Is there a Dive computer that gives you an ending pressure group?

[Matt S.]

wedivebc - Thank you for your explanation - although I am not sure I follow it completely. What I have been doing so far is looking at max depth, dive time, surface intervals from my computer and then using the tables to figure out the ending pressure group.

Then you are using your computer as a depth gauge and timer, and not a computer. Which is fine. But if you read your manual you will probably find that the computer tracks everything between dives automagically.

In other words, if you do a dive, and get in the water an hour later, the computer will show that you have less no-deco time than it would if you hadn't done a dive previously. And when you go back in for dive #3, it takes into account the previous two... That's why computers are cool--they are doing all the "pressure group" stuff for you, and they don't calculate from max depth. They use much more accurate depth readings, which tends to give you more NDL time.

I do not want to start the computer/no-computer debate. Again.

 

[SteveAD]

A more useful pressure group substitution for computers is to put it into dive plan mode right after getting out of the water and find out the NDL for a given depth (say 60 feet). Cross referense this NDL with the adjusted NDL on table 3 (assuming you're using PADI DSAT tables) and that will give you your pressure group. Oceanic computers use an algorithm that is based on the DSAT tables, so this will be fairly consistant, and allow for resuming diving on tables in the event of a computer failure.

That being said, I have never used this practice except as a theoretical exercise.

 

[ReefHound]

A more useful pressure group substitution for computers is to put it into dive plan mode right after getting out of the water and find out the NDL for a given depth (say 60 feet). Cross referense this NDL with the adjusted NDL on table 3 (assuming you're using PADI DSAT tables) and that will give you your pressure group. Oceanic computers use an algorithm that is based on the DSAT tables, so this will be fairly consistant, and allow for resuming diving on tables in the event of a computer failure.

That being said, I have never used this practice except as a theoretical exercise.

Maybe what the OP would like to see is the computer to do this automatically and flash the PG letter on the screen along with the elapsed surface interval time.

 

[Bubbletrubble]

There really isn't a good way to get your ending pressure group from a dive computer. I guess that, immediately following a dive, you could enter the "Plan" mode of your dive computer which will display your NDLs for given depths...and then match this up with the adjusted NDLs listed on the Repetitive Dive Timetable. For instance, immediately after you finished your first dive, if your dive computer Plan mode said that your NDLs were 55 min at 40 fsw, 20 min at 50 fsw, 8 min at 60 fsw, and 2 min at 70 fsw...then the ending pressure group of your first dive was probably Group S.

A more useful pressure group substitution for computers is to put it into dive plan mode right after getting out of the water and find out the NDL for a given depth (say 60 feet). Cross referense this NDL with the adjusted NDL on table 3 (assuming you're using PADI DSAT tables) and that will give you your pressure group. Oceanic computers use an algorithm that is based on the DSAT tables, so this will be fairly consistant, and allow for resuming diving on tables in the event of a computer failure.

That being said, I have never used this practice except as a theoretical exercise.

Great minds think alike!

 

[darry_99]

Thank you every one. This has been helpful. I am going to let go ... and leave empty space in my dive book

 

[Charlie99]

Thank you every one. This has been helpful. I am going to let go ... and leave empty space in my dive book

Or you can do like me and jot down the number of bars on the N2 bargraph. I also note the ascent indicator bars and if on nitrox, the O2 bars. So into the pressure group space will go some notation like "N9, O4, A2".

When looking at the logbook later, it gives me a rough indication of the level of loading (the N2 bars) and how well behaved I was on the ascent -- the ascent bars log.

If my maximum loading during the dive was significantly higher than the bars upon exiting, I might make a note of that too.

 

[TheGoast]

Thank you every one. This has been helpful. I am going to let go ... and leave empty space in my dive book

Have you seen what I did?

http://www.scubaboard.com/forums/do-yourself-diy/193341-tim-s-recreational-dive-log.html

 

[Peter Guy]

A question for SB'rs from this topic:

IF you are using a computer/bottom timer, would plugging in the "average depth" and time into a Table give you the square profile the tables assume? Would that yield the "real" ending pressure group thought to be wanted by the OP?

 

[ArcticDiver]

A question for SB'rs from this topic:

IF you are using a computer/bottom timer, would plugging in the "average depth" and time into a Table give you the square profile the tables assume? Would that yield the "real" ending pressure group thought to be wanted by the OP?

Two answers

"real"? None of it is real in the sense of actuality. Both Tables and the Dive Computer use models of reality that are expressed in each algorithm. One algorithm is presented on paper and the other is in the electronics of the dive computer. One does a better job of measuring actual times and depths and is therefore presumably more a more accurate representation.

Computer vs. Ending Pressure Group: My understanding is that computer manufacturers have different algorithms than the paper tables contemplated by the OP. So, unless the particular table and computer used by the OP used the same algorithm data would not be transferable unless some conversion calculation were done.

So, it is best to pick one or the other and stick to it. To allow for possible computer failure it is best to either have a backup computer, or plan on a sufficient SI to enter the tables as a first group diver.

 

[Charlie99]

A question for SB'rs from this topic:

IF you are using a computer/bottom timer, would plugging in the "average depth" and time into a Table give you the square profile the tables assume? Would that yield the "real" ending pressure group thought to be wanted by the OP?

It's a much better approximation than just using max depth and time. The pressure group calculated using average depth + time will be a bit higher than the actual pressure group --- i.e. in the conservative direction.


Charlie Allen

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ONLY READ THE REST IF YOU WANT TO SEE THE FOUNDATION FOR THE ABOVE STATEMENT.

Pressure groups are simply an alphabetical representation of the loading in ONE and ONLY ONE compartment. For the PADI RDP, this is the 60 minute compartment. For the USN and USN-derived tables (SSI, YMCA, non-RGBM NAUI, etc.) the repetitive groups show the loading of the 120 minute compartment.

At low loadings (pressure groups near A), the ongassing is directly proportional to depth. In other words, 5 minutes at 100' loads the 60 (120 for USN) minute compartment the same as does 10 minutes at 50'.

As the calculated loading in the 60/120 minute compartment goes higher, the loadings are no longer as close to proportional. For example, if you have been at 100' for 18 minutes or 50' for 41 minutes the 60 minute compartment will have an N2 load of 40fsw-absolute. (Note that 41min is a bit more than twice 18min, but here I'm just calculating the rate from an equal starting point)

When breathing air at 50', the ppN2 is 65fsw-absolute and at 100' ppN2 is 104 fsw-abs. What is driving additional loading into the 60 minute compartment at 50' is the 65-40=15fsw difference between inspired N2 and the 60 min loading. At 100' the difference is 104-40= 64fswa ---- a little over 4 times as much rather than the expected twice as much.

Comparing average depth caculations vs. what the PADI wheel calculates, you will see that same discrepancy caused by the changing loading level in the 60 minute compartment causing the ongassing to be exponential rather than linear, thereby making the average depth calculated pressure group slightly conservative.