Small Pony 6,8 or 13 cf. When are they a good option

[c5diver]

I don't plan on diving beyond 100 fsw. has anybody used the lp 8cf for a pony?

 

[Scared Silly]

Please do a search as this question gets asked, discussed, (master) debated, argued, etc. ad-nausium ...

Calculating the amount of air needed to make an ascent from XXX to surface is left as an exercise for the diver.

 

[Spimon]

A few threads to skim over... Horses for courses, of course, but I think the general (as in NDL diving without any extreme conditions) consensus seems to be that anything is better than nothing but the bigger the better. This is my interpretation of the threads I've read on the topic, not my personal opinion.

6' is silly
13' is too small
19' is ok
30' is good
40' is great

Remember once you're in the water you won't notice the weight/bulk anyway.

I've written a spreadsheet that will approximate the bottom time for a diven SAC rate, tank size, pressure and depth. It's available on my website (see sig) in the 'Tools > Diving Tools' section. I'm sure it wouldn't be too hard to find an imperial one either, if you don't speak metric.
This might help you decide how big you need your pony to be.

For example, if you're using an 8' (@3000psi) pony at 100' and breathing 20 litres/min (in a stressful situation), you have about 2.5 minutes on the bottom (a bit longer if you're ascending) until the tank is empty. Up to you to decide how long you want and how quickly you'll use it up.


A few threads to skim over. LOTS more available with some searching.
http://www.scubaboard.com/forums/basic-scuba-discussions/358816-pony-size-when-required.html

http://www.scubaboard.com/forums/advanced-scuba-discussions/318453-advice-pony-bottles.html

http://www.scubaboard.com/forums/basic-scuba-discussions/340671-pony-bottle-set-up.html

http://www.scubaboard.com/forums/advanced-scuba-discussions/337284-what-size-tank-pony.html

http://www.scubaboard.com/forums/tanks-valves-bands/333551-looking-purchase-pony-tank.html

http://www.scubaboard.com/forums/tanks-valves-bands/325766-pony-bottle-recommendations.html

http://www.scubaboard.com/forums/technical-diving-specialties/321472-pony-bottle.html

http://www.scubaboard.com/forums/introductions-greets/320213-pony-bottles.html

 

[TSandM]

The question is actually fairly easy to answer, if you do some math. When you deploy a pony bottle, you are assuming that your primary gas supply has been exhausted or has failed, so you need enough gas to get to the surface from wherever you are. How much gas that is can be calculated, as the link above shows, but it's important to be conservative with your estimates of the time needed to cope with the situation at depth, and with your gas consumption on ascent -- which is not going to be normal at all!

We generally, using conservatively padded numbers, calculate 20 cu ft per diver, to come up from 100 fsw. This allows an ascent which slows as you approach the surface, to minimize decompression risk. You may make a determination that such an ascent isn't necessary (after all, in the good old days, everybody went up at 60 fpm and there was no such thing as a safety stop) and that in a gas emergency, all you want is enough to get to the surface as fast as is not excessively risky.

But as pointed out, the difference between carrying a 13 cf and a 19 cf bottle is negligible, so unless you are worrying about minimizing weight for airline travel, you may as well use the larger one and have bigger reserves.

 

[knowone]

Beware your plans and allow for variables.