Re-Evaluating My GF

[dmaziuk]

Why 5%? Why not 10%? Or 20%?

I'm assuming you're not asking why 100 - 95 = 5 and not 10 or 20? I really wouldn't know how to answer that one.

 

[dmaziuk]

Thus, while the biggest difference between a 95/50 and 95/95 would be at the last stop, there would be some changes to the intermediate stops as well, very small at first, but progressively larger until you got to the surface.

It's a straight line, "progressively" tends to refer to increasingly larger differences. "Minor implementation details" aside, given e.g. 2 stops at 18 and 6 metres, it's 95% at the first one, 50% at the surface, and ((50 - 95)/(6 - 18)) * 6 + 50 = 72.5, which is equidistant from 50 and 95, at 6 metres.

 

[jgttrey]

It's a straight line, "progressively" tends to refer to increasingly larger differences. "Minor implementation details" aside, given e.g. 2 stops at 18 and 6 metres, it's 95% at the first one, 50% at the surface, and ((50 - 95)/(6 - 18)) * 6 + 50 = 72.5, which is equidistant from 50 and 95, at 6 metres.

It is a straight line. The progressively larger difference to which I was referring is that between the stop times of a 95/95 and a 95/50 as you shallow up as the two lines defined by those gradients diverge from a common GF Lo to very different GF Hi points. If your point is that "progressively" implies a rate of divergence that is nonlinear, that wasn't my intent.

 

[EFX]

I was misunderstood. Stuartv mentioned two divers who dove 85/85 and 95/95. One or both stay longer at the last stop which makes the dive more conservative. These divers mentioned that going with high GFHi's and high GFLo's get them shallow faster than differential GF's with GFHi's that were traditionally lower. My question wasn't to dive reverse GF's but to lower the GFHi and GFLo to avoid the longer hangs. So the GF's would go from 95/95 to 90/90. My rational is if you're going to make dives more conservative then set the PDC to a more conservative setting rather than monkeying around with longer stops and reverse GF's. Why add complexity to the dive plan.

 

[jgttrey]

My question wasn't to dive reverse GF's but to lower the GFHi and GFLo to avoid the longer hangs

I think you meant the opposite. Lowering the GF means more deco.

So the GF's would go from 95/95 to 90/90. My rational is if you're going to make dives more conservative then set the PDC to a more conservative setting rather than monkeying around with longer stops and reverse GF's.

I think this is missing the point because it begs the question of what "conservative" is. Those guys think the more conservative/safer setting is a high GF Lo, to minimize deep stops, and a lower GF hi, to extend shallow stops. But, the computer won't allow that, so they have do the second part manually by padding the stop and watching their surfacing gradient until it falls to whatever it is that they deem acceptable.

 

[dmaziuk]

I was misunderstood. Stuartv mentioned two divers who dove 85/85 and 95/95. One or both stay longer at the last stop which makes the dive more conservative. These divers mentioned that going with high GFHi's and high GFLo's get them shallow faster than differential GF's with GFHi's that were traditionally lower. My question wasn't to dive reverse GF's but to lower the GFHi and GFLo to avoid the longer hangs. So the GF's would go from 95/95 to 90/90. My rational is if you're going to make dives more conservative then set the PDC to a more conservative setting rather than monkeying around with longer stops and reverse GF's. Why add complexity to the dive plan.

@jgttrey : I misunderstood you too.

The point was that M-values are different for different tissues and also depth-dependent. Faster ones react to pressure change faster and thus control first stop, by padding it you protect the fast tissues more. Conversely, by padding shallow stops you off-gas the slower tissues more. If you want to do the latter, Shearwater interface apparently won't let you. As for why, I only fix computers, we'll need a meat doctor to answer that.

 

[stuartv]

I'm assuming you're not asking why 100 - 95 = 5 and not 10 or 20? I really wouldn't know how to answer that one.

You said this:

At 95/95 you should be padding every stop with extra 5% conservatism on top of M-values that were good enough for Dr. Buhlmann.

I interpreted that as "you should pad every stop that GF95/95 gives you by an additional 5%."

I think I misunderstood you.

 

[J-Vo]

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.

 

[stuartv]

I would amend this a bit (and you probably didn't mean it literally). I don't think you'd ride 95 all the way to the last stop.

While the GFHi is the "surfacing" gradient, it affects more than just your last stop.

A 95/95 would simply create a parallel line offset from the M Value, offset by 5%.

But, when the GFHi it is not the same as the GFLo, it is creating a "line" with a different slope than the M value line, and it will change more than just the shallowest stop. A 95/50 would, like a "normal" GF, draw a line from 95% of M value at first stop to 50% of M value at surface. So, the actual limiting GF (the % of the M value that controls when you clear the stop) at a given depth on deco changes linearly as you ascend between the GF lo and the GF Hi.

Thus, while the biggest difference between a 95/50 and 95/95 would be at the last stop, there would be some changes to the intermediate stops as well, very small at first, but progressively larger until you got to the surface.

Actually, that is exactly what I meant.

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.

The way you have described it (a reverse slope to the line, from 95 at the first stop, to 50 at the last stop) is an alternative way to do it. But, that doesn't match the actual practice being used by the people I've talked to. I believe it would result in a more time deeper, compared to the procedure I proposed.

 

[stuartv]

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?