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)