Ascent rates

[JonG1]

We were lucky enough to be able to spend a week in Scapa (N Scotland) last week.

Mixed group of oc and ccr depths from 25-45m 2 deco dives per day on average and either ascent via the shot or on SMBs depending on the dive, conditions etc.

A relatively hot topic for the week was how hard it is to actually hit and maintain a 9-10m ascent rate to the first stop.

Logging dives daily and comparing profiles we were all mainly averaging circa 6-8m per min.

If we did hit 9m a minute it felt pretty fast and unstable and was really only achievable on the shot ascents generally not during an SMB ascent.

Just wondered how others hit the prescribed 9m/min in ow.

In OHE the issue could be more pronounced given the profile of the cave or mine, and the slower the ascent the closer the deco profile strays towards a deep stop style ascent.

So does the computer adequately pad the shallow to reflect the slower ascent, or do you pad it to compensate?

 

[100days-a-year]

That was a prerequisite for getting tech certification for me.
We had to manage ascents in 3 increments 60fpm, 30fpm and 10fpm depending on depth. By visual reference and by computer. Computers are far more difficult for me. Visually with bubbles and particulates in the water was a lot easier on the bottom and mid water. Once close to the surface, I could see the the surface and that was pretty simple but I had to ignore a little variation due to swells offshore sometimes.

This was all done in wetsuits, wearing doubles and stages.

 

[boulderjohn]

I had a discussion on this topic with some pretty experienced tech divers a couple years ago, and the consensus was that it was extremely common for tech divers to ascend too slowly, sometimes extremely so. Your computer should adjust for that, though.

On a tech diving charter, the DM went around to the different groups asking their run times, and pretty much everyone was looking at something between 70-80 minutes. The two rebreather divers next to my teammate and me had exactly the same planned run time we did, and they were using the same GFs. As you might expect, we arrived at the ascent line only seconds ahead of them, and our groups were the last to ascend. My teammate and I ascended pretty carefully at the prescribed rate, using the chevrons on our Shearwaters as a guide. When we got to our first stop, I looked back down the ascent line and could not see the rebreather divers at all. I was concerned. As s we moved through the first stops, I kept looking back for them. Eventually they came into view. We got out of the water within a minute or two of our plan, got on the boat, and began to break down our gear. People started to get concerned about those two divers, who were well past their planned run times. When they finally got on the boat, the DM asked them what had happened. They said they didn't know--all they knew was that their computers kept adding time. I told them that their initial ascent had been awfully slow, and one of them said, "Well, you're supposed to ascend slow!"

In my early tech diving, we did not use computers. Our dives were planned ahead of time using the UTD version of Ratio Deco. We had the plans laid out in our wet notes, and when we got to the first stop, we followed those plans. A team of divers got bent. They were surprised--they had followed their plan perfectly. One of them was using a computer in gauge mode as a bottom timer, so he downloaded the dive profile and found that they had not been as perfect as planned. The biggest problem was that their ascent to the first stop had been extremely slow, so they had actually piled on a fair amount of extra bottom time during that ascent. They had made two other errors they did not know about, but the computer did. If they had been following a computer, it would have adjusted their ascent profile accordingly.

 

[dmaziuk]

So does the computer adequately pad the shallow to reflect the slower ascent, or do you pad it to compensate?

Short answer: yes to the former, long answer it can in theory get interesting if there is a switch in leading tissue compartment caused by a too slow ascent. I think you'd have to come up with a really special profile and ascent rate for that to happen.

 

[PfcAJ]

Gotta go fast.

 

[Vicko]

It's just practice, depending on the computer you use it's ascent speed reading might be delayed by up to 5 seconds, so dont accelerate to fast and once you hit the right speed use your feet to sense how much tension there is on your fins. If you feel them getting pushed down to much, exhale a bit or if you feel the tension slipping inhale a bit more.

 

[rjack321]

Too slow is far far too common, you gotta move and don't use ascent rate indicators as a guide. You need to be proficient enough to nail it based on the minutes (and seconds in gauge mode). If you are looking at the little chevrons almost guaranteed you're going to look back at the minutes and be way too slow.

 

[_Ralph]

9m/min on OC to me feels ‘controllably’ fast.... if that makes sense....

I’m not there yet with the mCCR... trying to keep my loop PPO2 up along with everything else is a skill I’m working on to keep up with mixed dive teams... I’d suspect
I’m around 5m/min.... but places where I’m deep on the CCR, most of that is multi profile wall shore dives, and I start bumping up the O2 during the later half ‘boring’ parts...

The shearwaters account for it..


_R

 

[Celt]

The secret to controlling a fixed assent rate is to maintain a negative buoyancy and fin up, we used to half our depth very fast and then slow to 60 fpm to the first stop with no computer.

 

[Gareth J]

I know when I was still teaching ERD and then Trimix on OC, getting people to ascend at 10m/min was a challenge. It was the most common reason for failures at the final assessment.

I do find ascending on a CCR far more complicated than when I was on OC. Managing three airspaces, and the falling PO2. Not so bad from deep to the deeper stops. But if your first stop is quite shallow I really struggle, I actually do a slightly deeper stop to get everything back on point before moving to the critical shallower stop.
It's one thing ascending up a shot line, but under a DSMB is much more difficult, with a reel, do-able, but a spool, impossible.

I was doing some bailout ascents yesterday, just for the hell if it, that was certainly under 10m/min. Although, being able to use your lungs to adjust buoyancy is a great luxury, although the trapped counterlung volume is a complication (, I normally vent out of the side of my mouth when on the loop).

The last few dives have felt great, like I was properly back in the zone. Just as we go back into another lockdown .

It's not as much of a problem now I uses computers in preference to runtimes. But it does mean you have to allow for the slower ascent in the gas planning, both the gas used, and the extra stops the slower ascent potentially incur.