Safety stops

[jonnythan]

I never said I was Mr. DIR. I just gave a profile that I was taught in a DIR class and explained what *I* do.

Was this presented as something appropriate to do every dive? Did they explain to you why to choose the depth you chose? Do you know why you'd choose the depth you did? Again.. at how many minutes would you choose to do the deep stop? 15 is not enough, but 30 is? How do you determine this for each dive?

Honestly... do you guess?

Ask yourself this...is what I am doing more or less safe than ascending straight to 30 ft (a difference of 2 ATM, in this case) and just doing 3 minutes of total deco time?

I think as you read more and progress more, you'll realize that decompression is a lot less black and white than you think. *Nobody* really understands how it works, just that certain types of profiles seem to work.

Just because decompression is still more a less a mystery to us doesn't mean that you can generate almost any profile you want without reasons behind those profiles, then explain them off by saying that deco is imprecise.

No offense, really, but I get the impression that you'd do a deep stop because your DIRF instructor presented that type of profile.. and you don't really understand why you'd do it that way.. and the only reason you can give is because you were told to do it and do it at a certain depth.

I don't care what you do, but I feel the information you're giving here might be the result of some sort of miscommunication or misunderstanding and I don't want that sort of thing to propogate.

 

[jonnythan]

If you don't like it, don't do it. You can continue to do 1@30, 1@20, 1@10 on every dive you do regardless of duration.

That's not necessary. I don't even use that profile, and I said so in my first post in the thread.

 

[Soggy]

Was this presented as something appropriate to do every dive? Did they explain to you why to choose the depth you chose? Do you know why you'd choose the depth you did? Again.. at how many minutes would you choose to do the deep stop? 15 is not enough, but 30 is? How do you determine this for each dive?

The deep stop is just a 30 second pause and 30 second move within recreational profiles. It's not like I'm staying at 70 ft for 15 minutes. It's just a way to start offgassing sooner rather than later. I do know a *little* about it. Do a little extrapolating and I'm sure you can figure it out yourself. I use a combination of what I've learned from reading, from my GUE classes, and from running Vplanner profiles. Deep stops have to do with pressure gradients in the various tissues that offgas at different rates. If you want more information, do the research yourself. I have.

I do a stop at 75% depth on almost every dive. Almost every dive I do that's below 60-70 ft or so is at or near the NDLs, as I dive big-arse doubles. I don't worry about it too much on dives where I'm at 60' for 30 minutes because I simply have not ongassed *that* much, especially since I almost always dive nitrox 32%.

If you want to read up on deep stops, there are plenty of papers out there.

Honestly... do you guess?

Yup, pure guesswork. I don't use my brain at all.

Just because decompression is still more a less a mystery to us doesn't mean that you can generate almost any profile you want without reasons behind those profiles, then explain them off by saying that deco is imprecise.

How am I generating "any profile want?" What I'm doing isn't particularly far from the realms of the traditional...it just follows modern science and is adding a bit of time deeper vs doing all the time shallow. I'm not sure what it is you find so objectionable. Like I said, I frequently "shape the curve" a bit differently from that profile by adding a couple minutes shallower.

No offense, really, but I get the impression that you'd do a deep stop because your DIRF instructor presented that type of profile.. and you don't really understand why you'd do it that way.. and the only reason you can give is because you were told to do it and do it at a certain depth.

Well
1) I don't think doing deep stops based on what you are taught by highly experienced instructors is a particularly bad idea...these guys have a *bit* more experience than you or I.

2) I'm not doing extended deep stops, just short pauses

3) I do have some understanding of it from readings I've done. Tons of this information is public domain.

I don't care what you do, but I feel the information you're giving here might be the result of some sort of miscommunication or misunderstanding and I don't want that sort of thing to propogate.

You're welcome to think that. I have email communications with a few different instructors clarifying the issue because I was initially confused on it myself. Brando has come in here and posted a *slightly* different profile a couple times, which includes only 30 sec pauses every atmosphere after the first deep stop at 75% depth. This reduces the overall ascent by 2 minutes, which, in the grand scheme of things, is nominal.

What do *YOU* think is an appropriate GUE Compliant profile?

 

[scottfiji]

Jonnythan, Soggy

- I just wanted to say thanks for all your postings, I'm following them and it's great to see different theories behind how to ascend. In the end, every diver has the freedom to ascend how they want. Isn't this country great! This is all the more interesting because decompression theory is so little understood by scientists. Anyways, thanks for postings your opinions, having open debates like this is what promotes learning.

Scott

 

[lragsac]

Um... OK.... I'll start.....
Before I started using the much slower ascent profiles and paying attention to "shaping the curve," I would usually feel quite fatigued and sometimes even downright crappy after a dive or two near the "NDL" and using the industry standard 30fpm ascent rate and 3 min at 15'. Since I started changing the shape of my ascent curve, I actually find I sometimes feel better after the dive than I did before the dive!!

Yep, me too. I happen to be one of those people with a physiology that seems to be very sensitive to bubbling. I've gotten mild skin bend hits on NDL dives (and, no, I don't have a PFO, and I'm not overweight or unfit, but I do seem to have crappy circulation and some areas of tissue that are poorly perfused). Since altering the shape of my ascent curve I've greatly reduced the incidence and severity of these hits. Of course, gas selection is another really important factor. I rarely do deep dives (90'-100') on air.

 

[Scuba_Vixen]

If I'm getting a good read on what Soggy's suggesting, it's that the schedule he posted is more a guideline, and should be considered somewhat flexible within logical bounds.

A 70' stop for a 100' dive (NDL dives) is still going to be eating away at the NDL time. (you'll still typically be ongassing) That's not that big an issue. If you use the 70' point as more of a "hollywood" stop, or slow rolling pause, it gives the team a chance to check gas and make sure everyone's on the same page and ready to ascend together.

