Have I understood the basics of decompression theory, GF99 and SurfGF?

[David Carron]

If the DC is "throwing" away old predictions because the "idiot" diver is not following them then what's the point in making new predictions? The fact that a "good" DC has the power and memory to do parallel multiple predictions is beside the point. Again, what is gained by assuming an ascent rate or by doing a prediction for NDL or for deco stops?

Here's an example of the kind of mess you can get yourself into if you don't do predictive calculations

Note that at 7:30, subsurface is reporting that NDL has been exceeded because the instantaneous ceiling, calculated without taking into account offgassing during ascent, is below the surface (or alternatively, because the instantaneous GFsurf is greater than GFhi) but that almost one minute later at 8:20 a direct ascent to the surface is still possible.

Note also that the instaneous ceiling continues to get deeper due to ongassing during the first half of the ascent; I'm sure that it wouldn't be difficult to contrive a profile where a dive reported within NDL turns into a deco dive, or the first stop gets deeper, during the ascent.

This type of behaviour, possibly acceptable in a simulator, is clearly unacceptable in dive computer whos reported ascent profile must be smooth and continuous.

 

[ofg-1]

I cannot for the life of me figure out how anyone that was diving in the 60's,70's,80's,90's got though it without falling over dead from DCS every dive. Back then, Shearwater was a bird.

 

[dmaziuk]

I cannot for the life of me figure out how anyone that was diving in the 60's,70's,80's,90's got though it without falling over dead from DCS every dive. Back then, Shearwater was a bird.

Imagine all those caisson workers who died of old age. Inconceivable!

 

[boulderjohn]

I cannot for the life of me figure out how anyone that was diving in the 60's,70's,80's,90's got though it without falling over dead from DCS every dive. Back then, Shearwater was a bird.

Up through the 90s, everyone was using tables, and those were very conservative. That was, in fact, the impetus for the PADI RDP. The Navy tables were very conservative, with no ability to do multi-level diving and with extremely long surface intervals. The scientist who led the RDP research (Spencer) was personally frustrated because he felt the limitations on his diving were ridiculous.

The RDP research showed that the Navy's use of the 120 minute compartment to control surface intervals was far longer than necessary. Tightening up the rounding by making more pressure groups made a huge difference as well. They also created the Wheel, which made multi-level diving possible. Unfortunately their timing for that was a bit late--the wheel came out just as computers were starting to be widely used.

 

[lowwall]

I cannot for the life of me figure out how anyone that was diving in the 60's,70's,80's,90's got though it without falling over dead from DCS every dive. Back then, Shearwater was a bird.

In the very early years, even if you didn't bother with tables, dives were also limited by the widespread use of 72 cu ft tanks. Especially since the early divers were either hunting or looting instead of sightseeing, so they burned through that air relatively quickly.

Cousteau's triples were about 80 cubic ft total, but one of the tanks was used as a reserve. When they cracked it they would start their ascent.

 

[ofg-1]

Up through the 90s, everyone was using tables, and those were very conservative.

Yes, I started diving in 1967. I was trying to be ironic. The amount of mental calisthenics some of these guys are going through to squeeze 3 minutes off a deco schedule is amazing to me.

 

[dmaziuk]

Yes, I started diving in 1967. I was trying to be ironic. The amount of mental calisthenics some of these guys are going through to squeeze 3 minutes off a deco schedule is amazing to me.

I'm sure you remember that most of "these guys" are navies and their excuse is to get the divers out and start steaming away from the site pronto.

On a serious note, the "DCS risk" we speak of is not easily quantifiable on its own, you need to anchor it to something else for it to make practical sense. Traditionally that something is Time To Surface, so... yeah.

 

[Capt I]

This suggests you are misunderstanding what "theory" means here. In a lot of everyday usage, "theory" is the same as saying something is just a guess and pretty much useless. But the sciences, a theory is an encapsulation of the available knowledge into a coherent, useful package that can be used (for example) to make predictions. The decompression theory you are talking about is NOT a representation of the detailed workings of our physiology, but rater a model based on several (for example, 16) "tissue compartments, each with a time-response to on-gassing and off gassing, and each with an allowable maximum gas overpressure before bubbling occurs (the m-value). The point is, WHICH of those compartments is closest to bubbling at any moment in your dive varies with the dive profile and time, so the computer has to track ALL of them and focus on the "most-likely-to-bubble" compartment -- called the "leading" compartment -- at the moment. So GF99 and SurfGF are talking about the leading compartment, and that keeps changing.

Only SurfGF. If you want GF99 you need a Shearwater.

I suggest you do a little more reading, for example:

• ask Google AI "what is the meaning of GF99 and SurfGF?"
• Shearwater Dive Computer Settings: Become A GF Power User!
• Surface GF and Other Teric Musings

Your explanation of the term "theory" is spot-on right. Many misunderstand the term and throw that title around to discount the real significance and meaning of a topic (often because they disagree with it or don't understand it), as in "that's just a theory." Thank you for succintly providing that clarification!

Important to this discussion is that ALL computers run an algorithm (commonly Buhlmann's ZHL 16C or RBGM that are often modified by individual computer manufacturers) - a mathematical model that provided projected values based on a diver's depth, time, gas exposure, and other parameters. I believe many (less experienced?) divers are under the misconception that it is a true representation of the gas conditions within their bodies - which is patently NOT the case. The computer isn't sophisticated enough to take biometrics into account - ex: your hydration, how rested you are, whether you are hunover, if you are cold, your BMI etc. That is why you apply gradient factors (GF low & GF high) - to adjust the output of the algorithm and give yourself some buffer for that uncertainty.

 

[boulderjohn]

Yes, I started diving in 1967. I was trying to be ironic. The amount of mental calisthenics some of these guys are going through to squeeze 3 minutes off a deco schedule is amazing to me.

I'm not sure what you mean by trying to squeeze 3 minutes off a deco schedule.

 

[tursiops]

They also created the Wheel, which made multi-level diving possible. Unfortunately their timing for that was a bit late--the wheel came out just as computers were starting to be widely used.

I have two Wheels; still haven't had to change the batteries on either one.