Would you really know what was going on if your computer went into Deco...?

[fins wake]

Ok, here's my analysis: [snipped]Therefore, using only the Haldanean approach, the diver who has done only 5 minutes of deco is more likely to take a Type II hit, while the diver who did 30 minutes of deco is more likely to take a Type I hit.

I personally agree with your analysis. I think your approach is very close to that expressed by Peter Bennett in several Alert Diver editorials and examined by Bennett, Marroni et al in the recent DAN study.

(It's nice to find common ground with Genesis again. )

 

[MikeFerrara]

What does surprise me considering your occupation is the illogic portrayed by your posts. I've snipped one quote to illustrate. It rather directly says that if they follow their computer they are letting someone else make decisions for them. Implied is that if they don't follow the computer; but follow something else they aren't letting someone else make the decisions.

In fact, whether a diver follows tables, or a computer they are trusting someone else to establish the parameters of their decisions. The only difference is whether the presentation in an electronic display, or on paper.

Yes but I don't really use tables for calculating decompression either. I use decompression software to provide an outline. I can vary setting in the software that drastically changes the schedule. Therefore the decompression that I use for a given dive may be very different from what some one else is comfortable with even when the schedules are generated by the same software using the same basic model.

Even given that we still modify the outline that we have from the software. I decompress differently in cold water of when I have to make 6 trips up a Missouri mountain to get me gear back to the truck. Neither a computer or a table are aware of such additional risk factors. I am. The computer or table also has no knowledge of what decompression I have done before or the results. I do.

Anyway this is what I mean by making decisions.

Now much of your posting, and others, comments on whether this or that computer is too conservative, or too aggressive. Putting that kind of subjective information in this discussion just fogs the issue instead of putting light on it. What is too conservative for one person may not be for another. That perspective has little to do with science and a lot to do with personality and personal experience.

Decompression is a very cloudy issue and that's exactly my point. Just go pick up 4 or 5 different tables and look at the difference in NDL's. In some case you can see like a 40% difference from one table to the other.

In software, as I pointed out before, changes in user settable parameters make unbelievably huge differences in the schedule generated. Which is the right one? Will the real decompression schedule please step foreward?

Anyway, thanks for the insight so I can better evaluate how much of what you post to accept and how much to chuck. Especially since you are exceptionally outspoken about your views.

Thanks

When it comes to decompression don't accept anything I say except that it's not an exact science.

 

[MikeFerrara]

Ok, here's my analysis:

My answer is that the diver who has undergone 5 minutes of deco is in more
danger.

The reason is that the diver who has undergone only 5 minutes of deco
almost certanly has his "leading" compartment as the fastest (one or
two, excluding his blood) compartments in the body - which are unfortunately compartments that can really f#$@ you - the "fast" compartments include the tissues that produce neurological bends, as "faster" tissues are (generally) those that are best-perfused - neurologically-related organs, of course, are among the best-perfused organs of all. (Actually, blood is probably the "fastest" tissue, and bubbling on the arterial side of the circulation will severely hose you.)

The diver with 100% loading in his leading compartment after 30 minutes of
deco PROBABLY has a "slower" leading compartment - he has LESS overpressure in the "fast" compartments, because slower compatments tend to control the shallower stops, and you've specified that the diver has already done 30 minutes of stops - which would be the deeper ones. If you were to look at that diver's fast compartments, they would be WELL below critical tension.

The slower compartments are less likely to produce a Type II hit.

Therefore, using only the Haldanean approach, the diver who has done only 5 minutes of deco is more likely to take a Type II hit, while the diver who did 30 minutes of deco is more likely to take a Type I hit.

This is with all other things being equal, which of course they never are, and this also assumes no PFO - if a slow compartment bubbles into the venous circulation it may not produce a hit at all (the lungs are a pretty decent filter, and aleovi can obtain oxygen by direct gas exchange with the air in the lungs), provided there is no shunt.

If there IS a PFO, then slow compartments can easily produce a Type II hit, since bubbling into the venous circulation can cross over - in this case there is may be little or no difference in risk profile.

(BTW, the true "answer" for the diver who had 5 minutes of required deco is to do another 5 minutes, so as to allow those fast compartments to desaturate - because those are the ones you REALLY don't want to have a problem with.)

So how'd I do, teach?

I'd tend to agree although I wasn't putting so much into it. I'm not sure that in practice the longer required decompression wouldn't result in a type 2 just as often as the shorter obligation.

My main point was that in theory either dive has the diver cleared to surface with the leading compartment at or very near it's critical tension regardless of which compartment that is. I suppose that the greater the loading the further we may get from tested profiles but...I don't believe the diver should believe that the risk is minimized because the required decompression is short.

To hedge my bet I prefer a slower ascent with short stops starting deeper even for "no-stop" profiles. The deeper/longer the dive or the more dives I'm planning the more important it is to me.

A direct ascent to a shallow decompression however short is NOT something that I want to do. Others can do as they please.

