Recreation diver:pony bottle?

[robzr]

Well, you just completely backed off and now agree just about precisely with the green areas of the chart that you were critiqueing... ( 13 cu ft min down to 66 fsw and 19 cu ft min at 100 fsw)

According to http://cemu.org/scuba/ponycalc.php, based on a sac rate of .87, bottom depth of 66 feet, 30 seconds spent at depth in order to assess the situation, then a 60 fpm ascent to 60 feet, followed by a 30 fpm ascent to 15 feet where a 3 minute safety stop is conducted, followed by a 30 fpm ascent to the surface, 7.68 cf of air is used.

The same parameters, with a starting depth of 100 feet, shows 9.92 cf of air is used. So in theory, this diver could spend 80 seconds assessing the situation on the bottom and still make it to the top within the recommended ascent rate, with a full safety stop, on a 13 cf pony.

I'm not nearly as experienced as you folks here, but it seems to me the best thing to do is know exactly what you have, what you are dealing with, and make a choice based on the tradeoffs and safety margins you are comfortable with. Keep in mind, there is a tradeoff to having a larger than necessary pony tank, which is that one is more apt to not dive with it, not bring it on a plane to travel, etc due to it's size and perceived inconvenience, as well as initial cost of the bottle and mounting options.

If it were as cheap, convenient and easy to use a 40 cf pony as it is a 6, this would be a no-brainer for all....

Rob

 

[lamont]

According to http://cemu.org/scuba/ponycalc.php, based on a sac rate of .87, bottom depth of 66 feet, 30 seconds spent at depth in order to assess the situation, then a 60 fpm ascent to 60 feet, followed by a 30 fpm ascent to 15 feet where a 3 minute safety stop is conducted, followed by a 30 fpm ascent to the surface, 7.68 cf of air is used.

The same parameters, with a starting depth of 100 feet, shows 9.92 cf of air is used. So in theory, this diver could spend 80 seconds assessing the situation on the bottom and still make it to the top within the recommended ascent rate, with a full safety stop, on a 13 cf pony.

80 seconds is not a long time, particularly for a recreational diver who is not trained to get off the bottom, and has never been in even simulated situations with task loading where there's been a requirement to get off the bottom.

I'm not nearly as experienced as you folks here, but it seems to me the best thing to do is know exactly what you have, what you are dealing with, and make a choice based on the tradeoffs and safety margins you are comfortable with. Keep in mind, there is a tradeoff to having a larger than necessary pony tank, which is that one is more apt to not dive with it, not bring it on a plane to travel, etc due to it's size and perceived inconvenience, as well as initial cost of the bottle and mounting options.

If it were as cheap, convenient and easy to use a 40 cf pony as it is a 6, this would be a no-brainer for all....

The cost is probably remarkably similar between all the different sizes, the 40 cf bottle does not cost 7x the 6 cf bottle because it holds that much more air. Scuba diving is also not cheap, and you shouldn't cheap out on something which is clearly perceived as being essential safety equipment. I don't understand how people who buy pony bottles justify them as being essential safety equipment, and then justify small ones because the big ones are too expensive and inconvenient... Which is it?

As for perceived inconvenience of a 40cf bottle, that argument really isn't going to go anywhere with me. This is what I consider to be inconvenient:

The only real argument you've got is the airplane issue... I solve that one by diving locally... *shrug*

 

[Jibeho]

Well, you just completely backed off and now agree just about precisely with the green areas of the chart that you were critiqueing... ( 13 cu ft min down to 66 fsw and 19 cu ft min at 100 fsw)

I felt a gang raping coming on

 

[Jibeho]

As for perceived inconvenience of a 40cf bottle, that argument really isn't going to go anywhere with me. This is what I consider to be inconvenient:


The only real argument you've got is the airplane issue... I solve that one by diving locally... *shrug*

See I would think a 40 to be to much to pack. But some may think it is nothing, I have never had doubles on my back nor to I intend to. The previous poster and I seem to agree that having a gear bag that doubles ones body weight to be excessive. My 13 fits in my bag so I pack and dive it all the time. If it was much bigger I may start to think is it worth it.

 

[dhampton82]

It cost will be a factor, http://scubatoys.com/store/Scuba_Tanks.asp , check out the price difference on the 3 tank sizes. 6cf= $86, 13cf= $93.50, 19cf= $103.95.

I dont know about you, but my peace of mind is worth more then a $20 max. price difference.

Tank size, and not carrying it can be a valid arguement, thats why I'm (possibly mistakenly) going with the 19 and not the 40.

 

[robzr]

It cost will be a factor, http://scubatoys.com/store/Scuba_Tanks.asp , check out the price difference on the 3 tank sizes. 6cf= $86, 13cf= $93.50, 19cf= $103.95.

