Re-Evaluating My GF

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Buhlmann set the limits of each compartment by bending men & goats. I feel that if 70% is where I want to be on surfacing there is no reason I don't want to stay below that all the way up. Just because someone else is getting away with it doesn't mean I want to get closer to the line.
 
I interpreted that as "you should pad every stop that GF95/95 gives you by an additional 5%."

I think I misunderstood you.

Looks like I had a bit of a braino: I guess I meant to say "at 95/95 every stop should -- ignoring implementation details and round-offs and such -- be padded by 5%", but typed something not quite that.
 
Given that Buhlmann already compensated for fast tissues being more tolerant with the default M-values, it seems to me more appropriate to stick with a flat GF and maintain the original empirically tested deco curve.

I take it by extending the shallow stop(s) you would be targeting slow tissues (as fast ones will off-gas fast anyway).

Buhlmann did e.g. note that off-gassing seems to be slower during sleep, but there's nothing in the model to account for that. On- and off-gassing are strictly symmetrical and the only way to saturate slow tissues is a saturation dive. My impression is a few people, from Thalmann to Wienke, are not entirely happy with that symmetrical off-gassing part.
 
A setting of GF95/50 would mean (in my version of how I want it to work) that it gives you an ascent to your Last Stop Depth (10 or 20 feet - I also requested an option for a 30' last stop) that is the same as if you had your computer set to GF95/95. That would mean that you are bumping up against 95% all the way to your last stop. Then it would hold you at that last stop until your SurfGF drops to 50.

That profile would correspond to what these fellows you talked to are doing, but that isn't the way a 95/50 would work.

You wouldn't be bumping against 95% all the way to your last stop at 95/50. You'd only bump against 95% on your first stop. You'd bump against something a bit less than 95% on the next one and so forth until by the time you got the surface, you'd be bumping against 50%.

Changing the GFHi doesn't just change the GF limit at the surface, it defines the endpoint of a line between your GF limit at your first stop and your GF limit at the surface and thereby influences all the intermediate stops.

It is true that GF Hi sets the length of your last stop. It is not true that it only has an effect on your last stop. Those guys diving 95/95 and padding the last stop until their surfacing gradient is 50% are not diving a 95/50. They are diving a non-linear GF that looks like a hockey stick. 95% and parallel to the M value line until last stop and then a sharp right turn.
 
I had a thought. I feel like there is a flaw in here somewhere, but I'm going to put it out for critique.

Consider the Coke bottle analogy for DCI. The faster you open the bottle, the more likely it is to fizz. This is analogous to the faster you ascend, the more likely your body is to "fizz".

As we all know, the closer you are to the surface the more important it is to ascend slowly. In other words, if you ascend at 30 feet per minute from 100' to the surface, the ascent from 100 to 90 corresponds to opening the Coke very slowly. The ascent from 90 - 80 is opening the Coke a little more quickly. And so on. Ascending from 10 to the surface, still at 30 fpm, corresponds to opening the Coke much more quickly. The last 10' is always the most important part of the ascent.

Consider an ascent from depth where you are bumping up against a GF99 of 95 the whole way. As you go up (presuming a constant ascent rate), the rate of pressure change increases steadily. You go up 10' every 20 seconds, but the change in pressure every 20 seconds gets bigger and bigger.

The change in pressure from 100' to 90' is going from 4 ATA to (approx) 3.7 ATA. That is around an 8% drop in pressure. The change from 10' to the surface is going from ~1.3 to 1.0. That is a ~23% drop. So, 8% per 20 seconds versus 23% per 20 seconds.

My thought is that bumping up against GF95 during that last 10 feet is where the real danger is. Perhaps bumping up against 95 all the way up TO the last stop (of 10 or 20) is much less dangerous than it might seem on initial consideration. Then staying at that stop until your SurfGF is, for example, 60 really makes the last 10' very safe (statistically speaking).

In other words, maybe the way the folks that I have talked to are doing their dives is not at all as crazy as it might sound?

I think we're in danger of the simplification getting too simple. The coke bottle is a good metaphor for new divers, but as we all know the physiology is a lot more complex.

Yes, for the reasons you discuss, it's really important to use the ascent rate on which the models are based especially when shallow, but it is conflating concepts to say that "10 feet is where the real danger is" based on the fact that relative pressure change is greater with depth changes at that point. That's true but we're talking about a stationary diver at 10', who by definition (in this case) has the leading compartment at 95% of the M value or less. Physiologically, I'm not sure why it would be any worse to bump against 95% at 10' versus 20' or 30' or 40'.

