Rock Bottom for Solo Divers

[bkotheimer]

Question: Do rock bottom calculations and tables for pony bottles (i.e., one reg, one diver) seem applicable to one's primary tank when solo diving? In other words, disregarding one's ACTUAL pony bottle for a moment, as well as the need to reserve air for one's buddy (not necessary), do the pony bottle figures apply well to determining rock bottom for a solo diver's PRIMARY tank?

If so, since the general tables stop at an AL40, would the pressure for an AL80 be half of that?

No, I'm not going to take the table and just jump into solo diving, but I am curious.

 

[fire_diver]

The size of your pony needs to be large enough to let you spend a couple minutes at max depth resolving any issues, plus the time to make a normal ascent, including safety stop, while breathing at a very hard rate.

You need to know what your heavy exertion SAC rate is to calculate this out.

The same applies to your rock-bottom backgas calcs.

 

[rjack321]

do the pony bottle figures apply well to determining rock bottom for a solo diver's PRIMARY tank?

If so, since the general tables stop at an AL40, would the pressure for an AL80 be half of that?

Yes and no.

Yes the pony bottle numbers are for 1 diver. But taking that same amount of gas and applying it to your primary tank misses the fact that 1min to "solve a problem" by yourself on the bottom is pretty tight. It can take a min to even figure out how you're entangled nevermind how to extricate yourself solo. Or lost, unable to find the anchor, how long would you search? etc.

You might want to make your own set of realistic worst case assumptions and plan for those. Honestly sudden "out of gas" is probably fairly low on the list of issues compared to other site-specific issues which take time to resolve, how much time are you going to budget for is the key.

 

[Teamcasa]

I would not count the pony in on any dive gas plan. It's there strictly for emergencies.

 

[tbone1004]

I don't factor it in on anything. I don't usually dive with a pony because I'm in doubles, but I'd factor
10 minutes at depth at your working sac rate
Total time for ascent plus a liberal safety stop, with that much breathing going on you up the risk for DCS so I'd plan on 5 mins instead of 3.
Then add another 10cf or so for the oh sh!t factor. AL40's seem to be somewhat convenient for that.
That doesn't get factored into backgas though, it's a pony not a stage bottle

 

[Jax]

The pony is never part of the gas plan when solo diving. (SDI)

 

[lamont]

A lot of divers when they look at bailout plans start to cut corners assuming that they'll blow off their stops and do a 60 fpm ascent so they only need a few breaths to be able to make it to the surface and can take some massively undersized pony bottle.

I think most of them fail to take into account how difficult it can be to get moving sometimes, particularly when narcosis is involved. If you haven't had an instructor show you how bad you really are under task loading at making decisions and have practiced that a lot, you will most likely need a lot more time than you think. A common mistake would be to attempt to swim back to the upline as you realize that you suddenly realize that you're not that good at blue water ascents and in the heavy current you really don't want to get blown away from the boat. That's a good reason to pack something like an Al40 so that you've got tons more options.

Options are very good things to have. Once you've run out of options underwater you have a minute or less to live. I like protecting as many options as I can.

 

[GrumpyOldGuy]

I will toss out a different perspective. It may not be "correct" by some standards. I also cannot contest, a large AL40 gives you more options.

The rock bottom for my pony is for 1 issue only, I went OOG either due to total stupidity on my part or less likely some unexpected gear issue. There is nothing to work through, it is a simple ascent. I am not going to search for an anchor line, etc.... I drill at least one a month on this scenario, I have it down cold. I even did it once without a mask (had my buddy check my ascent rate since I could not monitor myself). So, by some divers standards my pony is too small, but I think it is enough for a single failure scenario.

The rock bottom for my primary is a lot larger. I use the traditional calculations for 2 divers even if I am solo, taking into consideration a possible uninvited buddy. This also gives me plenty of time to work through other possible issues such as entanglement, lost anchor, mask recover, whatever, etc...

 

[bkotheimer]

Right about the pony bottle not being part of the plan. As I said I am ignoring my pony bottle for this. I'm merely trying to find a good time to consider an ascent when/if I happen to be alone with my AL80 as my backgas. I only mentioned pony bottles because the pony bottle table is based on one diver and one reg. Maybe it was a confusing way to ask the question.

Anyway, I suppose - having found my own RMV and done my own tables based on a standard buddy dive - it would be more conservative (though only slightly so) to halve the AL80 pressure from the backgas table than to halve the AL40 pressure from the pony bottle table.

That said, I really wouldn't want to find myself at 80 ft by myself with only 548 psi (half the pressure of the example table based on a 2.0 for two stressed divers). The simplest calculation calls for depth x 10 + 300 (=1100 @ 80 fsw), half of which is 550 psi, while the "mental table" values cited call for a diver at 80 fsw to use the 100 ft value, which is 1300 (/2=650). I rather like this better I think.

Having thought about it, perhaps it's not too conservative to do what GrumpyOldGuy says .... use the buddy diving numbers even when solo diving. This makes up for the additional stress of being alone and the additional need to self-rescue in the event of any problems.

 

[lamont]

I use the traditional calculations for 2 divers even if I am solo, taking into consideration a possible uninvited buddy.

I think that's the answer right there.

If you're diving somewhere that you're not likely to ever run into another diver, then you just need enough gas to deal with whatever issues you may have as a truly solo diver. If you're at any kind of halfway popular divesite then one of the issues that you may have is a diver you don't know with really big eyes showing you a gauge way into the red. I know it has happened locally around here before. That means you need all the gas you need as a buddy.