Safety stops

[Soggy]

Yeah, that's what I wonder about Soggy's example. If it was a short dive you could easily still be taking on gas at a deep stop like that (I would think). I'm not sure what I think of that but I would appreciate if someone versed in the subject spoke up.

I wouldn't do all that for a short dive, but on anything coming close to the NDLs, you would be offgassing some compartments and possibly still ongassing others...My understanding is that deep stops help prevent bubble formation and also help to offgas the tissues that cause Type II hits, so if you were to get bent, it'll likely be something more minor, like a skin bend.

If you run a Vplanner profile on a 100' dive for 30 mins on 32%, you'll find that the offgassing point is right around 60' (if I recall), so the stops are supported by current models.

Basically, what novadiver said

 

[Clive]

I know PADI teach (initially) recreational diving should be all no decompression diving and the safety stop of 3 mins at 5 metres at the end of every dive is just that and in theory divers should be able to surface without it from dives within table limitations, except in the cases specified on the tables.

Having also trained with the BSAC (British Sub Aqua Club) system I know that they teach "decompression diving" from the start with tables which give multiple stops at deeper depths for longer and deeper dives. For example you may stop at 12 metres for 1 minute and at 9 metres for 1 minute and at 3 metres for 2 minutes at the end of some dives.

I don't think this is what has been in the news more recently which is a suggestion that basically slowing the total ascent by having much deeper stops probably starting at 30 metres from a 60 metre dive and again at 15 metres and then 10 metres could be safer as it allows off gassing before larger nitrogen bubbles form in the blood. It should reduce deco time for deeper dives. The new theories tend to get reported when used to develop dive plans for some of the deep dive record attempts but bear in mind the divers involved in these may be non typical and safe surfacing by one or two is no guarantee that it will be safe for everyone to follow the same plan (Just as there are no guarantees against DCI in any event).

I often plan deeper dives with a fairly rapid descent to max depth and a very gradual ascent without stops apart from a final safety stop, which both extends total enjoyable dive time and I believe offers reduced risk of DCI.

 

[lamont]

if you plan a dive to the NDL at 100 fsw (at least 20 mins even on air) you're going to be doing some fairly serious loading in you fast compartments. for a 5 min half-time compartment you are going to have loaded that compartment to 93% of 4 ATAs of pressure (of .79% N2). those fast compartments can withstand more pressure differential before bubbling (have higher M-values), but in my mind, you shouldn't immediately bounce up to 15 feet (1.5 ATA). you want to go up slowly, but at the same time you're mostly concerned about fast compartments at that depth, so you don't have to spend too much time coming up because they're offgassing as fast as they ongassed. when you get close to the surface you want to come up really slowly because you're more concerned there with slower compartments (slower offgassing) that withstand less pressure differential (smaller M-values). the natural curve that you want to hit is going to look sort of like a decay curve to the surface. a "straight-line" 30 fpm ascent with a "straight-line" 3 min @ 15 ft stop, followed by a "straight-line" ascent to the surface isn't a curve.

 

[pete340]

the natural curve that you want to hit is going to look sort of like a decay curve to the surface. a "straight-line" 30 fpm ascent with a "straight-line" 3 min @ 15 ft stop, followed by a "straight-line" ascent to the surface isn't a curve.

So all those people who've been diving those "straight-line" profile have been fooling themselves into thinking they haven't gotten bent? <g> Ascending at a varying rate in order to match a theoretical optimal profile is hard; fixed rate with a safety stop is easy to describe and easy to do, and it works.

 

[Charlie99]

I don't know how other agencies teach it, but GUE teaches a 'minimum deco' ascent which includes on ALL dives 1 minute at 30', 20', and 10'. Additionally, deep stops are started at 80% of your max atmospheres or 75% of your maximum depth (they come out to be about the same number) and continued every 10' thereafter.