 

[Genesis]

Yep....

 

[ArcticDiver]

Yes but I don't really use tables for calculating decompression either. I use decompression software to provide an outline. I can vary setting in the software that drastically changes the schedule. Therefore the decompression that I use for a given dive may be very different from what some one else is comfortable with even when the schedules are generated by the same software using the same basic model.

Even given that we still modify the outline that we have from the software. I decompress differently in cold water of when I have to make 6 trips up a Missouri mountain to get me gear back to the truck. Neither a computer or a table are aware of such additional risk factors. I am. The computer or table also has no knowledge of what decompression I have done before or the results. I do.

Anyway this is what I mean by making decisions.

Thanks for providing some clarity to your position. Essentially what you have said is that you apply an unscientific and untested fudge factor to your dive planning.

This is akin to the "Kentucky Windage" rifle shooters use when they don't have any better data to aim their rifles. Sometimes it works, if you guess right. Other times it doesn't, if you guess wrong.

So, you are taking the accepted tables and applying a guess to them to plan your dive. That makes the dive plan guess work. This may work for you. But, to encourage others to dive by guesswork I think is at best misdirection.

On the other hand if you indeed do have a large enough data base from using your Fudge Factors on a wide variety of divers to make it statistically valid, then please publish the information. You owe it to the dive community to give them tools for better planning; if indeed you have them.

Or, when it is all said and done does all the language in your posts boil down to two sentences? "Use the dive tables or your computer with caution. Your dive may result in conditions different from those contemplated by the dive table or computer." If so I think that is what everyone else is saying too.

 

[Snowbear]

Just wonderin' - Are you a tech diver?

 

[fins wake]

A direct ascent to a shallow decompression however short is NOT something that I want to do. Others can do as they please.

Depends what we're talking about. If we're talking about technical divers with full redundancy, you're absolutely right.

If we're talking calm recreational divers with a plentyful gas (almost invariably air) supply, I fully agree with you. They should strive to do a slow, preferably multilevel ascent to stop depth.

If we're talking a newbie OW diver, LOA or near panic, I don't agree with you. DCS accounts for far less diver fatalities than drowning. Out-of-gas situations on the other hand are heavily overrepresented in diver fatality statistics. OC gas waste is considerably less in the shallows. There is therefore a reason most recreational agencies strongly suggest divers to spend extra stop or emergency decompression time in the shallows.

 

[hvulin]

hi,
this is a good thread to read (lot's of info) since I'm only a rec. diver that goes into deco regularly (I'm AOW) and dive by my comp.

I have read my comp. manuals and asked around before doing deep dives (and passed the AOW test) and have never had problems, but I have to say:
1) my comp. (aladdin smart pro) gives me a couple of configurations to run by (first is the default mode, and others are more and more restrictive, so I can choose the one I like (depending on my physical state))
2) the manual explicitly states that you should have additional education prior to using a comp.
3) the manual uses 2 terms, one is min. depth for deco stops and other is min. time to get there... (notice the "minimum" in this...) It also say to obey both...

When I dive, I usually get to the specified deco stop depth when I have already done it (diving very slowly to the deco stop...), but I still do at least one more stop at around 5m (at least to check my buddy is ok one more time)

This way I have NO problems with going to deco, but as I said:
I have read the manual
I have gone through additional education
I have asked around

Btw. this 3 things are the best way to know what your comp is saying (and not only when in deco mode)

 

[MikeFerrara]

Depends what we're talking about. If we're talking about technical divers with full redundancy, you're absolutely right.

If we're talking calm recreational divers with a plentyful gas (almost invariably air) supply, I fully agree with you. They should strive to do a slow, preferably multilevel ascent to stop depth.

If we're talking a newbie OW diver, LOA or near panic, I don't agree with you. DCS accounts for far less diver fatalities than drowning. Out-of-gas situations on the other hand are heavily overrepresented in diver fatality statistics. OC gas waste is considerably less in the shallows. There is therefore a reason most recreational agencies strongly suggest divers to spend extra stop or emergency decompression time in the shallows.

ok, point taken but maybe it should be stressed more for these divers to stay well clear of the problem in the firsrt place. Which is sort of what I've been saying.

 

[-hh]

Sorry. Both divers have completed their required decompression. One had a total of 30 minutes and the other 5. In both cases their computer/table has cleared them to surface.

IMO, the 30 minute diver is at lower risk, because the time remaining is based on the controlling compartment, not all compartments, so all of his non-controlling compartments have had more time to reduce their loads, thereby reducing his overall risk.

Do you prefer to make those choices or to just get into deco and hope the guy who wrote the firmware made the right choices for you?

True, but all models place a certain amount of trust into the guy who wrote them, which means that the same model expressed as a computer versus a plastic table has a minimal difference in risk/safety. The underlying problem stems from us not knowing enough about what's going on so as to refine our current probabilistic models to be "adequately" precise.


-hh