Required Safety Stop a decompression stop?

[Reg Braithwaite]

It's "required" only to impress on you how important it is to add that safety cushion when diving near the model limits.

So are you speaking in your capacity as an instructor for PADI when you say that sometimes when PADI says something is "required," it isn't really required?

Seriously, no need to answer, I'm not trying to troll up a fight about agencies. I'm just trying to highlight something Rick Inman said much better than I: This simple question leads one to a lot more research about diving profiles and safety, and I believe that learning more is a good thing.

In the mean time, my understanding as a student is that conforming to the recommended ascent rate is "more required" than the "required stop."

 

[Walter]

Nevertheless, I and many other OW divers were taught that for certain profiles, a "safety stop" is "required." So by your statement, either the agency is wrong to call it a safety stop or the agency is wrong to tell us it is required.

Exactly. It is either a deco stop or it is not required. I would err on the side of caution and assume it is a required deco stop. Well, I actually err on the side of caution and stay away from the RDP.

Do I infer from this that you are saying the problem is the nomenclature and therefore the required stop is actually a decompression stop? If so, the answer to the OP's question would seem to be "yes." Is that what you are saying?

Correct.

2 things:

1. PADI teaches no stop (no decompression diving) diving with a recreational model. So, if you miss it.....it's not like missing a stop when deco diving. It's "required" only to impress on you how important it is to add that safety cushion when diving near the model limits. Especially now that research into the RGBM model discovered micro bubble nucleation with doppler flows.

2. Semantics. If we profess no stop, no decompression diving....it's silly to put in a deco stop. Some lawyer, somewhere, would use that in accident litigation.

You're pretty smart. That's exactly right. PADI says don't make dives requiring staged decompression. Then they come up with tables where dives they allow do require staged decompression. What to do, what to do?... Ah, let's change the terms and call a deco stop a required safety stop and pretend we still advocate not making dives requiring staged decompression. A jury won't understand the difference, we can pull this off.

 

[Reg Braithwaite]

PADI says don't make dives requiring staged decompression. Then they come up with tables where dives they allow do require staged decompression.

Another, unfounded (I heard it on ScubaBoard!) explanation is this: "Dives requiring staged decompression" is a phrase that refers to dive profiles where the underlying model (US Navy Tables, Buhlmann, whatever) requires a stop. Cue discussion of m-values and ambient pressure gradients. Anyhow, if you go with this narrow definition, then what happens if stops are added to the table without any basis in the model? Say by a lawyer?

In that case, one can argue that the dive doesn't require staged decompression in the sense that the underlying model doesn't require it. For example, if we start with US Navy tables and add conservatism with a simple rule (like adjusting depth and time ratios), then any dive that doesn't run into deco is "doesn't require staged decompression."

Now we add stops for some other reason, lawyers, we feel like it, or we are concerned that our divers are violating ascent rates. We call these "safety stops." Whether we require them or not, the underlying model doesn't require a deco stop, so we say the dive is not a staged decompression dive.

I used to do this, in a way. I own a Suunto Gecko. It doesn't do deep stops. I would add a short stop at 1/2 maximum depth on any dive below 60'. I still dove according to what my Gecko thought was my NDL for the dive, my deep stop was above and beyond what my model told me I should be doing. You could call my deep stop a "safety stop," even though I required it on every one of my dives below 60'.

 

[don Francisco]

Often these discussions become questions of semantics. In the broad definition of the word all dives are decompression dives, but we draw a line between those where one can (statistically) ascend directly to the surface, and those that require a stop to decompress before surfacing.

Likewise the required safety stop is still a safety stop, in that it isn't required for a safe ascent to the surface and therefore isn't a decompression stop. But it is required to remain within the no-decompression model when planning multiple dives.

It pays to remember that the decompression tables are based on mathematical models, yet our bodies aren't. Some folks will suffer DCS within the NDL, while others might violate it and suffer no consequences. Use common sense and take opportunities to extend safety stop times after deep or long dives, and plan dives with a margin of error within the NDL.

In a sense diving is like playing in traffic. You can do it for years and not get hit, you can even do it in heavy traffic and not get hit, but if you do it in heavy traffic often enough eventually it'll catch up to you.

 

[fdog]

Although you can make a NDL dive without a safety stop, there's significant bubble production. Silent bubbles. No symptoms.

Simply doing a 2 minute stop at 10' reduces bubble production to 1/6.

Doing a stop of 1 minute at 20' and 4 minutes at 10' reduces this to 1/12, and there are no traces of the silent bubbles at 45 minutes after surfacing.

For a single NDL dive this may not be significant, but has big implications for multiple repetitive dives over a day.

