Do you still calculate pressure groups if you use a computer?

[1_T_Submariner]

I tried this when I first got a computer. Alot of my dives where off the table so it did no good. I carry a spare computer it is a on without any bells and whistles but, I can still dive if the primary quits. I bought 2 of them from Scubaboard Classifieds.

 

[Blackwood]

Big bag of worms, this thread.

When using a computer to compute multilevel decompression, 'calculating' pressure groups (which exist to facilitate the tracking of inert gas loads across repet flat profile dives) makes little sense. They provide absolutely no useful information, as they were formulated based on dives you didn't make.

 

[Thalassamania]

Big bag of worms, this thread.

When using a computer to compute multilevel decompression, 'calculating' pressure groups (which exist to facilitate the tracking of inert gas loads across repet flat profile dives) makes little sense. They provide absolutely no useful information, as they were formulated based on dives you didn't make.

Before you rest on that statement download this PDF from Rubicon and look at PDF Page Numbers 110 (bottom) and 197+.

 

[petronius]

I do when it's a shore dive (i.e just sinlgle tank then done). On multi-dive boat trips I'm usually in group ZZZ by dive 2 according to the tables, so it's pointless...

 

[emttim]

So here's a simple question...why don't dive computers tell you what pressure group you're in at the end of each dive? I'm sure they have the potential of calculating PGs...sure every agency probably does PGs and tables slightly differently, but dive computer manufacturers could just adopt the most conservative PGs they're willing to.

 

[Thalassamania]

Since no one seems to be willing to go get the paper, read the critical extract here:

LOST RESIDUAL NITROGEN DATA PROCEDURES

When the data in the computer is lost, regardless of the cause, there are two considerations. The primary consideration (assuming the data is lost while in the water) is effecting a safe return to the surface and the secondary consideration is how to safely begin diving again. Getting back to the surface is easy if the diver is never in a decompression status, which is one of the reasons that U.R.I. requires that the diver always keep 5 minutes of no-decompression time available. The failure of a dive computer while a physiological ceiling is in effect could be a very serious problem (I suggest carrying a backup computer for all planned decompression dives).

Getting a diver back in the water after the failure of a computer is the second problem. The best way is to follow the manufacturer's recommendation on how long to wait to clear out the residual nitrogen and begin diving again with a restarted computer.

This is not always possible. On occasion, a diver would prefer to get back into the water without waiting twenty-four to forty-eight hours. There are several techniques in use, all of which depend on keeping good dive records, especially records of the scrolling nodecompression limits that are displayed by most all computers in their surface mode. None of the currently available computers are set up to permit user access to set levels of tissue saturations. Thus it is impossible to "get back on the computer" without waiting for residual nitrogen to clear. So the first question is, "How long must I really wait?"

There is no complete answer to this question. Clearly, a diver who made a few, short (even deep) dives will clear out excess nitrogen before a diver who made many, long (even shallow) dives. If all the divers in the group have similar past profiles, it becomes a judgement call to permit a diver to reenter the water with a computer when other computers in the group are clear. This has been done successfully and usually involved waits of 12 to 18 hours, but the ultimate safety of this procedures is clearly problematical. More to the point is the question, "How can I immediately get onto a set of written tables?" A technique that was independently developed by Mike Emmerman of LifeGuard Systems and myself has been used. It requires that the diver record the time of day and scrolling no-decompression limits after every dive. Refer to M. Emmerman - "Dive computer log for the EDGE or SkinnyDipper" in these AAUS Dive Computer Workshop Proceedings for the complete procedure and warnings.

If a computer fails the diver must then:

1. Look back at the scrolling no decompression limits and subtract the available time for each depth to come up with a series of time/depth pairs describing dives that would have resulted in equivalent nitrogen uptake.
2. Determine the repetitive dive designator for each of the time/depth pairs determined.
3. Use the designator that represents the greatest amount of nitrogen to enters the tables and continue on.

There are some potential problems with this approach, especially through the use of a set of tables that is inappropriate for the model used by the computer to determine the scrolling no decompression limits. This technique seems to yield reasonable results using the Huggins Tables with the EDGE or SKINNY DIPPER or the Swiss Tables with dive computer programmed with the Buhlmann Model. This technique has not been proven, so use it at your own risk.

This technique should work if you use some care to match the tables that you wish to transfer to with the decompression model used in your computer.

 

[Charlie99]

While the procedure that Thal has posted is more accurate, I have a much simpler but almost as effective dead-computer plan.

Assuming that I have stayed within no decompression limits, I simply assume that I exited the previous dive at the highest pressure group on the PADI table, pressure group Z. The penalty for overestimating my N2 loading is a bit of extension of the surface interval. For example, if my true pressure group is Q, my surface interval will be 29 minutes longer (the time it takes to go from Z to Q) than if I had more precise information available.

There are some W,X,Y,Z rules that require an extended surface interval of either 1 hour or 3 hours in a series of 3 or more dives, but one can only truly reach these pressure groups with relatively shallow dives much longer than I can achieve with a single tank, even with 0.4cfm SAC, so I know that I can safely assume that those rules are not invoked in my typical dives.

 

[El Orans]

Just wondering if you still calculate the pressure groups in your dive logs if you dive with a computer? I understand that it is still good to know how to do it in case your computer fails, but as a general rule do you still do the calculations and note them in your log book?

Yep.

It gets kinda tricky though when calculating multi-level dives...

 

[Peter Guy]

Just one more reason to dive "Minimum Deco Rules" so that you are "doing deco" on each dive.

As someone else wrote, of course, most our our recreational diving bears very little resemblance (multi-level) to the type of diving for which table were designed (square profiles).

This seems to me to be one of those questions that sounds good on a BB but is not terrible useful in the real world of recreational diving. (Note -- I am not making any statements as to how useful this is to Scientific Diving -- which we all know is special!)

 

[yyidus]

So here's a simple question...why don't dive computers tell you what pressure group you're in at the end of each dive? I'm sure they have the potential of calculating PGs...sure every agency probably does PGs and tables slightly differently, but dive computer manufacturers could just adopt the most conservative PGs they're willing to.

Because different schools/scuba systems teach different tables. I have both PADI and SSI tables, and those two are completely different (SSI is more conservative in time allowed, and has fewer pressure groups). I don't know what other systems teach, but I presume there's quite a lot of different tables and pressure group values out there...

Also, if you already use a computer the whole point of the pressure group/tables is to give you a conservative limit that you can deal with when your computer breaks. So it's almost useless to have your computer give you the pressure group information when it's working... and it can't do it when it's not.

Finally, as was mentioned previously, even if everyone used the PADI table, and computers gave you the pressure group info according to the table, you will discover that a large chunk of the time the computer would just say something like 'pressure group INVALID', since computer dives almost always go beyond the limits of the tables.