Required Safety Stop a decompression stop?

[lamont]

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. Even on a recreational NDL dive you can have an overpressurization gradient and you will bubble before you hit the surface. Slowing down the rate of ascent will slow down bubble formation. Physiologically it will make some kind of difference after the dive if you decompress 'better' from an NDL dive (not all of them, but ones closer to the NDL limits) -- it is debatable if that leads to actual lower risk of DCS, and it is debatable as to which curve is actually 'better'.

[*]What is more important on a "recreational dive"? Ascent speed or a safety stop?

My guess is the safety stop. A rapid ascent to 15 feet will drive a gradient, but the stop will help to take that off.

[*]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?

There is evidence that rec divers who follow the ascent rate and do the stop bubble less on surfacing than divers who only follow the ascent rate. I can't cite chapter and verse offhand though. But a 30 fpm ascent with a 3 min safety stop is a measurably 'better' decompression than just 30 fpm alone.

[*]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'?

On a recreational dive it probably doesn't matter much, and typical recreational divers aren't going to be able to hold +/- 2' unless situations are very good. The more important point is to do 3 mins at 15 +/- 5 rather than at exactly 10, 15 or 20.

[*]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?

Near the surface. That is where the overpressurization gradient is highest. At the bottom of the dive there's no point in being slow.

For example, if you are diving 21% at 100 feet, then even in fully saturated compartments they only have 3.16 ata of PPN2 -- which is 72 feet in pressure depth ( ( 3.16 ata - 1.00 ata ) * 33 feet / ata ). That means that no matter how long you've been at 100 feet on that gas, there's no point in doing a slow ascent between 100 feet and 72 feet from a decompression standpoint -- you aren't offgassing into bubbles at all, so its better to get up to 72 feet to maximize the diffusive offgassing.

As a general rule, move faster closer to the bottom and slower closer to the surface -- and look at any decompression curve generated by any program or model, and it has that this general kind of shape.

[*]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?

No useful relevance for recreational diving.

DCS seems to have a time lag before it hits, so you can use that to your advantage and climb out of the water before you are done with your decompression (blowing off decompression) and then climb into a chamber aboard a ship which will recompress you in order to finish the omitted decompression. This probably puts the diver at some additional risk, but gets them out of the water and back on a ship. It has more relevance in technical diving in discussions of the relative merits of in-water recompression 'treatment' for missed deco or treating DCS, and for lost-deco-gas situations.

 

[lamont]

Also, people seem to be throwing around the words "mandatory" and "required" decompression diving without recognizing how fuzzy the limits are even there. I know of one diver who recently blew off from a 20 min @ 150 foot dive at about 50 feet and missed all the "mandatory" and "required" decompression above that level, but just sucked down O2 on the boat and didn't need to take a chamber ride. Clearly for that diver, on that day, the remaining missed "mandatory" decompression was, in fact, optional (and the diver presumably just got a free PFO test). But the risk of that kind of a profile is much higher than the risk run by a typical NDL dive and today if the diver ran the same schedule they might wind up in the chamber.

There is no clear line between "optional" and "mandatory" decompression, there is only increased or decreased risk. So all dives *are* decompression dives. But in recreational diving the bar for skills and gas management in order to stay within a zone of relative safety is much, much lower.

On a recreational dive, its is commendable to treat the safety stop as a mandatory decompression dive, but the risk run by blowing off that decompression is acceptable (and, indeed many recreational divers have run this risk their entire 'career' and it used to be SOP, so the risk is low). Recreational divers need to be more concerned with running out of gas or embolising on ascents, and in the event of an emergency should (safely) get onto the surface and not overly worry about ascent rates or decompression stops. Even on a 20 minute @ 150 foot dive, mistakes can be made and the risks are just a lot higher, but not guaranteed. On a 30 minute @ 240 foot dive missed decompression becomes more explosive and massive DCS and death become almost certain (e.g. the Rouses on U-869), so mistakes that are tolerable at the recreational level can't happen at that level without severe consequences.

 

[scubadada]

A safety stop is a nonmandatory decompression stop. It occurs during a nonstop dive. It provides an additional safety factor, particularly if the ascent rate may have been exceeded. It gives additional protection against the M value for the controlling tissue being exceeded prior to surfacing. Please see Deco for Divers, Mark Powell, 2008, Ch 2.

Good diving, Craig

 

[Mr Carcharodon]

A safety stop is a nonmandatory decompression stop. It occurs during a nonstop dive.
Good diving, Craig

The problem is that a stop during a non-stop dive is nonsense. If you do a stop to decompress it is a decompression stop. And I expect no one here or elsewhere can give a valid reason why a safety stop is done other than for decompression. Safety stops are decompression stops.

 

[jtivat]

Holy crap this is still going on!

Whatever you want to call it hanging for three minutes from 20' to 10' at the end of a dive is a good idea!

 

[scubadada]

The problem is that a stop during a non-stop dive is nonsense. If you do a stop to decompress it is a decompression stop. And I expect no one here or elsewhere can give a valid reason why a safety stop is done other than for decompression. Safety stops are decompression stops.

Correct, safety stop = nonmandatory decompression stop. One electively stops to decompress during a dive that was done within NDL. Outside NDL, the stops are no longer elective and would be mandatory decompression stops. Once I had committed to a decompression algorithm for a dive (computer, software, table), I would follow through with its recommendations regarding my exposure. We seem inordinately caught up in semantics.

Good diving, Craig

 

[boulderjohn]

I think people will find this DAN report helpful:

DAN Divers Alert Network

 

[milkathecow]

great link on the various reports john, it's good to be able to download the yearly ones and to analyse year by year and see the variations and the knowledge we can learn from it. thanks

 

[DBailey]

Hi All,

Forgive me if this has already been covered. When the RDP lists a required safety stop is this essentially a decompression stop?

Thanks,
Sean

My answer is sort of. I view it as a generic, catch-all decompression stop. A "real" (do the quotes with your fingers) decompression stop is planned out carefully based on your depth, time at depth, travel time, breathing gas, etc.

PADI recommends making one of these generic, catch-all decompression stops after EVERY SINGLE DIVE. Under some circumstances, PADI pushes it from recommended to required.

In all diving, safety stops, deep stops, decompression stops, etc. are used to help manage the risk of DCS. The RDP is set-up on the theory of "no decompression diving". Simply stated, no decompression diving means that if you are diving within the limits of the RDP, then at any time you can ascend to the surface with minimal risk of DCS. Doing the safety stops lowers the minimal risk even more.

 

[boulderjohn]

great link on the various reports john, it's good to be able to download the yearly ones and to analyse year by year and see the variations and the knowledge we can learn from it. thanks

I'm a bit confused. The link should send you to a specific report on ascent rates and safety stops.

The report indicates that stops are more important than had been previously thought.