So, for a 100 foot dive, the stops would be
70' - :01
60' - :01
50' - :01
40' - :01
30' - :01
20' - :01
10' - :01

the natural curve that you want to hit is going to look sort of like a decay curve to the surface. a "straight-line" 30 fpm ascent with a "straight-line" 3 min @ 15 ft stop, followed by a "straight-line" ascent to the surface isn't a curve.

I agree wholeheartedly with LAMONT. A straightline 10fpm ascent, which is effectively what Soggy posted above as the GUE deep stop method is definitely NOT the most effective ascent.

That sequence of 1 minute stops from 70' to 10' (which is effective a Hill style linear ascent of 10fpm) spends too much time deep and not enough time shallow.

The "natural curve" that Lamont refers to can be seen in just about any decompression program if you either enter a very agressive dive profile that generates deep required deco stops, or if you crank of the conservatism so high that deep stops are generated. You will also see the "faster at depth, slower and slower as you near the surface" curve if you use low gradient factors in a program that has that tweak.

 

[jonnythan]

So all those people who've been diving those "straight-line" profile have been fooling themselves into thinking they haven't gotten bent? <g> Ascending at a varying rate in order to match a theoretical optimal profile is hard; fixed rate with a safety stop is easy to describe and easy to do, and it works.

You're right, it does work, but there are better ways.

 

[jonnythan]

I agree wholeheartedly with LAMONT. A straightline 10fpm ascent, which is effectively what Soggy posted above as the GUE deep stop method is definitely NOT the most effective ascent.

That sequence of 1 minute stops from 70' to 10' (which is effective a Hill style linear ascent of 10fpm) spends too much time deep and not enough time shallow.

The "natural curve" that Lamont refers to can be seen in just about any decompression program if you either enter a very agressive dive profile that generates deep required deco stops, or if you crank of the conservatism so high that deep stops are generated. You will also see the "faster at depth, slower and slower as you near the surface" curve if you use low gradient factors in a program that has that tweak.

I agree there, too. 10 fpm straight up from 70' isn't the way your ascent should look, IMO.

 

[pete340]

You're right, it does work, but there are better ways.

If you never drive more than 70 mph you don't need a car that can go 150. "Better" isn't necessarily better; sometimes it's just more complicated and wasteful.

 

[Charlie99]

If you never drive more than 70 mph you don't need a car that can go 150. "Better" isn't necessarily better; sometimes it's just more complicated and wasteful.

It doesn't have to be all that complicated. IIRC, the Pyle method of deep stops is to stop at 1/2 the distance between your starting point and your 1st deco stop (or the 15' safety stop). Once you've finished that first deep stop, then you split the difference again. Then perhaps split it again, but only if the difference is more than 15 or 20'.

Or you can just have a couple precanned ascents ... one for the 90-120' range, another for 60-90' range.

If I were to spend 7 minutes on an ascent from 100',rather than 1 minutes at each 10' from 70-10', I'd do something more like:

70' 1 minute
50' 1 minute
30' 2 minute
15' 3 minutes.

In practice, the 15' stop really starts closer to 20', and slowly changes into a 10' stop over the 3 minute period.

My computer shows only whole minutes, and usually I don't bother timing anything with my watch, which does show seconds. I just pick out a series of whole minute runtimes that are the endtimes for my deep stop / 2nd stop / 15' stop and go from there. On most of my dives there is something of enough interest a bit shallower than max depth that I'm starting from 80' or shallower rather than 100+', and there are just 2 deep stops in addition to 15'.
It sounds a lot more complicated that it really is.

 

[lamont]

So all those people who've been diving those "straight-line" profile have been fooling themselves into thinking they haven't gotten bent? <g> Ascending at a varying rate in order to match a theoretical optimal profile is hard; fixed rate with a safety stop is easy to describe and easy to do, and it works.

yeah, actually i think that's true. try going slower and shaping the curve better sometime. you should find that you've got less fatigue and/or are more alert after diving. the reason why you felt bad doing it the industry standard way is that you were bubbling too much when you go out. none of those bubbles were getting stuck anywhere to cause tissue necrosis, but the immune response was slowing you down.