay there a long time. That's what the Buhlmann-type decompression algorithms come up with.

However, we do know that not all nitrogen remains in solution. Some of it forms bubbles, and bubbles are more difficult to get rid of than dissolved gas. They are also thought to be the agents of tissue damage in DCS (the absolute link between the two is not as clear as you might think). Bubbles form and grow when the gradient is high, so getting shallow fast is not advisable, if you are using a bubble-based model. But, if you stay too deep, you don't have an adequate gradient to drive dissolved nitrogen out, and you may even be deep enough to cause some parts of your body to continue to ongas.

So, if you are going to decide what ascent strategy you want to use, you first have to define an endpoint. Are you looking to minimize bubbling on Doppler, or are you looking to avoid symptomatic DCS? They are NOT the same thing.

 

[MXGratefulDiver]

To understand the reasoning behind deep stops for recreational diving, you have to keep in mind that your body is made up of hundreds of different types of tissues all of which ongas and offgas at different rates ... and even for the same tissue types, the rates will vary from dive to dive, and day to day, depending on a variety of factors. So there is no real science that can be applied here ... it's all a matter of "playing the odds".

For the recreational diver, what's sufficient to understand is that as you begin your ascent, some of your tissues will start offgassing while others will continue to ongas. The offgassing tissues will be those that ongassed the fastest, and therefore came closest to saturation at whatever level you were diving. Those will mostly be your blood and nervous system. As you begin your ascent, because these tissues came closer to saturation than the tissues in your muscular and skeletal systems, they will offgas faster as you get closer to the surface. Continuing your ascent makes them offgas even faster.

So the whole point of the deep stop is to give these faster tissues a chance to "catch up" with your lung's ability to expel the offgassed nitrogen. Also keep in mind that these are the same tissues that are involved in the more serious Type II DCS ... so why wouldn't you want to "play it safe" and keep the offgassing rate at a relatively low level?

The point being that what you are doing with a deep stop is "playing the odds" of a benefit ... there are simply too many variables to know whether it's really helping or not. But like the levels of conservatism built into your dive computer, they help you reduce the odds of having a really bad day and overloading your system beyond your lung's capacity to expel the offgassed nitrogen. There are, of course, tradeoffs (there always are). In this case it is that the same deep stop may increase the level of nitrogen loading in your slowest tissues ... but the stop is brief (about a minute or so), and because those are slow tissues, the price you pay for the potential benefit is a small one.

There are, of course, far more specific explanations about bubble formation etc. that can be applied here ... but frankly what the recreational diver needs to know is simply that a deep stop is intended to keep the rate of gas exchange coming out of your fastest tissues at a reduced level where your body can more safely deal with it. And like anything involving decompression, it's mostly a game of deciding how much of a safety factor you're comfortable with ... because the process of decompression involves too many variables to quantify whether it's helping you or not. The only thing you can know for certain is that, barring a low on air situation or some factor involving being cold or really needing to get out of the water and pee ... there really isn't a downside.

... Bob (Grateful Diver)

 

[Mike Boswell]

I think the deep stop is a great idea. It makes us rec divers feel like we are playing with the big kids.

 

[fnfalman]

For one, I don't think most people would call a 1-minute stop to be a "level."

But in any case I think you are a victim of flawed terminology' in specific: "ND(L)".

Why "can" spend 20 minutes at 100 feet, 10 at 50, 10 at 30 and 10 at 10 while you can't spend all that time at 100 feet?

Because, on those higher 'levels' at least some of your tissues are offgassing (i.e. decompressing).

Okay, in that case, then why bother holding a 1-minute stop at half-depth? Let's just reduce ascension rate even more.

The problem is that there are no studies, no empirical data to back up the theory of deep stop.

 

[TMHeimer]

Certainly makes sense to me. Another way of putting it is say you dive 30 mins. at 70 feet. You do the 3 min. shallow safety stop even though it's not required. If you dive to 120 feet and do a deep stop at 70 you are doing a safety stop at a depth from which you should do a safety stop.[/QUOTE]

My post was too simple. As some have said, there is offgassing and ongassing depending on the compartment. Perhaps offgassing from the fast ones is a bit more important as they are closer to saturation at depth and more likely to create bubbles.

 

[lamont]

A half-max depth stop in recreational diving probably has more benefit to simply slowing and controlling the ascent rate, which could help prevent blowing to the surface ballistically.

The possible unintended consequence that i see is of recreational divers that are low on gas, who focus on the deep stop instead of getting shallow.

Its probably better for divers to just gradually let off the pressure from 30 feet to the surface and add more total decompression time there.

 

[dumpsterDiver]

To understand the reasoning behind deep stops for recreational diving, you have to keep in mind that your body is made up of hundreds of different types of tissues all of which ongas and offgas at different rates ... and even for the same tissue types, the rates will vary from dive to dive, and day to day, depending on a variety of factors. So there is no real science that can be applied here ... it's all a matter of "playing the odds".

