Improving Ascent/ Safety Stop Practices

[Scuba_Steve]

I think we can rationalize this until we're blue in the water, but I believe that his rationale was, in esence, that trimix is a better gas, but that it requires stricter offgassing techniques, which if you use Nitrox, you can achieve a similar result by slowing your ascent to 10 f/p/m.

I took a mini-hit leading a drift dive to 70 fsw, on 36%, having done 30 fpm ascent to 35 fsw, 1 min stop, 30 fpm ascent to 15 fsw, 4 min stop, and slow ascent from 15 fsw to the surface. So the point is, that deserved or undeserved, DCS does not discriminate and can hit the most careful diver. Fortunately my hit manifested itself after what was somewhat strenous physical activity that happened about 1 hr after the dive. It only caused extreme fatigue and it did not require a chamber ride, but the doctor classified it as a minor dcs hit solely because of the extreme fatigue that followed the dive.

Again, you have your reasons for the hit (As best as we know), plus, it didn't require a chamber, making it at least an extremely mild one.

Heck, I think we all have had one of those!!!

 

[Scuba_Steve]

So, let me just summarize something for clarity here based on my understanding of what you just wrote. I'll try to fill in a few white spots to get them on the table because people will be doing that anyway and it's better to get the assumptions cleared up.

If you're at 100ft and you're close to your NDL and want to ascend. You go:

100
90 - 20 seconds
80 - 20 seconds
70 - 20 seconds
60 - 1 min
50 - 1 min
40 - 1 min
30 - 1 min
20 - 2 min
15 - 1 min (my assumption)
10 - 2-3 min (we used to call this a safety stop) <SNIP>
R..

The poor guy is trying to keep it simple, even though it would be impossible to answer your question since you've attached no MIX, or BT (for sure) to the calcs. Both matter (as I know you're well aware of it) and I could give you a 100ft dive on Air, with a BT of XX (As you say "close to the NDL") and get your numbers to work quite well for what I would do. (12 minutes for ascent, with a BT of 20min on AIR, for one)

Steve

 

[ABQ]

Yep; 20 minutes or less at 100' gets you back.

 

[Diver0001]

When you add gas management like that it does add another dimension to it... However I think RTodd was just trying to stress the importance of a slow ascent rate and how it is much more benificial compared to rocketing up to your safety stop to off-gas.

Well, there is a lot of room between what Richard described and rocketing.

The important thing about that post for the rec diver is the 20 second stops. He explains the "trick" to guaranteeing a 30fpm ascent by going 10' - 20 sec all the way to your safety stop depth. So a more typical ascent for the rec diver might look like this:

100
90 - 20 sec
80 - 20 sec
... and so on ....
30 - 1 min
20 - 1 min
10 - 3 min
5 - 20 sec
0 - 1 min hidden stop

That's about 1/2 the gas and gaurantees a 30fpm ascent rate that still slows towards the surface. It's not how you would do it if you were over the NDL's but we're not in this scenario. Your deep stop, if you were throwing one in, might be at 30ft (2ata off the bottom) so you might want to extend that stop a bit. This ascent is probably already a massive improvement over the common practice.

R..

 

[Scuba_Steve]

Also Diver0001, just to touch on your gas management, yes, to me it does seem reasonable, if the guy wanted to END the dive with 750PSI, he would have to start up with 1500+- for this dive. I see nothing wrong with that, since his SAC would determine that for this 100ft near NDL dive anyway. (Roughly figured of course).

We figure (Who's "we" anyway?) to call the dive @ 100ft with not less than 1000PSI, as we know we'll have enough reserve for two people with a safety stop, on one tank alone.

 

[Charlie99]

From the article “Deep Stops” in Alert Diver May/June 2004:
“What is interesting, and not necessarily intuitive, is that an in-water stop with a relatively rapid ascent rate appears to be more effective at eliminating inert gas than a very slow ascent rate.”

The above quote is a somewhat misleading simplication of some DAN testing. The full articles are on the DAN EUROPE site.

What those tests did show is that a slow, LINEAR ascent is not as good as a faster ascent with a couple stops. The slow linear ascent spends too much time deep, and not enough time shallow.

A slow linear ascent is better than a fast linear ascent. A nonlinear ascent, faster deep, slower shallow, is better than a slow linear ascent that takes the same amount of time. "Faster deep" is relative --- typically only 30fpm. The DAN tests also shows that instantaneous ascent rates of 18m/min or 60fpm with stops had better doppler results than 3m/min (10fpm) linear ascents. In other words it doesn't matter if you do a 10' move in 5 seconds, then wait another 25 seconds, or of you do a continuous gradual move of 10' in 30 seconds -- those two are almost equivalent.

