Deep Stops Increases DCS

[PfcAJ]

Of course a 40 min bottom time at this depth is beyond most people, and well beyond the normal reach of OC divers,due to the gas volume required. The 40 min profile demonstrates how long ZHL/GF become badly out of proportion due to the extrapolation and GF compounding issue. No one needs that much extra time - look at the 100/100 plan !

If you ignore the GF expotential error in the 40 mins plan,which involves the Last stop only, then rest of the profile is all pretty similar. Go back and look at the more typical 20 min profile. A few minute difference only.

Argument being imagined here, here is they are pretending to "not' be doing deep stops. But in reality, the real world slow ascents (that are not actually planned), mean they are doing the deeper stops.

So they pretend to not to do deep stops, buy in fact really are. And if doing normal length dives, then the finish time about the same as a real VPM-B+3 plan. therfore... still doing a VPM-B style profile.

Well, since you brought up an OC plan, now, lets take a gander at how 'similar' they are.

240' for 40mins, 15/55, 35/25, 50%, 100%
40/70 = 137mins of deco, VPM+3 = 114mins.
4mins of stop time before the 1st switch with 40/70, 9mins with vpm+3.
68mins of oxygen vs 49mins of oxygen.

VPM has over double the amount of stop time before getting on a deco gas and 20mins less on oxygen.

I'm havin a giggle. We can do this all day. Even the 20min exposure has 4mins of stop time before getting on a deco gas with vpm+3 and 5mins less oxygen time.

Can we do another?!?! They're SOOOOOOOOO similar!!!

 

[rossh]

What you should do is stand back and look at the bigger picture.

In the space of just 20 mins bottom time, your GF has gained 30 mins extra time (on top of the actual deco time increase) and is clearly out of sync with the real underlying requirements. If you planned this in 60 min bottom time size, it gets even worse, and continues.


40/70: 40 mins = 183min, 60 mins = 302min ( +120 mins)
100/100: 40 mins = 138min, 60 mins = 226min (+ 88 mins)
VPM-B+3: 40 mins = 161min, 60 mins = 245min (+ 83 mins)

In RD terms, for this depth: 100/100 & VPM-B = 4:1, GF = 6:1

Clearly the GF plan is grown to ridiculous amounts. Why is that?

Why does GF grow exponentially wrong with added tissue load? 1/ GF is applying a linear multiplier, to values that are logarithmic. In decompression, time and tissue load values are typically logarithmic, not linear. Hence this GF error will grow when GF is used as an absolute measure. 2/ GF was never intended to be an absolute measure - it was intended to be used as a relative adjustment method only.

I know you love GF and take it at face value, but sadly its being used in a way that's neither correct or accurate to the real underlying decompression. Large GF's like this x/70 has taken the result way off scale, and its wrong.

Further, you do realize that GF is not a model? GF is only two (2) lines of code, patched onto the end of a finished ZHL calculation. There is no way on earth that such a simplistic design, can stay proportionally relevant to the underlying decompression. What you really get from such simplicity is this exponential growth style that can be seen above.
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[rossh]

Can we do another?!?! They're SOOOOOOOOO similar!!!

And compared to the 100/100 - where you actually started from??? We find you have totally abandoned the ZHL concepts. Instead you have adopted the same attributes of VPM-B model (deeper stops, longer shallow time). Yes, you are similar.

 

[PfcAJ]

What you should do is stand back and look at the bigger picture.

In the space of just 20 mins bottom time, your GF has gained 30 mins extra time (on top of the actual deco time increase) and is clearly out of sync with the real underlying requirements. If you planned this in 60 min bottom time size, it gets even worse, and continues.


40/70: 40 mins = 183min, 60 mins = 302min ( +120 mins)
100/100: 40 mins = 138min, 60 mins = 226min (+ 88 mins)
VPM-B+3: 40 mins = 161min, 60 mins = 245min (+ 83 mins)

In RD terms, for this depth: 100/100 & VPM-B = 4:1, GF = 6:1

Clearly the GF plan is grown to ridiculous amounts. Why is that?

Why does GF grow exponentially wrong with added tissue load? 1/ GF is applying a linear multiplier, to values that are logarithmic. In decompression, time and tissue load values are typically logarithmic, not linear. Hence this GF error will grow when GF is used as an absolute measure. 2/ GF was never intended to be an absolute measure - it was intended to be used as a relative adjustment method only.

I know you love GF and take it at face value, but sadly its being used in a way that's neither correct or accurate to the real underlying decompression. Large GF's like this x/70 has taken the result way off scale, and its wrong.

Further, you do realize that GF is not a model? GF is only two (2) lines of code, patched onto the end of a finished ZHL calculation. There is no way on earth that such a simplistic design, can stay proportionally relevant to the underlying decompression. What you really get from such simplicity is this exponential growth style that can be seen above.
.

Its not 'out of sync' with anything. VPM+3 just doesn't provide enough deco time. I already posted links regarding the non-linearity of risk in regards to longer deco times. If you want to assume more risk, do a shorter time. If you want to assume more risk, spend less time on deco gases. If you want to assume more risk, do less time on oxygen. AKA all the things that VPM wants you to do.

RD has nothing to do with it. At all. But in terms of understanding your decompression requirements, it highlights just how short VPM is.

I don't think any algorithm is 'correct', nor do I 'love' any algorithm. I do think some are better than others, and I think GF is better than VPM so reasons I've stated over and over (as have others). They're all wrong. GF might just be less wrong.

 

[PfcAJ]

And compared to the 100/100 - where you actually started from??? We find you have totally abandoned the ZHL concepts. Instead you have adopted the same attributes of VPM-B model (deeper stops, longer shallow time). Yes, you are similar.

VPM doesn't have the longer shallow time! THAT'S THE POINT!!! It only has deep stops (silly ones before deco gases) and truncates the oxygen time.

The similarities end once you get past 'stop for some minutes every 10ft'.

 

[Jax]

I'm confused . . . @PfcAJ -

Are you arguing only that the two plans "aren't similiar"?

Or are you arguing that Deeps stops don't increase chances of DCS?

 

[PfcAJ]

I'm confused . . . @PfcAJ -

Are you arguing only that the two plans "aren't similiar"?

Or are you arguing that Deeps stops don't increase chances of DCS?

I think that they are not close enough to be called 'similar', and I'm suggesting that the deep stops prescribed by VPM+3 (topic pf conversation) are too deep and too long, coupled with not enough time shallow thereby increasing DCS risk. Double whammy.

Ross seems to be trying to say that 40/70 is 'similar' to VPM+3, and that 40/70 generates a VPM+3 profile (or close approximation).

 

[Jax]

Given all the vagaries in deco, the point is probably moot.

The two configurations would have to be run for each depth, and somewhere in there, one would have to use their own interpretation of what would work.

Then some people would never run a VPM +3, because they would like (or not) a VPM +2 or something.

Then you have all the differing body physiology . . . More loading in the fat cells?

You are both factually correct, but this sounds like a case of arguing 10 significant digits, measuring with a micrometer, and cutting with an ax. Physiology just is not that precise.

 

[PfcAJ]

Yea yeah, micrometer axe thing. Heard it before.

I'm actually interested in maintaining a certain level of risk, not just willy nilly with it.

If you think 30mins (or more) of deco time is moot, then when is it not moot for you? 30mins is something you can get into at JB. Care to just blow it off? We both know the answer to that.

 

[rjack321]

Given all the vagaries in deco, the point is probably moot.

9 minutes of additional time on deep gases before a switch and 21 mins less time on O2 is not trivial at all. That's a lot less shallow time and its compounded by the delay getting to the first deco gas.