What ascent speed(s) do you use after gas switching on ascent?

[thetechnicaldiver]

Thanks all. Does anyone know when/where/how etc these variable ascent speeds were first outlined, and whether there are recent studies that have looked into variable ascent rates as they relate to decompression diving? I'm aware of numerous ascent rate studies on recreational and saturation dives, but haven't seen anything relating to bounce deco dives with different mixes. I'm sure there will be some people that vary their ascent by such and such (or not) just because that's what they were taught and so that's what they do, but others may have some mathematical or practical reasoning based on factors such as relative pressure change, supersaturation in leading tissues during ascent (and how they all tie in with 3m stop depth intervals), or extrapolation from recreational studies showing different bubble counts across different ascent speeds, or, some other reason.

 

[Billy Northrup]

Real slow, depends on the traffic getting back on the boat I'll hang out or go overly slow as the line clears up. As long as I got gas and respect the run time I left with the captain, meh.

 

[lermontov]

relative pressure change,.


thats what i was taught - shallower the depth the greater the ratio

 

[JohnnyC]

9 m/min to first stop depth. 3 m/min through stops. 1 m/min after deco clears to surface.

 

[Diver0001]

10m/min then 3m/min. As slow as practical from 6m and up.

This is what I've been doing too. I don't think 3m/min is quick enough because even minor deviations (slightly slower) can cause the computer to accumulate a bit of deco from 21m to your first required stop.

I've talked about it with my buddy and we've been experimenting with 6m/min and it seems to fit more comfortably with what the software does. I'm still not ready to try 10m/min after the switch.

R..

 

[rjack321]

Thanks all. Does anyone know when/where/how etc these variable ascent speeds were first outlined, and whether there are recent studies that have looked into variable ascent rates as they relate to decompression diving? I'm aware of numerous ascent rate studies on recreational and saturation dives, but haven't seen anything relating to bounce deco dives with different mixes. I'm sure there will be some people that vary their ascent by such and such (or not) just because that's what they were taught and so that's what they do, but others may have some mathematical or practical reasoning based on factors such as relative pressure change, supersaturation in leading tissues during ascent (and how they all tie in with 3m stop depth intervals), or extrapolation from recreational studies showing different bubble counts across different ascent speeds, or, some other reason.

If you look at the myriad of NEDU "deep stop" threads the common theme is that slowing or stopping deeper (basically extra time in the intermediate portions of the ascent profile - whether you actually stop or just move slow doesn't really matter) leads to increased medium tissue accumulations and greater surfacing gas loads overall. These increased gas loads lead to a higher probability of DCS. This is why people have been bumping up their GF lows and moving up shallow faster.

 

[Michael Guerrero]

I don't really pay attention or track once I'm on my deco stops. It probably takes a minute or so to get from 10ft to the surface. I'm always below the modified m-value so I don't really worry about it.

 

[thetechnicaldiver]

Interesting to see two distinct approaches.

If you look at the myriad of NEDU "deep stop" threads the common theme is that slowing or stopping deeper (basically extra time in the intermediate portions of the ascent profile - whether you actually stop or just move slow doesn't really matter) leads to increased medium tissue accumulations and greater surfacing gas loads overall. These increased gas loads lead to a higher probability of DCS. This is why people have been bumping up their GF lows and moving up shallow faster.

Hi rjack, i've read the NEDU study and Spisni study abstract, and the surrounding threads on Scubaboard and Dive forum. I don't know whether the Spisni study also had a constant 9m/min ascent rate as with the NEDU study, or if both groups of divers had a slower ascent from the 50% switch (not sure what UTD teaches in this regard).

In the context of this thread and assuming a higher GF low, and no deep stops prior to switching. I wonder just how much slowing from 9m/min to 6 or 3m/min after switching to a 50% influences slow tissue loading and greater surface loads. In turn, is this worth doing in order to reduce the chance of bubbles on the ascent compared with a constant ascent rate. People's different approaches to the ascent after switching seem to be based on one or the other. The software keeps you within your GF percentages and adds or removes deco accordingly on both approaches. So is one ascent method more effective at reducing overall DCS risk than the other, or are the changes too subtle to really say anything if using higher GF lows? I will always try and add 10 mins to my 6m deco on top of the plan if possible, maybe that's more important than the difference between either of the above?

Those that teach either methods on your deco courses, what do you say when explaining why you do it one way over the other?

Thanks in advance and sorry to the imperial divers!

 

[Michael Guerrero]

it's an inexact science at best. By the time you're in the shallows, slow tissues are off-gassing as well, so loading isn't the issue, it's the rate of off-gassing. People obsess about the very dynamic makeup of human physiology. Don't overthink this. Give yourself a minute or two to get from one stage to the next and you'll be fine.

 

[rjack321]

Interesting to see two distinct approaches.
In the context of this thread and assuming a higher GF low, and no deep stops prior to switching. I wonder just how much slowing from 9m/min to 6 or 3m/min after switching to a 50% influences slow tissue loading and greater surface loads. In turn, is this worth doing in order to reduce the chance of bubbles on the ascent compared with a constant ascent rate.

Sorry, I guess I was thinking you were asking about the ascent rate before a gas switch.

After can make a relatively "big" difference, but its of course relative to the total time you'll be on that gas anyway - which is of course related to how much deco you have to do. ie If you only have 10 mins of 50% time then moving slower adds a significant percentage of time - but so what if its +30-40%, that's only 3-4minutes. (gas volumes not considered). If you have 90mins on 50% to do, 3-4 mins extra is a much smaller percentage of your total time anyway.

While most people I dive with use a total runtime on a gas approach, there certainly are schools of thought that use a stop time approach. My perspective is that the stop time people are chopping wood with a scalpel... But yes if you use a stop time approach and move slower between stops, and reset the clock at the beginning of every stop - you are likely adding slow tissue gas loads despite being on deco and offgassing other tissues. In this case you need to ascend at the assumed rate in the tables or software used to derive the schedule. Moving too slow is actually a common theme I see as a DM in beginning tech divers - emotionally they think they are "safer" but the reality is that the combination of slow tissue accumulation with finite gas supplies they aren't quantitatively safer and might actually be increasing their overall risk.

Its all hypothetical anyway and it took a few hundred person-dives to discern the difference in the deeper vs shallower schedules in the Nedu and Spisni studies. Adding or subtracting a cumulative extra ~4 minutes between 50% stops is too fine of a change in the variables to be worth testing since in would probably take tens of thousands of person dives and a whole ton of money, plus still may never reach statistically significant differences in DCS. As with the Spisni study, you could probably find some changes in biomarkers (with enough divers and close enough examination of their physiology) but you'd be stuck trying to understand what those biochemical differences actually mean clinically.