I dont know about you, but my peace of mind is worth more then a $20 max. price difference.

Tank size, and not carrying it can be a valid arguement, thats why I'm (possibly mistakenly) going with the 19 and not the 40.

Not only tank size, but breathing and mounting options. Zeagle has a pretty nice mount which appears to integrate nicely into the side of the BC; but it's max capacity is a 13 cf. EAS makes a nice relatively inexpensive 1st/2nd stage regulator with no hose, but it's really intended for a 6cf. You can't argue that the 3cf Spare Air wouldn't be easy as hell to carry around, fill, travel with, etc - and thats exactly why people buy them. As you point out, there is a moderate price difference in the bottles themselves. There is a convenience factor when travelling, realistically, more folks are going to leave a 19 cf behind than a 6 cf because of luggage space, or just convenience. There is a perceived filling issue; you can fill a smaller cylinder by bleeding off full tanks easier than you can a larger cylinder.

In short, it's a trade off, and you can't ignore that. Yes, sacrificing safety for convenience = bad, but thats a very black and white way of looking at this, and it's not such a black and white issue (as can be evidenced by peoples opinions being all over the place).

Seems to me, the best thing to do is know what you're requirements may be, decide what safety margins you want, what the limitations will be, and make sure you're comfortable using your system. Do what you can to minimize falling into a false sense of security. Perhaps ponys are not a good idea for a personality that will get lazy/cocky knowing they have a get out of jail free card. Knowledge and practice!

Rob

 

[dhampton82]

Someone correct me if I am wrong with my calculations and theory......

But wouldnt a 3cf spare air have roughly the same amount of air in it as an AL80 with about 112.8 psi left in it?

And wouldn't a 6ct have roughly that same amount of air in it as am AL80 with about 225.6 psi left in it?

I dont know about you but I dont see my self coming up from 60 ft and doing a 3 minutes stop with only 112-226 psi left in my tank.

Please correct me if I am wrong, this was just a theory of mine......

 

[robzr]

Someone correct me if I am wrong with my calculations and theory......

But wouldnt a 3cf spare air have roughly the same amount of air in it as an AL80 with about 112.8 psi left in it?

And wouldn't a 6ct have roughly that same amount of air in it as am AL80 with about 225.6 psi left in it?

I dont know about you but I dont see my self coming up from 60 ft and doing a 3 minutes stop with only 112-226 psi left in my tank.

Please correct me if I am wrong, this was just a theory of mine......

Not exactly, an 7.25" diameter (standard) "80 cf" tank is really 77.4 cf. Having 3000 psi in a 6 cf tank is the equivalent amount of air as represented by a 232.5 psi pressure differential in an al80. A 232.5 psi pressure differential is not exactly the same as having "232.5 psi left in a al80" (ie a pressure differential of 0 - 232.5).

According to http://cemu.org/scuba/ponycalc.php, and this scubaboard poll, a calm scubaboard diver with an median sac rate (median amongst the 87 respondants of the poll), could make a 30 fpm ascent from 60 feet with a full safety stop on 3.53 cf of air (equivalent of a 136.8 psi pressure differential in an al80).

Obviously that is assuming a calm diver, in average conditions (assuming the sac rate poll results were based on overall average sac rates of the divers), and the only safety margin here is assuming a 30 fpm ascent and full safety stop, as some have pointed out, many people dove for years on 60 fpm ascents with no safety stops without problems. If we plug [url="http://cemu.org/scuba/ponycalc.php?bottom_depth=60&bottom_time=0&rmv_rate=.50&breath_rate=15&stop_time=0&stop_depth=15&rate_deep=60&rate_shallow=60&rate_change=60&verbose=1&excertion_rate=1.0]those numbers[/url] into the calculator, this ideal diver in ideal conditions with no safety margin could make the ascent on 1.02 cf of air in 60 seconds. Probably not a good formula to use when planning, of course.

regards,

Rob

 

[rjack321]

Those numbers are about right. More precise than I would calculate since tanks dimensionally vary by those 100ths of a cf.

Unless either of you have a gauge reading plus or minus 7.1 psi I suggest rounding to the nearest 50psi

 

[MXGratefulDiver]

Richard,

You may also want to check his actual SAC rate on Sunday. I bet it's lower than he expects it is.

I about fell out of my chair when I first read the 70cf for a 100' dive. The Emergency SAC of 1.8 cf per minute helped to understand the numbers though.

~ Jason

Just catching up on this thread now ... yep, he'd do way better now.

Back when we did his class, I could just about see the sides of Doug's tank cavitating every time he took a breath ...

... Bob (Grateful Diver)