Computers vs. Dive Tables

[Mesa Nan]

This is probably a dumb question, but here goes.

So far I've done all my dives with a rented computer (not the same one). My first trips I dived according to the computer, then used my dive tables to calculate my pressure group. On my last trip, to Fiji and GBR, I logged my data from the computer and the divemaster's guidelines but didn't do my pressure groups till I got back home and located a metric dive table. That's when I realized some of the entries in my logbook were hinky. For example, max depth = 24m, bottom time = 49 min which is right off the chart.

I know that computers are more accurate than dive tables because they account for exactly the time spent at each depth, but would the two really be that far off? Or is it more likely that with an unfamiliar computer I wrote down the elapsed time rather than the bottom time?

 

[Blue Abyss]

The reason that you notice that the computer dives are "right off the chart" is that dive computers are constantly recalculating the decompression algorithm (table). This allows the computer to give you "credit" for the time you spend shallow on your dive. Tables on the other hand assume that you have spent the entire dive at your deepest depth. If you did a square profile (spend the entire time at one depth) on a computer it would calculate your no decompression time much closer to tables. But most divers do not dive square profiles, we usually go to our deepest depth then work our way back up the wall, reef, wreck, etc... This is why your computer allowed so much more time than your tables would have allowed.
Thus you cannot calculate multi level dives on a basic dive table. There are some multi level tables/calculators i.e. the PADI wheel but why bother just get a dive computer.

 

[Sideband]

Elapsed time should be bottom time. The tables are only using one depth to figure Nitrogen loading. Your max depth. The computer is keeping a running tally on where you have been and where you currently are. By tables, if you make a quick dip down to 130' for 1 minute then come up to 40', your total allowable dive time is about 9 minutes depending on which tables you have. The computer will only dock your NDL by the amount of ongassing you did during that 1 minute at 130' and then figures your new NDL for the remainder of the dive at 40' which is going to be WAY more than 9 minutes. IMO, unless you are truely diving a square profile you can't do a computer dive and then figure pressure groups on tables with any accuracy. You would have to break that dive down into several single dives with no surface intervals.
For example:
you dive to 130' for 1 minute then ascend to 60' for 10 minutes then ascend to 40' for 20 minutes. You could figure that out as 3 dives with 0 minutes as a surface interval in between and probably be pretty close to your pressure group. The computer will still probably show you having a longer adjusted max dive time for your next dive than the tables though.

Joe

 

[Mr Carcharodon]

Ever notice how what a computers does looks a whole lot like time weighted depth averaging? Of course a lot of people on this board think that could not possiblly work despite it working for many people over a long period of time.

I think what the others said is right. If you do not dive a square profile the computer will give you a different answer than the tables. If it gave you an answer that was different than the tables for a square profile by more than a few minutes that would be important to know. But usually the computers are more conservative than the tables.

 

[Tollie]

Just as an aside. Neither computers nor tables are really “accurate” measures of nitrogen uptake in your body. Both provide estimates based on theoretical assumptions and models. Your personal status will be effected by (in addition to time at depth) your ascent profile, environmental conditions (cold), how hard you may have worked under water, how much you may have drunk the night before… and your own biological/physiological condition.

So… for me bottom line. Dive with your head in addition to any other aids you may use (tables or computer).

 

[gangrel441]

Ever notice how what a computers does looks a whole lot like time weighted depth averaging? Of course a lot of people on this board think that could not possiblly work despite it working for many people over a long period of time.

I seen many on here say that depth averaging can't possibly work. What I have seen people say is that it is not just a simple average, and while it may be a good technique, it requires training and should not be attempted without proper training. An important distinction, if you ask me....

 

[Blackwood]

but would the two really be that far off?

Would? Not necessarily... depends on the profile.

Could? Definately.

I bet you wrote it down right.

 

[Charlie99]

Or is it more likely that with an unfamiliar computer I wrote down the elapsed time rather than the bottom time?

Most computers only have total dive time. While the exact depths vary a bit, a typical dive computer will go into dive mode and start counting time at say, 5', and continue to count dive time until you go above something like 3' depth. (1.5m to start, 1m to stop)

I you think back to your 24m 49 min dive, you may recall doing something like 15-20 minutes in the 20-24m range, and then shallower than 12 meters for at least the last 20 minutes of the dive. A good pre-dive briefing would include such information about the intended profile.

 

[donnad]

ok, i'm new at planning say 4 or even 5 dives in one day and don't have my own computer yet, but will be going on a two day liveaboard, and renting a computer. i know how to do that planning with the table. will be renting a vyper computer. it will have a planning mode and you can plan your day in advance and have a basic idea for your depth and times for the 4 or 5 dives, since you can't use the tables then to determine your pressure group appropriately. teach me oh diving gods!

 

[bradshsi]

But usually the computers are more conservative than the tables.

That's a pretty broad statement to make.

If you are comparing a computer with a table (both using the same algorithm), the table will always* be more conservative on anything other than a square profile (when they should be the same).

If you are talking about the computers vs tables in general, I still don't think the statement is correct, because the algorithms used in both vary considerably, depending on the supplier.


*Except some computers that include penalties for missed stops, excessive ascent rates etc, but then again doing these things, breaches the table rules in any event.