Your computer doesn't clear you to ascend to 10' until your GF ceiling is 10' or shallower. The only time you "bump" against your GF ceiling is when you first arrive a stop. That ceiling continues to lift, a little faster now what you've ascended and increased the pressure gradient for offgassing. When it lifts above the depth of your next stop increment, your computer will clear to go up again. For example, towards the end of your 30' stop, your ceiling will be 21'. When it's 20 or a bit less, the computer will let you ascend to 20'.

There may be some more complicated math and physiology about which compartments matter the most (are the least tolerant of supersaturation), etc., but to the extent that there is a difference between the risk associated with bumping against your GF limit at 10' versus some other depth, the story would be much more complicated than (and I believe unrelated to) the "rate of change" concept. For example, one good reason why it is more risk to surface at 95% of your m-value than to hang around on your stops at 95% of your m-value, is that you're now doing all sorts of things that could contribute to a hit, like climbing ladders and moving gear around.

Where the phenomenon you discuss is pertinent is when you're doing your deco on a fixed line in rough seas so your "depth" is changing as waves pass over or you're hanging on a deco bar and getting jerked up and down by the boat with the same effect. On the last stop, you might be a 5' one minute at 15' the next - and that's a problem.

Custom GFs allow you add conservatism by setting your personal GF ceiling at something less than the M-value. By allowing you to set the slope of the line, you can also bias deco towards deeper stops and have a profile that looked more like a bubble model. That was a thing for a while, but the NEDU study led many to question that practice. Some, like the folks you spoke to, seem inclined to throw the baby out with the bathwater.

As I think the researcher you spoke to seemed to imply, we have some good empirical data on "conventionally" sloped GF factors. Jumping to something like a 95% all the way to the last stop seems to me to be a layperson's "if I take twice the dosage, I'll get better twice as fast" approach to medicine.
 
Buhlmann set the limits of each compartment by bending men & goats. I feel that if 70% is where I want to be on surfacing there is no reason I don't want to stay below that all the way up. Just because someone else is getting away with it doesn't mean I want to get closer to the line.


Another note, Buhlmann's target DCS risk was way higher than I am willing to except in my personal diving. I think 95% is too close to that as well.
 
IIRC Baker in one of his texts quotes 85% as his personal choice.
 
Interesting discussion and something I have pondered, I am currently diving 50/75 but on my return to diving I started with 80/80, I reverted to the former due to the advices of Simon Mitchell, COMEX and also the nagging thought that Pyle, DAN, Pearl Fleets etc. might not be wrong about the half depth pause.

It almost seems like deep stops were a product of human nature, if a little bit is good lots more must be better.

That said there are perhaps cogent arguments for matching lo and hi in terms of gas management and bailout, because if nothing else you reduce deco stress by dragging less gear about and having to manage less gear reduces pyschological stress which has been shown to cause DCS in a recent DAN study.

I am going to have a play with modelling maybe a 75/75 plan, but if not bailed out (I am ccr) would stop at 50% of average depth for maybe a minute or 2.

That way I could have 1 pdc set to 75/75 to cater for bailout, the other at 90/90 for emergency but still add a 50% stop when still on the loop and let the pdc recalc.

The ccr ascent wouldn't match a pure 50/75 but would be close enough for me.

That said if gas savings aren't significantly different might not be worth it.
 
Why add a stop at 50% of max depth? The science doesn't bear it, at least as far as our current understanding based on the most recent studies. See NEDU and Spisni as far as deep(er) stops leading to higher levels of decompression stress. You've gotta make it up with additional shallow time, I don't see the point based on our current understanding of the resiliency of fast tissues to supersaturation.
 
I am no expert but the NEDU study didn't really reflect current tech diving profiles (e.g. air only) and not aware of any that model CCR in comparison with OC for example, some of the Doppler studies have shown big improvements in changing the Lo from say 30 to around 50, not sure if they have done the same with matched hi and lo though.

Simon Mitchell who knows his onions when it comes to decompression dives 50/75 or 50/80 AFAIK, and both Pyle's empirical experience and an early DAN study using Doppler and 50% of average depth stops showed beneficial results over and above matching GF's. COMEX also extrapolated their known data to provide an insight to sensible GF's and came up with the following:

80/80 Air
50/80 Normoxic trimix
20/70 Hypoxic trimix

At the end of the day no-one really knows and its a very subjective field with a huge range of variables, on multiple levels.
 
http://cavediveflorida.com/Rum_House.htm

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