...may I suggest a fantastic text, Deco for Divers by Mark Powell, as an excellent text that answers all the questions you poise? This is a book that should be in every diver's library, right next to the NOAA Diving Manual.


All the best, James

 

[eponym]

Notrab, I assume by now you're even more confused. Some of the recreational terminology is worse than muddled.

And yes, this gets discussed often. Use Search to search for thread titles containing "safety stop" and you'll find other threads, for example this one from last December.

Keep asking questions and observing what others do. At some point you (and your buddy) will feel able to make those calls yourselves.

-Bryan

 

[TSandM]

I think a lot of the confusion comes from an attempt to draw a thick, black line between dives requiring staged decompression, and dive which don't. And there simply isn't such a line.

As dives get deeper and/or longer, more nitrogen is absorbed, and more time is required to offgas that nitrogen. For short, shallow dives, the likelihood of developing DCS symptoms with a direct ascent (at a controlled speed) is close enough to zero to be considered that. For dives of sufficient depth or time, the likelihood of symptoms with a direct ascent approaches 1 as a limit. But there are dives in between. At some point, each model will "require" a stop, but the requirement may be generated in different ways. If you don't understand where the stop requirement comes from in the model you are following, you can't know how important that stop is to your well-being.

It is my belief, and my practice, that deeper and longer dives within the recreational range (pushing the NDL of whatever model you are using) ought to be considered more and more like staged decompression dives, in that you SHOULD do some in-water decompression stops. But for either these dives that push the NDLs, or dives that go into a "little bit of deco", the likelihood of you being seriously harmed by omitting the stops is still low.

We need to stop trying to draw lines, and simply consider a risk continuum that increases as you increase the nitrogen load, and gradually transform our behavior to a more and more conservative profile as the nitrogen loading increases.

And yes, Reg, the exponential nature of gas kinetics results in a curve shape -- from every model -- that has a faster ascent rate deep and a gradual slowing as you approach the surface. 20 and 10 foot stops are only equivalent if you are breathing 100% O2.

 

[Blackwood]

All squares are rectangles, not all rectangles are square. A decompression stop is a stop during which you decompress. A 'safety stop' is a special case decompression stop which may be required by table methodology and instructions but not by the underlying decompression model used to develop said table.

 

[MXGratefulDiver]

No, a required satey stop is just that. I beleve, technicaly your still in the NDL and can still assend to the surface without stoping. I becomes 'REQUIRED" because you have gone below a certain depth or come close the the NDL.
Basicaly is a very good idea to always do a safety stop, but if something happens where you can't. Its still ok.

This is my understanding of it.

Technically, any stop you make ... required or otherwise ... is a deco stop, since the reason you're stopping is to allow your body to decompress before continuing your ascent. It isn't a matter of semantics ... it's a matter of degree.

Staying within NDL doesn't make your ascent "safe" ... it merely maintains the risk within what is generally considered an acceptable limit.

The safety stop is not a required decompression stop, in the sense that if you skip it you are not at an increased risk for DCS if you are within the limits of the table. However, it does allow your body to offgas, so technically it is a decompression stop.

Yes, if you skip it you ARE at an increased risk of DCS. An ascent is like walking under an umbrella in a thunderstorm where there's this little metal tip sticking out the top of the umbrella. Is it likely to attract a lightning strike? Yes ... but the risk is incredibly small. Extend the metal tip and the risk goes up.

Diving's like that. Every ascent produces a finite risk of exceeding your body's capacity to offgas safely.

How risky is too risky? That's always your call. Coming up slowly and making a safety stop will reduce that risk ... coming up faster or skipping the stop will increase it. Will that increase be statistically significant enough to worry about? Your call ... frankly even the "experts" in decompression couldn't give you a finite answer to that question.

Here are some things I wonder about:

• Are all dives decompression dives since on all dives I on-gas Nitrogen and must off-gas it at some point?

Yes.

• What is more important on a "recreational dive"? Ascent speed or a safety stop?

I would say ascent speed ... and the closer to the surface you get, the more important it becomes.

• I read on the Internet that the safety stop was put in the tables because divers were either ignoring or lacked the skills to ascend at the table's recomended rate, so stopping them at 15' would slow them down. is this true? If so, is there any empirical evidence that safety stops matter for people ascending according to the table's ascent rate?

Probably ... and no.

• Why 15'? Why not 10'? Or 20'? Do surface conditions such as waves or swells have anything to do with this? How about buoyancy skills? If you could hold a stop at 10' +/- 2' would it be better than 15'+/- 2'? What if you tried to hold a stop at 10' +/- 5'? Is that worse than 15' +/- 5'?

