Pressure Groups

[JDostal]

When I fill out my log book, I used to be very attentive to filling out my pressure group when I started diving because I was only using tables.

Then, when I got a computer I would keep calculating my pressure group to keep up the practice and fill in the data. However, I found that if I were to have done the diving I did on tables I would be way off the tables. So I've kind of quit recording my pressure groups in my log book now....I still run the numbers after diving but I don't really record the information at all.

 

[Soggy]

Even though I dive with a computer, I still do the tables at the end of the day just to keep my chops up with them. It doesn't really matter if you write them down, since you will likely exceed the tables anyhow, but the point is to try and do them. You will end up with a better feel for how they are structured and that one time when you need to do them, you'll still be able to remember how.

 

[Arnaud]

Tables are meant to handle square profiles. By definition, a multi-level dive will generate less N2 retention than what the table provides at the max depth.

Walter and O-Ring, why do you consider it a no-no?

 

[O-ring]

...but I just don't get the point so I stopped doing it. The table wasn't designed to operate off average depth, so I figured the information I was getting from that exercise was useless anyway. The dive was done, my computer was ok with my NDL times, why do I need to know the pressure group post-mortem anyway? (so I quit doing it)..

 

[Arnaud]

O-ring once bubbled...
...but I just don't get the point so I stopped doing it. The table wasn't designed to operate off average depth, so I figured the information I was getting from that exercise was useless anyway. The dive was done, my computer was ok with my NDL times, why do I need to know the pressure group post-mortem anyway? (so I quit doing it)..

What if your computer breaks in the middle of a trip, you don't have a backup and you don't dive on the fly? You would need to use the tables. If you've logged your previous dives, would you start using the painful max depth or rather use the average depth to keep diving?

 

[O-ring]

Arnaud once bubbled...


What if your computer breaks in the middle of a trip, you don't have a backup and you don't dive on the fly? You would need to use the tables. If you've logged your previous dives, would you start using the painful max depth or rather use the average depth to keep diving?

What I would do is get another bottom timer from someone or a local shop and keep diving. I don't use my computer anymore, so it would be a non-issue. Otherwise, I would probably dive the old Navy 120 rule with a little conservatism thrown in here and there...maybe start my stops at 40' or 50' and go REALLY slow from 20' to surface.

Depends a lot on the profile and whether or not I am doing repetitive dives..

 

[Arnaud]

O-ring once bubbled...

What I would do is get another bottom timer from someone or a local shop and keep diving. I don't use my computer anymore, so it would be a non-issue. Otherwise, I would probably dive the old Navy 120 rule with a little conservatism thrown in here and there...maybe start my stops at 40' or 50' and go REALLY slow from 20' to surface.

Depends a lot on the profile and whether or not I am doing repetitive dives..

But with the 120 rule, doesn't the average depth get you in the exact same place than computing each level?

And if you use 110 instead of 120 to stick to the NDL table, aren't you in effect using the average depth concept with the table?

 

[Diver0001]

awap once bubbled...


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?

NOt all computers/tables are based on the same 1/2 times and number of compartments or even the same deco-model and not all tables use the same compartment for calculating surface intervals and therefore adjusted NDL's.

I'm sure it would work if your computer and your tables are based on the same model and same parameters but I'm not so sure that you can mix and match based on adjusted NDL's alone.

Having said that, there's a reasonable chance that it would work but since I'm not 100% sure I'm not going to advocate it outright. At least not for planning subsequent dives...... I'd feel terrible if I said it with authority and someone took it as fact and got hit because I was wrong.

It's one thing to write it down as a curiosity and it's another to use it for planning repetative dives. Personally if my computer crapped out between dives (or during a dive) and i was sure it was still within the nodeco boundaries then I'd call the dive a Z (PADI) for falling back to the tables for planning. That's probably more cautious than it needs to be but it's better to be safe than sorry in this game....

If you really wanted to know for sure then you could bump the thread into the deco forum and get an expert opinion.

R..

 

[wreckchick]

I don't assign pressure groups for my computer dives but if my computer died, I'd try and recreate it in a conservative way. Look at my max depth, overestimate it if need be, bottom time, overestimate if need be and look at the tables. If I'm way outside the tables, I'd be depressed, but I'd sit out until the next day when I could start fresh.

It's not the prettiest solution, but I'd rather be safe.

Rachel

 

[redacted]

Yes, I have looked at some other alternatives. I'm looking at a live-aboard that does 4 to 5 dives a day (with recreational NDLs) to a nominal max depth of 90 fsw. Bottom is pretty flat so profiles are fairly square. With a 2.5 hour SI between dives, I think it works out to a PG "C" at the start of each subsequent dive assuming max BT on each previous dive. (I'd have to look again.) I keep a spare computer (air only, more conservative algorithm) in my save-a-dive kit, but would like to make the transition without missing a dive. I figure that means switch computers but dive tables while the new computer is working on a repetative dive N load. After a couple dives, or maybe the next day, I could abandon the RDP and go with the new computer. Your suggestion seems to present another option that should at least be considered.