What are Mathematical Decompression Models?

[Doc]

Okay, that's settled.
How about "Question #2" ?

 

[Akimbo]

…Why should a dive requiring a decompression stop be the last and preferably only, dive of the day?...

This was a general rule of thumb long before dive computers. I learned it in my first Scuba class in the early 1960s and again in Navy dive classes in the early 1970s.

The great majority of divers used the US Navy air decompression tables and the actual data backing up repetitive dive tables, especially involving decompression stops, were not all that well tested. There also was not that much reliable quantitative data from the fleet to correct them over time.

There were several reasons. It wasn’t all that important to the Navy since diving crews were relatively large, there are so many possible variations, most of their work was pretty shallow, and human testing is very expensive. Deep diving in the Navy also required a chamber onboard to treat DCS, which was accepted as being in the 1% ballpark. DCS treatment efficacy is much greater when treatment is within minutes of detecting symptoms. Besides, the cost of treatment is a little oxygen so was no big deal.

Navy dive supervisors are also pretty conservative and would bump the diver to the next deeper or longer table at the slightest provocation, like workloads or thermal stress that might exceed the parameters the tables were designed for. The main thing that made these tables acceptable in practice for recreational divers was they typically didn’t work that hard or spend their entire dive at the maximum depth like a working diver. A 1% hit rate for recreational divers would have killed the sport.

A lot has changed since then in decompression tissue theory and the ability to calculate complex models. It may still be the most conservative approach, but isn’t nearly as justified as when the guideline was created. Rules of thumb can hang around forever, especially when nobody remembers how they came about.

 

[TSandM]

There is also the theory that, if you do a staged decompression dive first, and a shallower dive afterwards, that you may compress any venous bubbles you have (and we know that people can have them for hours after a dive) and allow those bubbles to pass the pulmonary filter to the arterial side. If the subsequent decompression is inadequate, those bubbles may expand in the arterial circulation, which is very bad. If you do the deeper dive second, you have to deco out the nitrogen from the second dive and whatever residual nitrogen you have from the first one, so your ascent is likely to be slow and your deco longer, and this may be a safer approach. I do not know that this has been tested outside of the WKPP, and I do not know if the math supports it.

 

[Akimbo]

Since this is the Basic Forum, we should point out that general terms like “deep” and “shallow” are wild variables. There are enough dives near recreational depth limits that require a few minutes at 3M/10' stops several dives a day that we know they are very low risk for properly trained divers.

That is not remotely comparable to dives to 50M/165' with two hours of decompression. Obviously, deeper and longer have major ramifications. Throw age, water temperature, and work load in the mix and that old rule of thumb can be most prudent to follow — especially the “preferably only, dive of the day” part.

 

[scagrotto]

any Suunto computer ... will track your deco on one dive and compensate for it on a repetitive dive

I would hope that every dive computer does that or locks you out. Accurately tracking (theoretical )N2 load from the very beginning of the first dive until you've completely outgassed, whether you do subsequent dives or not, is a rather important function for a dive computer.

Whether I went into deco or not, I'd be rather disturbed if my computer wasn't accounting for what happened on the previous dive(s) and any surface intervals.

 

[TSandM]

I wasn't sure where the OP had encountered this concept, and it is written in one of the articles on the WKPP site (or at least it used to be there).

 

[Redshift]

Due to the tides, in the UK it's common to have first a deeper and possibly deco dive when it's slack (for instance if wanting to go to a wreck) and then do a second shallower dive somewhere else, usually drifting with an SMB up.

There is also the theory that, if you do a staged decompression dive first, and a shallower dive afterwards, that you may compress any venous bubbles you have (and we know that people can have them for hours after a dive) and allow those bubbles to pass the pulmonary filter to the arterial side. If the subsequent decompression is inadequate, those bubbles may expand in the arterial circulation, which is very bad. If you do the deeper dive second, you have to deco out the nitrogen from the second dive and whatever residual nitrogen you have from the first one, so your ascent is likely to be slow and your deco longer, and this may be a safer approach. I do not know that this has been tested outside of the WKPP, and I do not know if the math supports it.

How would those bubbles pass to the arterial side (excluding PFO)? If they were not eliminated and are then compressed, chances are they will be more easily eliminated, no?
And if we are talking about the possibility of a diver having more residual nitrogen than what's assumed by models, in the form of venous bubbles, would that quantity of gas be relevant when compared to the new gas being dissolved during the second dive?