Most texts do say somewhere between 20 and 10 feet ... 15 is chosen because it's halfway in between. No ... I don't think the pressure difference at that point is going to be critical, because of the mechanics of what it's actually attempting to do.

• Is a slow ascent more important near the surface, half way up the ascent, one quarter of the way up the ascent, or near the bottom. Why?

The closer to the surface you are, the more important it is ... because it's the RATIO of pressure change per time unit that matters, not the absolute amount of change. And the closer you get to the surface, the faster that ratio changes.

FWIW - stopping or not is less important than what you do AFTER the safety stop ... too many divers will watch their bottom timer carefully, then hit the surface about 5 seconds after their safety stop is over. Taking a full half-minute to surface after completing your stop is AT LEAST as important as making the stop at all.

• Some tech divers were talking about deco the other day and they said that older deco protocols had them decompressing at the surface, while newer deco protocols have them decompressing in the water. What does this mean, and does it have any relevance for recreational diving?

What they really meant is that they are less ignorant today than they were back then ... but they're still generally ignorant (we all are ... although the mechanics of decompression have been generally understood for more than a century, there's an awful lot we STILL don't know about what causes it, and why).

Nevertheless, I and many other OW divers were taught that for certain profiles, a "safety stop" is "required." So by your statement, either the agency is wrong to call it a safety stop or the agency is wrong to tell us it is required.

Neither ... they're trying to give you some easy-to-remember rules of thumb so that you aren't required to go wade through Weinke's calculus equations to come up with your own conclusions.

Often these discussions become questions of semantics. In the broad definition of the word all dives are decompression dives, but we draw a line between those where one can (statistically) ascend directly to the surface, and those that require a stop to decompress before surfacing.

Likewise the required safety stop is still a safety stop, in that it isn't required for a safe ascent to the surface and therefore isn't a decompression stop. But it is required to remain within the no-decompression model when planning multiple dives.

It pays to remember that the decompression tables are based on mathematical models, yet our bodies aren't. Some folks will suffer DCS within the NDL, while others might violate it and suffer no consequences. Use common sense and take opportunities to extend safety stop times after deep or long dives, and plan dives with a margin of error within the NDL.

In a sense diving is like playing in traffic. You can do it for years and not get hit, you can even do it in heavy traffic and not get hit, but if you do it in heavy traffic often enough eventually it'll catch up to you.

It's also important to remember that even if you're playing in very light traffic ... even if there's just one car coming down that road per day ... there is still a statistical probability of getting hit. Therefore, it's incumbant on us when we play to pay attention to what we're doing, and actively take steps to protect ourselves.

Although you can make a NDL dive without a safety stop, there's significant bubble production. Silent bubbles. No symptoms.

Simply doing a 2 minute stop at 10' reduces bubble production to 1/6.

Doing a stop of 1 minute at 20' and 4 minutes at 10' reduces this to 1/12, and there are no traces of the silent bubbles at 45 minutes after surfacing.

For a single NDL dive this may not be significant, but has big implications for multiple repetitive dives over a day.

...may I suggest a fantastic text, Deco for Divers by Mark Powell, as an excellent text that answers all the questions you poise? This is a book that should be in every diver's library, right next to the NOAA Diving Manual.

Mark Powell's book is, by far, the most comprehensible read on this subject I've come across. I think it should be required reading for ANYONE who expects to take diving beyond the level of following a divemaster around a shallow reef.

It will answer all these questions, and much more ... in a way that anyone capable of reading it can comprehend.

... Bob (Grateful Diver)

 

[wve]

Here are some things I wonder about:

• Are all dives decompression dives since on all dives I on-gas Nitrogen and must off-gas it at some point?
• What is more important on a "recreational dive"? Ascent speed or a safety stop?
• I read on the Internet that the safety stop was put in the tables because divers were either ignoring or lacked the skills to ascend at the table's recomended rate, so stopping them at 15' would slow them down. is this true? If so, is there any empirical evidence that safety stops matter for people ascending according to the table's ascent rate?
• Why 15'? Why not 10'? Or 20'? Do surface conditions such as waves or swells have anything to do with this? How about buoyancy skills? If you could hold a stop at 10' +/- 2' would it be better than 15'+/- 2'? What if you tried to hold a stop at 10' +/- 5'? Is that worse than 15' +/- 5'?
• Is a slow ascent more important near the surface, half way up the ascent, one quarter of the way up the ascent, or near the bottom. Why?
• Some tech divers were talking about deco the other day and they said that older deco protocols had them decompressing at the surface, while newer deco protocols have them decompressing in the water. What does this mean, and does it have any relevance for recreational diving?

You may find some answers within this thread from a few months ago. There is a lot of the same debating as is going on here, but near the end of that thread are some references you may find interesting.