Another Tables vs. Computers Thread

[H2Andy]

lol

 

[TheRedHead]

10@ 130' is H. H at 40' is 37 minutes. Spend 20 minutes there. 20+37=57 = K.

This is useful for calculating the repetitive dive group at the end of the dive.

 

[Charlie99]

Say you had 2 wrecks that were close to each other: one was at 122 feet and the other was at 40 feet. You want to spend 15 minutes on the first wreck and you want to ascend to 40 feet and head out on a compass heading where you'll find the 2nd wreck and spend 20 minutes including travel time. That would be 35 minutes at 150/2=75 feet. Doesn't work on a table. How can you plan this dive?

On a practical basis, I know that the 40' portion doesn't really matter on a single tank dive, and I'd just plan for the deep wreck.

NDL for 130' is 10 minutes, for 120 is 13 minutes. Assuming I take a couple minutes getting down to the bottom, I should start heading back up towards my first stop at 60' somewhere after 10 minutes runtime, by 15 minutes runtime at the very, very latest.

For gas planning, I figure that 5ata x 15 minutes max is only 75 surface equivalent minutes, not even half an AL80. (Alternative calculation is 5ata * 0.5cfm * 15 = <40 cu ft.) Absolute rock bottom is 1200psi, I'd probably leave if I hit 1500psi.

That's off-the-cuff dive planning in real life.

--------------------------------

If I really wanted to plan the deco a bit closer, it would go something like .....

122' 15 minutes is a bit outside of NDL dive. I'd do something along the line of -----
lets make max depth 120' (or ignore that 2', or spend some time a few feet shallower so that average depth for the 1st segment is still 120' or shallower. All are valid for small difference like this).

120' air NDL is 13 minutes. On PADI wheel/RDP that is pressure group K. Pressure group K on PADI table at 40' is reached after 48 minutes. If travel time and putzing around the wreck takes 20 minutes, then I have the same 60 minute halftime loading that I would from a dive at 40' of 48+20=68 minutes. Pressure Group P. That's what I would use to plan any repetitive dives later that morning.

In real life, I find a dive profile like this to be less agressive than a lot of other profiles I dive such as 20@80, 15@60, the up to 40' for a while. One way of looking at it is that the short, deep excursion rapidly loads up the fast compartments. Those fast compartments then do a lot of offgassing when at 40'.

 

[TheRedHead]

In real life, I find a dive profile like this to be less agressive than a lot of other profiles I dive such as 20@80, 15@60, the up to 40' for a while. One way of looking at it is that the short, deep excursion rapidly loads up the fast compartments. Those fast compartments then do a lot of offgassing when at 40'.

I think your system is interesting, but it doesn't account for a little deco and that's real life diving fo rme. The profile I came up with is similar to two wreck I did dive in St. Croix. I purposely put in a little deco because that is more likely the way I am going to dive it.

Deco Planner puts in a 1 minute stop at 50 feet and counts the 20 minutes at 40 feet as deco and adds a 1 minute stop at 30 feet and a 1 minute stop at 20 feet for a total run time of 41 minutes. I would probably add another stop at 10 feet just for my personal safety factor for a total run time of 42 minutes. That shouldn't affect my gas planning.

 

[Charlie99]

I think your system is interesting, but it doesn't account for a little deco and that's real life diving for me.

I don't go more than a few minutes into deco per my relatively liberal Oceanic computers. I have played around with some spreadsheets that not only show ongassing as do regular tables, but also show offgass. By just changing two numbers, I can regenerate the table for the faster compartments. What it did for me is to quickly confirm some hunches I had about what was going on during a variety of different profile without having to run dozens of profiles through VPM or GAP.

What probably isn't clear from my posts, is that this is just predive planning stuff. During the dive, I use a computer, coupled with a pretty good idea of what it should be telling me from some seat of the pants tracking which really is more or less tracking the overall average depth, and also keeping track of average depth over the last 10 minutes. I don't go so far as to actually try to numerically track average depth. If I did choose to do so, I'd mentally ballpark average depth over each 5 minute segment, keep a running total of those 5 minute depths, then divide that running total by the # of 5 minute marks to get average. I don't bother with the rule of 120 stuff, since I have no difficulty remembering NDLs from 50 to 120 feet.

