Square Profile Dives

[Jerryg]

I've just been reading my Mares M1 dive computer owners manual and there is a warning message that I don't understand. Can anybody explain this. It reads:

"NEVER use your COMPUTER or any other dive computer for repetitive "square profile" dives (dives to the same or nearly the same depth) deeper than 60 feet (18m). This is an unsafe diving practice which will greatly increase your risk of DCS, regardless of what your COMPUTER reads."

 

[Uncle Pug]

... computer or not.

Every dive should be a multilevel dive since every dive is really a deco dive. Spend time shallow for time spent deep and make the ascent slooooower as you get closer to the surface.

As for your manual... they are just trying to cover themselves liability wise. They advertise and sell these devices as tools to maximize your bottom time and then to bring you at a safe ascent rate to the surface.

But of course the reality is contained in the last phrase: "... regardless of what your COMPUTER reads."

 

[Dr Deco]

Hello jerryg:

Square Profiles

I cannot say that I have ever heard of this injunction. Possibly it was written by lawyers.

During the testing of the PADI/DSAT tables, all of the dives were performed to the limits of the tables, both square wave and multilevel. The tables did not yield any more Doppler-detectable bubbles for square or multilevel dives performed to the limits.

There was a time when it was thought that tables were safer than computers because the gas loads were always less than indicated by the tables; this was because of rounding to the deeper depth and longer time. When computers were introduced, divers were always diving to the table limits (or algorithm limits) if they stayed to the limit for that dive and/or reentered the water for the second dive and stayed the limit at that dive.

In practice, the DCS incidence has not increased with computers. Why MARES would say that square dives are more hazardous is somewhat of a mystery. I have not seen any injunctions against this practice alluded to by DAN or anyone else. As Uncle Pug indicates, it is prudent to ascent slowly. This prevents the Boyle’s law-expansion of tissue micronuclei that can then be transformed, with inward diffusion of dissolved nitrogen, into decompression bubbles

Gas Control vs. Nuclei Control

I suspect that one reason DCS has not risen since computers were introduced is that diving practice is still the same. The avoidance of nuclei generation by the diver is not stressed. One can help themselves by not climbing onto the boat will full gear, not hauling heavy tanks during surface intervals, etc. Gas control is "easy" since it only requires the money to by the device. Nuclei control requires diver savvy and is not technology driven.:mean:

Dr Deco :doctor:

 

[Charlie99]

Dr Deco once bubbled...
Hello jerryg:

Square Profiles
During the testing of the PADI/DSAT tables, all of the dives were performed to the limits of the tables, both square wave and multilevel. The tables did not yield any more Doppler-detectable bubbles for square or multilevel dives performed to the limits.

There was a time when it was thought that tables were safer than computers because the gas loads were always less than indicated by the tables; this was because of rounding to the deeper depth and longer time. When computers were introduced, divers were always diving to the table limits (or algorithm limits) if they stayed to the limit for that dive and/or reentered the water for the second dive and stayed the limit at that dive.

In practice, the DCS incidence has not increased with computers.

Dr Deco :doctor:

I've seen estimates of something like 1.5% DCS rate for diving right to PADI (or NAUI) table limits. This is obviously much, much higher than the actual observed DCS rate. I assume that the difference is that the 1.5% DCS probability is for a square profile and 60fpm direct ascent to the surface without a safety stop. Correct??

Or am I misreading the risk estimates as presented in BRW's TDID, and in the proposed (but not accepted) 1% & 5% bends probability USN tables.

Thanks in advance,

Charlie Allen

 

[chrpai]

While I agree with most points here ( every dive is a deco dive, deep stops, safety stops, multilevel dives that are effectively stops, don't dive to the limits of a table or computer ) I'm not sure I would go so far as to say a deep square profile should completly not be done. Low profile wrecks for example often require a square profile. Is it really *that* much risk to do a couple of dives on a 100' wreck and do a couple minutes 40' or so followed by a few minutes at 15'. I don't think so.