If you think in terms of 1 minute intervals between 10' points, half the minute being a pause, and half being the next 10' ascent, then the idea of starting at 70' is more a way to keep the ascent slow and controlled, which is the idea in the first place.

Keep in mind here that we're talking NDLs and while all dives are decompression dives, that since this decompression is being done on backgas, (bottom mix) That there is no accellerated deco from partial pressure vacancy (oxygen window). Offgassing is driven by the gradient between ambient and dissolved gas pressures. Ambient is of course lowest closest to the surface, but rushing to get there invites bubbling, So we slow down getting there and hang out for a bit once there. How slow and how long, of course are the big issues.

The traditional 3 mins at 15' is much akin to the buhlman "bend 'em and mend 'em" algorithm. ... It's just in the times before doppler testing, no one actually realized it.

Personally, I use 30/30 for dives in the 100' range. Whether NDL, or short (<20min) deco, I do a rolling pause about 70' and a short stop every 10' from 50' up, adding a bit to each one if there's no deco obligation. If there is deco to satisfy, then those ceilings become hard stops once I get to that depth.

Just as all dives are really decompression dives, there really is no such thing as NDL, it's just a catchy phrase that the agencies like to use to make prospective new divers feel safer and more likely to want to learn to dive since it's so easy if you don't have to think about decompression.


Darlene

 

[Charlie99]

The traditional 3 mins at 15' is much akin to the buhlman "bend 'em and mend 'em" algorithm. ... It's just in the times before doppler testing, no one actually realized it.

Actually, there HAS been doppler testing on 3m/min (10fpm) linear ascent vs 10m/min (33fpm) with Haldanian (buhlmann in your lexicon) stops.

https://www.daneurope.org/eng/whatascent2.pdf

After a 25 minute 25m (83') dive, the constant 10fpm ascent of 8 minutes was the very worst profile (as measured by doppler testing) of the 8 tested, even though, with a total time to ascend of 8 minutes it was NOT the shortest time.

The DAN test showed that a 10m/m (33fpm) ascent with 5 minute stop at 6m (20') was both faster at 7.5 minutes, and performed better on the doppler testing.

In particular, 20% of the divers using the 8 minute constant 10fpm ascent has very high grade doppler bubbling, as opposed to only 5% of the divers using the 7.5 minute total time combo of 33fpm ascent with a single 20' stop.

------

Near the end of the article is a good summary: "The results indicate that slow, linear ascents (Hill approach) may be significantly more bubble-productive than ascents with even more rapid rates to reach intermediate stops and a similar total time to de-gas"

Please note that this DAN testing are reasonable dives well within the normal recreational limits at 83' 25 minutes, followed by a 83' 20 minute dive after a 3 hour SI.

 

[Scuba_Vixen]

Good link Charlie, thanks.
As I said, I'm not a believer in linear ascents, but do believe in making stops begining progressively deeper as the NDL time gets shorter, especially if you've been right on the edge of deco right up untill the gas transition point.

In the profile you quote, 83' for 25 mins, a 10'/min ascent is going to keep the diver ongassing for 2 or 3 of those minutes, with minimal time close to the surface where maximal offgassing occurs. It's not at all surprising that they tested worst. It would have been nice if they had tested divers doing 2 at 20' and 3 at 10' to see if it showed improvement over the 5 at 20' group.

The 3 mins at 15' safety stop came about back in the time when most agencies suggested 60 fpm ascents from any recreational depth. (and that was from before doppler) I consider that "bend 'em and mend 'em". If that wasn't basically true, they wouldn't be pushing 30 fpm ascent rates now.

It's definately a balancing act to be able to ascend to where maximal offgassing can occur while maintaining a low enough gradient along the way to preclude unacceptable bubble formation.

Recreational divers need to stop being scared to think "decompression". Realize that that stop before surfacing is a decompression stop, and that a well managed decompression has a curve to it.

[/ramble]


Darlene

 

[Charlie99]

To keep things in perspective, using just a 60fpm pop to the surface with no safety stop didn't have a really bad DCS rate. It's just that adding in a 3 min 15' stop is better.

Comparing the GUE DIRF linear ascent method vs. a proper shape of deco curve is taking things one more step.

I just figure that if you are going to spend X amount of time in an ascent, that you might as well get out of the water as clean as possible.

I agree with your comment that it would have been nice to have a 2@20' + 3@10' profile. That, or what is shown on www.dir-diver.com is closer to my understanding of what's ideal. The "half stop, then half again" Pyle stop method is both reasonable to calculate in the water, and generally comes in pretty close to what a VPM or RGBM model will come up with if you crank up the conservatism really high.

For me, that's "doing it right".

 

[CRDiver]

From 5thD's Michael Kane:

Rather then introduce certain concepts at the Fundamental level,
specifically 80% of ATA's, we recommend for an otherwise recreational
profile, to start the deep stops at 50' from the max. depth. In other
words, if you did a 120' dive, start the deep stops at 70'. Then do 30
second stops, with 30 second moves until you start your "safety" stops. In
lieu of the industry standard of 3 minutes at 15', we recommend doing an
additional minute at 30', then a minute at 20' and then a minute at 10'.

So to summarize, you do 30 FPM ascent rates, you start your deep stops at
50' off the max. depth, continue "deep stops" at 30 second stop- 30 second
move intervals until you get to 30'. Then do 1 minute at 30', 1 minute at
20' and 1 minute at 10'.

Bear in mind, this ascent rate strategy applies to no mandatory
decompression diving within otherwise defined recreational limits. If you
plan of progressing to deeper then "recreational" limits and/or doing
mandatory decompression diving, this strategy is inapplicable.