For the recreational diver, what's sufficient to understand is that as you begin your ascent, some of your tissues will start offgassing while others will continue to ongas. The offgassing tissues will be those that ongassed the fastest, and therefore came closest to saturation at whatever level you were diving. Those will mostly be your blood and nervous system. As you begin your ascent, because these tissues came closer to saturation than the tissues in your muscular and skeletal systems, they will offgas faster as you get closer to the surface. Continuing your ascent makes them offgas even faster.

So the whole point of the deep stop is to give these faster tissues a chance to "catch up" with your lung's ability to expel the offgassed nitrogen. Also keep in mind that these are the same tissues that are involved in the more serious Type II DCS ... so why wouldn't you want to "play it safe" and keep the offgassing rate at a relatively low level?

The point being that what you are doing with a deep stop is "playing the odds" of a benefit ... there are simply too many variables to know whether it's really helping or not. But like the levels of conservatism built into your dive computer, they help you reduce the odds of having a really bad day and overloading your system beyond your lung's capacity to expel the offgassed nitrogen. There are, of course, tradeoffs (there always are). In this case it is that the same deep stop may increase the level of nitrogen loading in your slowest tissues ... but the stop is brief (about a minute or so), and because those are slow tissues, the price you pay for the potential benefit is a small one.

There are, of course, far more specific explanations about bubble formation etc. that can be applied here ... but frankly what the recreational diver needs to know is simply that a deep stop is intended to keep the rate of gas exchange coming out of your fastest tissues at a reduced level where your body can more safely deal with it. And like anything involving decompression, it's mostly a game of deciding how much of a safety factor you're comfortable with ... because the process of decompression involves too many variables to quantify whether it's helping you or not. The only thing you can know for certain is that, barring a low on air situation or some factor involving being cold or really needing to get out of the water and pee ... there really isn't a downside.

... Bob (Grateful Diver)

I agree, From my personal expereince doing multiple recreational dives at depths between 60 and 200 feet, sometimes 5-6 dives per day and sometimes having required deco stops on multiple dives..the deep stops make me feel noticably better,

The way I look at it is: If you go to 100 feet and max out the no-deco limit, the faster tissues are now loaded up. If I stop at 50 feet for a few minutes, the fast tissues should be loosing some nitrogen (slowly) and the slow tissue is still absorbing MORE nitrogen. However, the Navy dive tables says my no-deco limit at 50 feet is 100 minutes. If I load up the slow tissue at 50 feet for maybe an extra 2% (2-3 minutes) of the allowable amount, I am not worried about it.


Plus, if I load up my slow tissue a little too much i will probably get a shoulder or elbow hit. If I exceed the limit and get bubbles in the fast tissue, I might get paralyzed. I will take the sore shoulder anyday.

I was very surprised by the DAN article.

 

[JeffG]

doing multiple recreational dives at depths between 60 and 200 feet, sometimes 5-6 dives per day

I was very surprised by the DAN article.

Context.

I doubt your diving habits are within the boundaries of which they are talking about.

 

[Slamfire]

Okay, in that case, then why bother holding a 1-minute stop at half-depth? Let's just reduce ascension rate even more.

First of all let me disclaim that I think I skimmed over the same article referenced here a couple of months ago. I probably should wait to get back to the mag and read it again before posting, so correct me if wrong. Was it that article that said that inspite of the new algorithms and the longer deco stop times the incidence of DCS has not significantly decreased? I think it then went on to suggest that perhaps we should look not into longer deco times but slower ascent rates. This is because recommended ascent rates have remained more or less stuck at 30 ft/min for a while.

It is not uncommon to find divers that plan their deco schedule using only one ascent rate i.e. the 30ft/min. A lot of divers will also plan trying to shape the ascent curve to resemble the green smooth curve seen below. Because they have a constant ascent rate, the means of shaping the curve are limited to the stops. Hence deeper stops will usually be shorter than shallower. The whole intent of shaping to the curve is to conform with safe off gassing of different tissues and handling of bubble mechanics. However even if you add a second ascent rate, you still have to resort to deep stops to shape the curve.

Now, imagine trying to shape the curve only using varying ascent rates and eliminating all stops. It means that your ascent rate will always be changing. I'd think that diving that perfectly smooth ever changing curve would be a fairly difficult and maybe unnecessarily impossible curve to produce in real life.

Another more plausible option would be to try to mimic the perfect curve using multiple linear ascent rates. It still may not be as easy to implement as the deep stop shapes, but it may be more plausible to implement. The problem then is that most tables and deco software are not set up to handle dives just that way. I guess you could give it a try doing some depth averaging to smooth out some deep stops.

 

[Bubbletrubble]

First of all let me disclaim that I think I skimmed over the same article referenced here a couple of months ago. I probably should wait to get back to the mag and read it again before posting, so correct me if wrong. Was it that article that said that inspite of the new algorithms and the longer deco stop times the incidence of DCS has not significantly decreased? I think it then went on to suggest that perhaps we should look not into longer deco times but slower ascent rates. This is because recommended ascent rates have remained more or less stuck at 30 ft/min for a while.

@Slamfire: You may want to read the article. The one that we're discussing just came out in the Winter 2010 edition of Alert Diver. It showed up in my mailbox last week.