Question Calculate repetitive dive group with safety stop on O2

[Florinda]

Hi, I am trying to use the Navy Tables to do a calculation that I can't quite figure out. I would like to do a 5 minute safety stop--NOT an obligatory decompression stop--breathing 100% O2. The reason for this is to add extra conservatism to my diving due to living at a higher elevation. Yes, I have advanced nitrox training, and yes, I am aware that this practice is not a substitute for offgassing at lower elevations. My question is simply mathematical. Can someone show me how to use the Navy Tables to calculate my repetitive dive group including this stop on O2? I know the EAD for O2 is -33 ft, but since that depth is not on the table, I'm stuck!

Let's assume the following profile: 42 minutes at 70 fsw on air, followed by 5 minutes at 20 fsw on 100% O2.

Thanks!

 

[tursiops]

Can't be done using the Navy tables. Sorry. It is trivial to make the calculation using various software.
ADDED: However, you will NOT get a repetitive dive group you can then use with the tables.

 

[inquis]

Since extra conservatism is the goal, why not use the normal / unaltered pressure group? That will result in a shorter -- more conservative -- repetitive dive. Hopefully you already are using the equivalent depth when using the table for altitude dives (which also adds conservatism compared to using unadjusted depths).

 

[Florinda]

Can't be done using the Navy tables. Sorry. It is trivial to make the calculation using various software.
ADDED: However, you will NOT get a repetitive dive group you can then use with the tables.

Thanks for the quick reply. Yes, the output is the problem, as I'd like to use the Navy ascent to altitude table subsequent to this calculation. When using software, I assume the output is something like GF99 as a percentage? Or what outputs are possible if running this in software? I tried it in MultiDeco but it doesn't tell me anything about my nitrogen, that I can tell, just CNS and OTUs.

 

[Florinda]

Since extra conservatism is the goal, why not use the normal / unaltered pressure group? That will result in a shorter -- more conservative -- repetitive dive. Hopefully you already are using the equivalent depth when using the table for altitude dives (which also adds conservatism compared to using unadjusted depths).

I could definitely just breathe the O2 without doing the calculations, but I am curious to do the math, if it's possible, that's all. And to clarify--the dives are being done at sea level

 

[tursiops]

Thanks for the quick reply. Yes, the output is the problem, as I'd like to use the Navy ascent to altitude table subsequent to this calculation. When using software, I assume the output is something like GF99 as a percentage? Or what outputs are possible if running this in software? I tried it in MultiDeco but it doesn't tell me anything about my nitrogen, that I can tell, just CNS and OTUs.

@EFX might be able to help you. His spreadsheet software tracks N2 in all the compartments. I don't know whether he can also calculate an ascent to altitude version of his software.

 

[OTF]

Tables are really meant as a quick reference tool for "relatively simple" dive planning. For something as complicated as multiple dives with gas switches and altitude changes you're much better off using something like multideco or subsurface.

"actually doing the math" (that the tables summarize and planning software does under the hood) is absolutely possible but it is a series of equations with many variables.

In general the idea of doing some extra deco (safety stop) time on O2 for added conservatism is valid. But if you're just doing a few minutes for buffer it's not really worth calculating - plan your dives as if you weren't doing it and you know your actual dive will be a little bit more conservative than the plan. Just like how people sometimes dive nitrox but limit themselves to air bottom times for added conservatism.

Are you diving using a computer or only a table+timer? A modern computer that will account for gas switches, altitude, and has customizable gradient factor settings would be useful in this kind of dive.

 

[inquis]

I gather you're concerned about driving home after a dive? Check out the Santa Rosa thread, which discusses the elevation gain when driving from a popular dive site in NM. Breathing the O2 during the drive is also an option.

Did you have a particular altitude about which you are worried? Timeline for that?

 

[Kendall Raine]

Breathing the O2 during the drive is also an option.

A very sensible option, although you'll likely drool all over yourself and get funny looks from other drivers (personal experience).

 

[Florinda]

Tables are really meant as a quick reference tool for "relatively simple" dive planning. For something as complicated as multiple dives with gas switches and altitude changes you're much better off using something like multideco or subsurface.

"actually doing the math" (that the tables summarize and planning software does under the hood) is absolutely possible but it is a series of equations with many variables.

Yeah, it's sounding like tables aren't going to cut it for what I'm trying to do. FWIW, I picked the Navy Tables to start because they have the most compatibility across the different kinds of calculations I'm trying to do. So how do I get my hands on all these equations?

Are you diving using a computer or only a table+timer? A modern computer that will account for gas switches, altitude, and has customizable gradient factor settings would be useful in this kind of dive.

I dive Shearwater computers. But based on reading several other threads on similar topics (e.g. this, this, this), it sounds like GF99 is not really translatable to these other calculations. It'll track my in-water gas switch, but not O2 on the surface, for example, or adjusted ascent to altitude times, or even off-gassing rates at different altitudes...