dive computers and reverse profiles

[TMHeimer]

Coincidentally I just finished that SSI section last night and was also confused about the reverse profile and computers. I rarely use my computer, so didn't give it too much thought. I figured it was something like -- the computer will track nitrogen regardless of your profile, but your body does things a little different if you sawtooth, therefore you can't really trust what your computer says. Thanks all for clearing up what I thought I was already clear on......

 

[dmaziuk]

I have a lot of issues discussing the validity of computers that have all of these idiotic "Safety factors" built in that is not based on any sort of research, heaven forbid when it is compounded by their refusal to disclose what it is they're doing to try to "keep you safe". The point remains that NONE of that should be discussed in the academic information provided by a training agency though with SSI and Mares being buddy buddy, it's unsurprising they're trying to bring some of that BS into their training materials.

Oh, come on: I bet it was written back when deep stops were all the rage and RGMB was shiny, and just never updated since. Until we all got locked up with too much time on our hands, and somebody actually went and read the thing.

The only research behind gradient factors is that lowering GF Low much is bad for you. Baker's goal was to let people add their own "extra safety" to ZH-L in case they wanted to, not because there was any research saying they should. If the user's an idiot, that'd make GFs "idiotic safety factors".

DSAT is in fact better validated than ZH-L16 for no-stop dives. I strongly suspect all its "safety" is limited to reducing the surfacing M-value, which in Buhlmann-Baker-speak is "GF Hi".

The next best researched model is actually Linear-Exponential; in a sense it is conceptually closer to "them bad models" than it is to Buhlmann.

 

[BlueTrin]

It’s good we have this thread.

I was confused by this as well when I read it.

I emailed SSI to ask for the source of this statement

 

[tbone1004]

@dmaziuk not sure of publication date, but even if they were talking about deep stops I have no issue with it, but the parts quoted were never applicable based on research.

The idiotic safety factors I was talking are things like simulating an altitude dive which is a very common way that safety factors are implemented. It's not adjusting something like bubble size in VPM or saturation levels like in ZHL, it's just telling the computer you're at altitude when you're not.

 

[Angelo Farina]

Coincidentally I just finished that SSI section last night and was also confused about the reverse profile and computers. I rarely use my computer, so didn't give it too much thought. I figured it was something like -- the computer will track nitrogen regardless of your profile, but your body does things a little different if you sawtooth, therefore you can't really trust what your computer says. Thanks all for clearing up what I thought I was already clear on......

I think this is the correct explanation. All the computer algorithms have been validated in a number of test cases, but when your diving profile becomes very "strange", far away from the tested profiles, the chances that the algorithm does not predict correctly what happens in your body are larger, hence you should thrust the computer with less confidence.
And this is the meaning of the increased safety factors embedded in some computers when they detect a "strange" profile. As there is no certainty that the standard algorithm is "safe enough" in these conditions, a "reasonable" additional safety factor is empirically added. A typical engineer's patch to lack of scientific data.

 

[wetb4igetinthewater]

SSI has some things out of date, such as quarter turn back for valves, in their open water materials. There is a video in their dry suit course which shows a diver in doubles and a snorkel.

I don't know the process of an agency in when/how they update all their training materials.

 

[dmaziuk]

@dmaziuk not sure of publication date, but even if they were talking about deep stops I have no issue with it, but the parts quoted were never applicable based on research.

Neither was the rest of VPM & RGBM, unless you count looking at bubbles in petri dishes full of gel.

Before Buhlmann, M-values were referenced to sea level and they had to do pressure correction on the side when diving at altitude. Since the net effect was adding conservatism, why not also use it to add conservatism...

Don't get me wrong, I'm a fan of ZH-L myself: it's simple, it's general, its open, and it gets you out of the water not bent -- all you need and nothing you don't.

 

[EFX]

See this response regarding the decompression section in SSI's book on the Science of Diving.

 

[Subcooled]

Stepfen found some really low quality text in the course materials.
I will comment on his findings.

I am not an expert in medicine and definitely not in hyperbaric medicine, but the following points should be quite widely accepted.

Section 3/Dive computers:
"...
Dive computers do have several limitations:
...
- The mathematical model works only with the correct dive profile, which is a multi-level profile with the deepest depth first, followed by subsequently shallower depths. "

That statement is false.

The Buhlmann algorithm for example tracks the absorbed nitrogen and its partial pressure based on ambient pressure and time. It does not remember any profiles, "correct" or not.

And a couple of paragraphs later:
" Going deep, then shallow, then deep again will yield unreliable results. Since a dive computer is an actual computer, an old axiom in the computer business is applicable here: “garbage in equals garbage out.”

In other words, the computer model must be supplied with the correct input to calculate decompression status properly. Since the computer gathers its input using a depth gauge and a watch, if a diver dives improperly the computer will calculate improperly (see Multi-Level Diving)."

Not true. Computers work well with reverse profiles.

If the quoted statement were true, then this would be false. But is is true. Hence, the statement cannot be true. It is false.

Which implies: New working hypothesis: the Multi-Level Diving chapter contains errors.

Then again,
Section 3/Different Dive Profiles That Impact Decompression Theory:
"... Dive computers were designed for multi-level diving"

True. Tables are sufficient and economical for square profiles.

Use the computer with deep-to-shallow profiles, not deep-shallow-deep profiles.

Use it on all dives.

The computer continuously computes nitrogen absorption based on the actual depth and time actually spent there.

True.

The text however contradicts itself ("works only with the correct dive profile").

As indicated earlier, the residual nitrogen affects that process, yet the computer does not accurately account for this affect.

That statement is false as Stepfen has highlighted.
Reference: DAN Europe - Reverse profiles.htm

Even the SSI dive tables account for residual nitrogen!!!
And every single recreational and technical diving computer does.

But hey, I found one dive computer that does not account for residual nitrogen: The SOS decompression meter (1959), fondly known as Bend-o-matic.

Residual nitrogen does not affect the process of computing.
It causes tissue compartment saturation(s) to be achieved faster. Hence shorter no-decompression-limit.

The physiological effects of gas bubbles beeing squeezed to smaller size which possibly allows them to bypass filtering in the lungs, and all that, should be answered by someone with training in hyperbaric medicine. Perhaps SSI should call DAN.

Therefore, the computer is not calculating absorption the same way your body is, which is potentially hazardous. "

Writing is an art indeed.

 

[Subcooled]

@dmaziuk
VPM-disproven by current research, don't use it
RGBM-disproven by current research, don't use it

That's interesting! Where can we find these papers (or references to them)?