 

[Dr Deco]

Hello Charlie:

DCS Estimates

These estimates always seem to be higher than is actually observed in practice. In the PADI tables ( Recreational Dive Planner ), the laboratory tests gave results that were nothing like that. In more than 1,000 controlled dives, there were no instances of DCS.

Risk is calculated the number of DCS events divided by the total number of dives. The DCS hits can be known but the number of dives actually performed is more difficult to determine. This last factor can give a big error in the actual percentage.

DCS in US Navy tables is based on stage decompression dives; NDL diving is really quite safe. The 1% and 5% DCS estimates were derived by US Navy scientists, but I have never read the report and thus am not familiar with the conditions under which these estimates were made. Suffice it to say, one does not encounter such incidences when the tables are actually used. It is well known among researchers in the US Navy (who must follow Navy procedures) that you will not get DCS in the test subjects by following the US Navy Standard Air Tables. If you move to the US Navy Exceptional Exposure Tables, the DCS incidence (and a chance to acquire data) increases.

Square Wave Dives

These are really the dives that are laboratory tested (e.g., in the DSAT/PADI) case. The US Navy, believe it or not, did not develop their own set of NDLs. They developed stage decompression tables and the NDLs dropped out of the calculations. These were later modified according to Doppler bubble measurements of Dr Merrill Spencer and these modifications were then incorporated into the DSAT tables (sometimes called the Rogers/Powell tables).

CNS Problems

What is curious in recreational diving is that the ratio of CNS to joint-pain only DCS is 1. That means that there are as many neurological hits and joint hits [bends]. Now, the curious thing is that, in laboratory tests, neurological hits are not ever encountered in a final test series approved for distribution. It is axiomatic that tables that will not produce joint-pain DCS will ever produce neurological DCS - - and that is true. What is different in the “wild?”

I have been proposing on this board for several years that it lies in the testing conditions versus the practice of very strenuous activity in the field. Open water tests were performed in the DSAT/PADI test series but these were limited to a couple hundred dives and divers were assisted onto the boat. Many divers are probably exerting considerable stress and strain while boarding the boat resulting in considerable flexing maneuvers on the spine that are nucleating events. Flexation will most likely result in hydrodynamic cavitation by Stephan adhesion (sometimes this aspect is called by the name of “tribonucleation”). Flexing the spine while wearing weights is never performed during tables testing. A Human Use Committee would not approve it, since it would be considered too dangerous. :tut:

Dr Deco :doctor:

 

[cornfed]

chrpai once bubbled...
Low profile wrecks for example often require a square profile. Is it really *that* much risk to do a couple of dives on a 100' wreck and do a couple minutes 40' or so followed by a few minutes at 15'. I don't think so.

You have a funny definition of square.

 

[Charlie99]

but that's a normal state for me .

Dr. Deco, your comment on CNS vs joint bends ratio is very interesting. Perhaps rapid ascents (loss of buoyancy control, bolting for the surface, etc.) might also partially account for the unexpectedly high % of CNS bends.

I take very seriously your comments on tribonucleation, and wait until well into the SI before changing out tanks or doing any other heavy work that I can avoid.

Actually, I always avoid heavy work when possible, but you have provided a nice, scientific rationale.

-------

I'll have to ping BRW with some questions, since much of my confusion is in trying to interpret part 8, "Decompression Risk and Statistics" of his TDID book.

Thanks,

Charlie Allen

 

[chrpai]

cornfed once bubbled...


You have a funny definition of square.

Not really. What I describe is a square profile for the bottom time portion of the profile. The rest is part of the "direct" ascent. (With a few of the "safety" stops that everyone wants to argue over. )

This is as apposed to a multi-level profile where much more time is spent at each level with no intention of directly surfacing.

I can go down to say 90fsw for 25minutes on air for a wreck. Stay at 90fsw for the whole time ( square ) then take a couple of minutes to come up to 40' do a 2 minute stop, come take another minute to come up to 20' for a 3 minute safety stop, then take a minute or so for the last 20'. And lets no forget about the worlds biggest water bed at the very end.

I guess I consider that a square dive since my bottom time was spent, on the bottom.

 

[cornfed]

I think you're missing to the point of what Pug and others are saying.