So far, I haven't found it of sufficient benefit to go to the bother of formally tracking average depth, since what I'm really doing is keeping track of N2 loading at sufficient accuracy during the dive to make sure my computer isn't lying to me. My mental bargraph just has 5 or so bars --- zippo, light, medium, med-heavy, heavy, loaded to the gills. Just as important as the relative loading levels is the gut feel as to which compartments are most heavily loaded. You will deco out / get large safety margin quite quickly from a square profile deep dive. Spend lots of time at 80 to 60' and or a significant multilevel dive and it will be much slower compartments that are most heavily loaded. Those will take a lot longer to offgass to get some safety margin.

To calibrate those comments to your deco program --- the yellow-to-red transition on my computer is right around gradient factor 100. the Yellow to Green transition is around 80 to 85%. I would like to leave the water with my Oceanic bargraph in the green, but usually get bored and exit just a bit before it backs off that far.

There are lots of different ways to dive safely.

 

[limeyx]

Say you had 2 wrecks that were close to each other: one was at 122 feet and the other was at 40 feet. You want to spend 15 minutes on the first wreck and you want to ascend to 40 feet and head out on a compass heading where you'll find the 2nd wreck and spend 20 minutes including travel time. That would be 35 minutes at 150/2=75 feet. Doesn't work on a table. How can you plan this dive?

I would make the assumption that I could see the wreck just fine at 110 feet, but could dip down to 120. I would do the dive on 30/30, and have about 20 mins at 110 feet (average) which might mean 10 at 120 and 10 at 100 or just 20 at 110 or whatever.

Then ascend (slowly, doing the proper min deco) to 40 feet and spend 20 or so mins at thw wreck there. I would dive the 30/30 like 32%, so it'd be like diving 20 mins at 90 or so feet, which gives me plenty of time at 40 or shallower.

Even better might be to plan a second dive for the shallow wreck. All provided enough gas etc. If I am planning a 120 dive, then I definitely prefer doubles.

 

[limeyx]

I think your system is interesting, but it doesn't account for a little deco and that's real life diving fo rme. The profile I came up with is similar to two wreck I did dive in St. Croix. I purposely put in a little deco because that is more likely the way I am going to dive it.

Deco Planner puts in a 1 minute stop at 50 feet and counts the 20 minutes at 40 feet as deco and adds a 1 minute stop at 30 feet and a 1 minute stop at 20 feet for a total run time of 41 minutes. I would probably add another stop at 10 feet just for my personal safety factor for a total run time of 42 minutes. That shouldn't affect my gas planning.

And best to be careful on the "little deco" thing .. as then it's a "little more" and then more and then .ooops.

Deco dives should be something that you plan on on the surface, not something that "just happens" during the dive.

Well, in my opinion anwyay.

 

[TheRedHead]

I don't go more than a few minutes into deco per my relatively liberal Oceanic computers. I have played around with some spreadsheets that not only show ongassing as do regular tables, but also show offgass. By just changing two numbers, I can regenerate the table for the faster compartments. What it did for me is to quickly confirm some hunches I had about what was going on during a variety of different profile without having to run dozens of profiles through VPM or GAP.

I am amazed that you have developed such as system for tracking and have such a comprehensive knowledge of deco theory and how to apply it on the fly. I started planning a little bit of deco on the deeper wrecks because it better to plan for it than to incur it unplanned, which happens too often when you have only 12 minutes on a wreck you've never dived.

I also dive with a computer in addition to my plan and I have the Navy tables in my pocket.

 

[Scubakevdm]

I just dive until I feel the nitrogen pressure gradient get too high, then I ascend a bit.

 

[TheRedHead]

And best to be careful on the "little deco" thing .. as then it's a "little more" and then more and then .ooops.

Deco dives should be something that you plan on on the surface, not something that "just happens" during the dive.

Well, in my opinion anwyay.

When you plan deco you had best dive your plan and have a contingency. I would probably dive that profile on a steel 100 or 120 and that leave plenty of gas for my buddy as well.

As far as a little deco, I find it unrealistic to plan a 10 minute dive to the NDL at 130 feet, particularly if I have traveled a long distance for a special dive. I am certified to make decompression dives and realized when I wasn't, that I was taking a risk I wasn't prepared to take.