What works best for me is to decide when leaving the bottom how many minutes I will take to surface. The heavier my loading the longer I take. Typically I use somewhere between 5 and 12 minutes.

My total ascent time + current runtime gives me the runtime at which I will surface. Initial ascent is typically 30FPM, so that tells me the runtime for at least another point. The transition from 30fpm to slower ascent is determined differently by different divers. For my diving, I find that 80% of ATA rule is too deep. 2ATA off the bottom or 40', whichever is deeper seems to work better. Distribute time between 40/20/15-to-surface such that your average ascent gets slower as you get shallower.

Diver0001's schedule above is a good example, although my preference would have been to continue at 30sec/10' up to 50', then add the extra couple of minutes to the 30' and 20-to-10' stop.

 

[Diver0001]

Also Diver0001, just to touch on your gas management, yes, to me it does seem reasonable, if the guy wanted to END the dive with 750PSI, he would have to start up with 1500+- for this dive. I see nothing wrong with that, since his SAC would determine that for this 100ft near NDL dive anyway. (Roughly figured of course).

We figure (Who's "we" anyway?) to call the dive @ 100ft with not less than 1000PSI, as we know we'll have enough reserve for two people with a safety stop, on one tank alone.

I was waiting for someone to pick that one up. Maybe we should conclude not that the ascent is wrong but that many divers wait way too long before they start to ascend and then (have to) rush it..... !?

R..

 

[Scuba_Steve]

Yeah, we're smart guys aren't we <Collective pats on the backs!!>

I think you're right, most folks wait way too long, and then, Zing, gotta hit 15ft now! <Beep, Beep goes Mr. Computer, all the way up>.

Good discussion guys.

 

[Charlie99]

100
90 - 20 sec
80 - 20 sec
... and so on ....
30 - 1 min
20 - 1 min
10 - 3 min
5 - 20 sec
0 - 1 min hidden stop

It's not how you would do it if you were over the NDL's but we're not in this scenario. R..

Actually for the short 10' ceiling deco obligation times I incur using an AL80 my ascent is still very similar to what you posted, but with additional time starting around 40', using the same "shape of deco" where the stop times get longer and longer as depth decreases.
Charlie

 

[RTodd]

If you're at 100ft and you're close to your NDL and want to ascend. You go:

100
90 - 20 seconds
80 - 20 seconds
70 - 20 seconds
60 - 1 min
50 - 1 min
40 - 1 min
30 - 1 min
20 - 2 min
15 - 1 min (my assumption)
10 - 2-3 min (we used to call this a safety stop)

and then you didn't specify it but a nice slow ascent from 10 might look like

8 - 30 seconds
5 - 1 min
3 - 30 seconds
0 - 1 min hidden stop

So from a 100ft dive your ascent is costing you 12 - 14 min of which only 2 min is deeper than 60 ft. Looks good.

Assuming this isn't a little overboard let's just take it a step further. Let's assume a diver with a well charged AL-80 and a SAC of 15 litres / min (.45 or so?) does this ascent. How much pressure will he need....



So he needs to turn his dive and start ascending at a little over 1300 psi.

So totally ignoring the descent then our diver with a 15 lpm SAC is breathing 10 bar or so (160 psi give or take) per min on the bottom which gives him a useable bottom time of 10 or 11 min max, in rough terms. (at least if I didn't make any major blunders in my finger-tip math).

So you're well away from the 20-25 min NDL you have at 100ft depending on what you're breathing.

I'm not trying to pick apart your post; I think these kinds of ascents are just fine..... However; when you look at the gas requirements it does add a different dimension to the idea of ascending like this, doesn't it.

R..

Okay, I hate to put numbers to this because the people that are truly being exposed to this concept for the first time may get too hung up on a set ascent practice. As others have said, exactly when the ascent slows depends on the profile. Personally, I generally do a 100/70 multi-level dive for open water dives. On a multi-level profile, I would slowly work my way up to 50-60' and then begin my "ascent". In this scenario the first 1 minute stops wouldn't begin until 40'.

However, lets take a real world dive I would do if the bottom dictated. Nitrox 32, bottom time of 35 minutes with an average depth between 95 and 100'. The ascent you posted is basically what I would do. However, I would spend 2 minutes at 15, 1 at 10 and spend 30 seconds moving to 5'. Then, I would surface to avoid getting hit by a boat. Really slow from 10' is much more practical on shore dives/ cave dives. Now, if we are just messing around with plenty of time or this is a repetitive dive, I would spend progressively more time at every depth from 40' up.

As for your gas usage calcs, that totally depends on the diver. My regular open water dive buddy weighs 95lbs. Assuming we messed around on the shallow stops because we had time scenario, we would both surface from this dive with more than 600 psi. For those with higher sac rates, they should be doing more of a multi-level dive to get more bottom time and then the "ascent" time is less.