ascent rate: how slow is too slow?

[KenGordon]

After owning my computer for 2 years, I finally took the time to download my dives and look at my dive profiles. This was prompted by a series of four dives to ~100 ft from my last dive trip, two of which were part of my AOW Deep/Wreck dives. All dives were on wrecks in water with moderate to high current. Consequently the descent took 4 minutes because we were fighting the current the whole way down. For all four dives I found that based on my gas planning and NDLs, I ended my dive and began my ascent up the mooring line at about 17 minutes bottom time. My ascent rate was about 10 ft/min with a 3 minute safety stop. I surfaced with a dive time of 29-31 minutes for all dives. I remember thinking at the time "that's a really long ascent", but it seemed to be about the same speed that other divers ascended so I assumed it was about right.

My question is this: At what point is a slow ascent unproductive? Is 10 ft/min too slow? I can't help but notice it's 3x slower than the recommended ascent rate and it takes 9 minutes to reach the safety stop from 100 ft.

How hot is that porridge? Too hot, too cold or just right?

Deco risk increases with exposure. Longer dealer dives are riskier than shorter shallow ones. By ascending slowly you are increasing your dive time and so risk. You will have on gased more. Since you are within the NDL time of your computer you ought to be able to ascend safely at its recommended rate. You would be spending an extra three minutes below 50ft.

 

[AdivingBel]

I think that doesn't quite works the same way if you're coming up the line from a 30m wreck in the middle of nothing.

Yeah... popping bubbles from fellow divers below you, barnacles on the rope, and maybe a barracuda or two.

Just used Bonaire as an example that long slow ascents won't hurt anything as long as you're within NDLs (and have enough gas). Not saying it is optimal or enjoyable. Just saying that if you ascend really slowly (as in your original post) your computer will treat it as multi-level bottom time and calculate accordingly. As far as unproductive...I've had some excellent shark & ray sightings while staring into the blue coming up a mooring line, but that doesn't mean I want to spend the bulk of my bottom time there.

 

[kafkaland]

I feel rushed when I plan my dive with an ascent rate of 30ft/min. I like to slow it down to about 20 ft/min. As an example, when I do a dive to 120ft., and have my first stop at 40ft., it makes subjectively a difference for me whether I try to hit that stop after three minutes, or after four. I am fine with taking a little penalty in terms of nitrogen loading and another minute or two to offgas in the shallows in exchange for not feeling rushed during ascent. So this becomes part of my dive plan, and it works for me.

 

[Bob DBF]

Your computer will account for slow ascent rates. Your are fine as long as you don't run out of gas. Picture diving on a wall. Nearly all of your dive can be ascending.

Based upon a recent thread regarding a surprise deco, if your computers NDL is still decreasing as you ascend, then you are going too slow.

Slow ascents aren't the same as deep stops, especially in this context. You can easily take 45 minutes to ascend from 100' to 20' on a wreck, wall, or pinnacle.

All of this can true so watch your NDL. I followed the bottom of the lake up to the surface as usual, and on a flatter stretch the computer profile showed under 10 fpm and I went into deco. I knew the rise increased shortly so I continued to rack up deco untill the slope increased and I could do close to 30 fpm ascent and the deco decreased until I was out and decreasing NDL. If I had continued at that rate of ascent, I would have continued further into deco, and if far enough I would not have had the air to cover the deco.

Granted, I had a prior dive and knew that I was going to be near NDL on that dive so I reserved more air for any issue that might come up. I never noticed the decrease of NDL at that point before because I was always well within NDL and paid little attention.

As long as you are well within NDL the ascent rate makes no difference, if you are near the limit of NDL you may well be put into deco by a slow ascent. I'd rather stay within NDL and do a long safety stop.


Bob

 

[reefrat]

Even allowing for the safety stop I ascend the last 20 or so feet very very slowly. I am always in horizontal trim and using my lung volume primarily to control the ascent rate. Maybe it's not really necessary but I've been doing it this way for more than 20 years and feel that the last 30 ft is where the greatest pressure gradient is, so the slower the betterer!
Deeper than this and my ascent rate is pretty average and typical.

 

[BRT]

I used to do that slow ascent thing. At 130 my computer would say I was at 1 minute to NDL so I would ascend 5 or 10 feet and wait till it said 1 minute again. This was then considered a bad thing.

 

[100days-a-year]

As long as your goat doesn't die then you are OK.
Unless you make me have to wait ,then you are not OK.
Seriously,if one is so risk averse as to consider 10fpm on NDL dives,one might consider golf instead.

 

[dmaziuk]

Just saying that if you ascend really slowly (as in your original post) your computer will treat it as multi-level bottom time and calculate accordingly.

I was nitpicking: it depends on the depth and profile and the computer. If you ascend really slowly from 10 metres it (or at least some computers) will treat it as off-gassing time. Bonaire is a good example: I got consistently longer NDLs after longer dives on Bonaire where we would follow the bottom all the way to the beach.

 

[stepfen]

I recently opened a related thread ( Past NDL. And then this??? ). In my case things were a bit more serious because I was already in non-planned deco and I did what I thought to be slow ascent. It turned out that I was far too slow. My computer didn't like that and it was trying to alert me, but I kept ignoring it - being stuck with the "slower the better" mentality.

You can go through that thread to find several good suggestions by fellow divers. In particular @DevonDiver sent me a link where your exact question is discussed: Best Ascent Speed for Scuba Diving
(BTW you can find several other very interesting articles in that blog.)

If interested a simplified physical explanation of what's going on is this: Some body parts accumulate and off gas Nitrogen fast (they are so called fast tissues). Other parts accumulate and off gas nitrogen much slower (slow tissues). As you ascent, fast tissues are off gassing accumulated nitrogen. The slow tissues though keep accumulating nitrogen as long as you are deeper than few meters or so. So during a (slow) ascent the nitrogen in your slow tissues keeps increasing. This can become significant especially when you do several dives a day over several days and hence your slow tissues might have significant nitrogen residue from previous dives. All these are taken into account from your dive computer in calculating NDL etc.

On the other hand, according to PADI OW manual dangerously fast ascent rates (for forming bubbles) that we should never reach are 15+m/min or so. So 10m/min has been considered a "good" or "suggested" ascent rate (see article above) balancing fast ascent rate (to prevent further on gassing in slow tissues) and safety (avoid bubble formation).

I hope these help.

 

[rsingler]

He meant "So 10m/min has been considered..."
Don't jump on him - he was very good to post "Past NDL. And then this???"