Differences in Ratio Deco

[boulderjohn]

Related to the topic of deep stops, we were also told something in our RD class that we had not heard before and was not in the written course materials. Andrew said that in calculating RD profiles, bottom time does not end until the first deep stop (75%) is reached.

I don't know GUE's position on this.

 

[ScubaFeenD]

I don't do those "75%" stops anymore. I haven't done them for years since most of my buddies agree. My personal rule of thumb is come up 50ft from the bottom at 30ft/min. Then do a slow slide on the first deep stop. 1st "1min" deep stop is 60ft (2ata) off the bottom. Assuming a square profile - once you start massively multileveling (which I occasionally do up a wall) its more fluid but the principle remains - "75%" is just too damn deep unless you're doing 250ft+ dives.

What is this information based on/how did you decide that this was the way for you?

 

[Blackwood]

Related to the topic of deep stops, we were also told something in our RD class that we had not heard before and was not in the written course materials. Andrew said that in calculating RD profiles, bottom time does not end until the first deep stop (75%) is reached.

I don't know GUE's position on this.

Thats certainly not what I learned when I took the ratio deco class (in the 5thd-x era, i.e. pre-UTD). Nor is it how I dive.

I'd love to know the rationale behind that since it seems to fly in the face of the efficacy of making a stop that deep.

IMO, if it's still BT, you're too deep to be stopping.

Maybe it's a way to build more conservatism into the deco obligation?

 

[Blackwood]

Alternately, maybe it's based on observations that new tech divers have a tendency to initiate their ascents too slowly, and this covers them.

 

[Rainer]

If you're unable to make an expedient ascent to the first deep stop (which a lot of new T1 divers have trouble with), then sure, count that ascent time in the bottom time for conservatism. If you can manage 30'/min off the bottom, then no reason to bother (IMO).

 

[rjack321]

What is this information based on/how did you decide that this was the way for you?

RossH published a discussion of this a long time ago. I read it, it made sense to me. I tried it and have stuck with the 50-60ft up (varies a bit depending on dive length). Unfortunately I can't seem to find his critique. The gist of it was that 75% (or 80% of ATAs) is a linear approximation of a curve. It works for the deepest dives (300ft) but the shallower you are the more absurd it gets. Stopping at 75ft (or even 70ft) from a 100ft dive is just nonsense no matter how long the dive. Even your fastest tissues are not offgassing, there's not enough gradient. Offgassing gradient consistently starts about 2ATAs up hence our choice to slow down 50ft up and start stops 60ft up.

The RD rules are a start to building your own experience base to know how much and where to deco out. Its intended to be an adaptive "algorithm" unlike a fixed computer generated mathematical scheme.

Algorithm. n. A set of instructions for calculating a function. The "function" in this case being how much deco to do and where to do it.

Alternately, maybe it's based on observations that new tech divers have a tendency to initiate their ascents too slowly, and this covers them.

Yes this is a recent pad based on the observation that ALOT of even fairly experienced "DIR" divers are way too slow off the bottom. It can very easily happen and having the default to ignore this extra time wasn't conservative. So the new default is to stop the bottom time clock at the 1st deep stop.

 

[boulderjohn]

Alternately, maybe it's based on observations that new tech divers have a tendency to initiate their ascents too slowly, and this covers them.

Actually, as I recall his exact wording, he said that is how he himself does the calculations.

 

[rjack321]

Courtesy of ian33 and charlie99 since my memory failed me.
Decompression myths and mistakes

And here (check out the date)
http://www.scubaboard.com/forums/846756-post33.html

 

[rjack321]

Actually, as I recall his exact wording, he said that is how he himself does the calculations.

Ask again and you'll get a difference answer. But really, so what?

 

[Blackwood]

Actually, as I recall his exact wording, he said that is how he himself does the calculations.

As he should if that's what he's teaching.

I still question it, though. If you know what you're doing, you know how long it will take you to reach your first stop. Ending BT at the stop level implies to me that the initial ascent time is an unknown quantity until after the fact, which is a symptom of either a) something unexpected happening - would be a red flag WRT deco, or b) inexperience.