Pressure Groups

Razorblade · Apr 25, 2003

Okay, so I dive with a computer but when I fill in my logbook one of the spaces in it is a little box that asks what my starting and ending pressure groups are. I realize that there's no real point in filling these out since I also record bottom time, surface interval (when applicable) max and avg depth, but I like to do so anyway. I determine my pressure group by using an RDP and the average depth instead of the max depth. I know that if diving using ONLY an RDP you're supposed to use the max depth, but as we all know that's unrealistic since we could only be at that max depth for say 20 seconds from an 70 minute dive.
What are the different opinions about this?

dsgobie · Apr 25, 2003

Interesting... I think this should be moved over to decompression forum though.

jonnythan · Apr 25, 2003

I go ahead and use the bottom time and max depth and figure accordingly. Most of the time, when doing more than one dive in a day, I exceed the table limits and make a little note.

cstreu1026 · Apr 25, 2003

Funny that this topic comes up as I was thinking about it today since I have yet to use the computer that I bought in december. It will be interesting to see where this goes.

Walter · Apr 25, 2003

If you must fill out the group designation, you'll use your max depth and total bottom time. There will be many times when using a computer, you'll have blown your tables. Rather than fake the tables with average depth or some other such silliness, it would be best to leave the group designation blank or write in "computer."

Diver0001 · Apr 25, 2003

Razorblade once bubbled...
Okay, so I dive with a computer but when I fill in my logbook one of the spaces in it is a little box that asks what my starting and ending pressure groups are. I realize that there's no real point in filling these out since I also record bottom time, surface interval (when applicable) max and avg depth, but I like to do so anyway. I determine my pressure group by using an RDP and the average depth instead of the max depth. I know that if diving using ONLY an RDP you're supposed to use the max depth, but as we all know that's unrealistic since we could only be at that max depth for say 20 seconds from an 70 minute dive.
What are the different opinions about this?

Well, you can do one of 3 things:

1) use the max depth and bottom time. This is pretty meaningless but it does follow the guidelines.

2) don't fill it in. YOu can't use the information for planning anyway so you really have to question the need to record it.

3) go to the planning mode on your computer, work out what your computer thinks your pressure group is by reading backwards on the table. It will only work if you have a Haldanian computer using more or less the same 1/2 times as your tables. You still shouldn't use this information for planning.

Personally, I don't fill it in.

R..

dsgobie · Apr 25, 2003

Walter once bubbled...
If you must fill out the group designation, you'll use your max depth and total bottom time. There will be many times when using a computer, you'll have blown your tables. Rather than fake the tables with average depth or some other such silliness, it would be best to leave the group designation blank or write in "computer."

Now that I've thought about it more, I would agree with Walter. I did just mark a "Com" entry when I used to fill that in. But after so many dives using the computer, I do not fill it in anymore.

O-ring · Apr 25, 2003

Walter once bubbled...
If you must fill out the group designation, you'll use your max depth and total bottom time. There will be many times when using a computer, you'll have blown your tables. Rather than fake the tables with average depth or some other such silliness, it would be best to leave the group designation blank or write in "computer."

You can try this at first, but will realize that you will be off the tables since you may have a max depth of 100' and a 45 minute runtime since you were on a multilevel profile that your computer could calculate, but standard tables are not designed to do.

I stopped recording the pressure group info when I stopped diving tables..

redacted · Apr 25, 2003

Diver0001 once bubbled...
SNIPPED

3) go to the planning mode on your computer, work out what your computer thinks your pressure group is by reading backwards on the table. It will only work if you have a Haldanian computer using more or less the same 1/2 times as your tables. You still shouldn't use this information for planning.

Why will this not work? I've been looking at options to transition from a computer to tables in the event of a computer failure and I don't want to carry a backup computer. This seems like a reasonable way to estimate your current pressure group, adjusted as desired for reduced risk, and make a transition to tables.

If I know that x time after my last dive I can do a dive to y ft for not more than z minutes; that will map to a PG in the RDP. Might want to look at a couple values of (y,z) which could yield different PG. Might even want to back off a PG or so. But it seems like a very feasible approach. Sure there is some risk. But is the risk simply that it is not a proven acceptable method (the risk of the unknown) or is there known quantifiable risk?

cstreu1026 · Apr 25, 2003

Would something like be covered in a class like Padi's multi-level/computer specialty?

Pressure Groups

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