Optimal Pony Bottle Size for Failure at 100ft?

[34109411]

This isn't really how pony bottles are calculated, however:

1200psi /3000 psi * 80 cu = 32 cu.

You effectively turn your dive having 32cu remaining. Typically, we also try to have 500psi or more, when we actually surface. How much PSI would you have remaining?

Just to shortcut the conversation, lets say you have 700psi remaining. That means you used (1200-700) 500psi to surface from that depth, including safety stop and whatever else you normally do.

500/3000*80 = 13.33 cu.

The above would be an empty (0psi) tank if it started at 13cu. You could still safely surface with a 13cu, but you might surface a little faster and skip the safety stop. However, a 19cu would have almost 900 psi remaining....

(19 cu - 13.3333 cu used)/19 cu * 3000 psi = 894.7 psi

It's easy to make incorrect assumptions, or things in the real world might work a little differently than mere raw math. Even incorrect gauges can throw things off. I also like to actually test those assumptions. Since you can rent a 30cu, why not do a dive to this "max depth" you intend to go, then switch to the pony-bottle, and surface normally, including safety stop. Check how much psi you have left on the 30cu. If my calculations and your measurements are correct, I'm guessing you'll have around 1700 psi remaining on the 30cu.

^ Plus this. We have to keep in mind other things that could go wrong, including entanglements and increased breathing rate, other equipment problems, cramps, etc. Those other things that went wrong may also have lead to the OOA situation, which is why they can't be discounted.

My guess is 30cu may be an appropriate size.

ok sorry i miss few post so ok i will stick with the 30 cf.

 

[34109411]

i don't won't to go off topic but how many people will likely remove the octo from the main tank ? (assuming your diving solo and you use a pony).

how many of you will take the reg from the pony and put it on a necklace instead of having it snug on the pony ?

Sorry if i go bit off topic but so far i had great answers from all of you here.

 

[SlugLife]

Sorry if i go bit off topic but so far i had great answers from all of you here.

Given how many questions you have, it might be worth starting your own thread for your various pony-bottle questions.

Give this comment a read, I wrote this to answer common questions and to give tips and advice for pony-bottles: Redundant Air

i don't won't to go off topic but how many people will likely remove the octo from the main tank ? (assuming your diving solo and you use a pony).

Octo or No-Octo: In most SideMount, you have two first-stages, and 2 second-stages (Each 1st stage has one 2nd stage). No octo. Assuming your pony-bottle is adequately-sized, you may consider ditching your octo. More hoses, means more clutter/entanglement/mess, and possible confusion (i.e. grabbing wrong regulator). You could even re-purpose your "octo" as a 2nd-stage for your pony-bottle, meaning you only really need another 1st-stage and SPG (or button-SPG).

how many of you will take the reg from the pony and put it on a necklace instead of having it snug on the pony ?

A top priority, is that your pony regulator can be very quickly, reliably, and easily accessed in any emergency. Take a few breaths from your pony every dive. Practice switching regulators on the shore if you haven't done it in a while (I was guilty of this), and remember to purge or exhale when switching. Switch regulators underwater too, at least once per dive. Also ensure the pony reg can be found, by feel (no looking!) almost instantly. Stuff the hose under some bungies, use a retainer, or whatever you need to do, to ensure the pony-reg is always in the same place.

In SideMount, often one regulator (left/main tank) is on a necklace or long-hose properly wrapped. The other (right/secondary) is usually clipped to a d-ring on the chest, about in front of your collar-bone. You don't have to do that configuration, but it is an example configuration that works.

Personally, lately, I've been getting "lazy" and instead of clipping the pony, I leave the hose stuffed in bungies on the tank. The tank is slung, the reg is easy for me to access there, and it's one less step when donning my gear. And of course, I practice using it every dive.

 

[34109411]

Given how many questions you have, it might be worth starting your own thread for your various pony-bottle questions.

Give this comment a read, I wrote this to answer common questions and to give tips and advice for pony-bottles: Redundant Air


Octo or No-Octo: In most SideMount, you have two first-stages, and 2 second-stages (Each 1st stage has one 2nd stage). No octo. Assuming your pony-bottle is adequately-sized, you may consider ditching your octo. More hoses, means more clutter/entanglement/mess, and possible confusion (i.e. grabbing wrong regulator). You could even re-purpose your "octo" as a 2nd-stage for your pony-bottle, meaning you only really need another 1st-stage and SPG (or button-SPG).


A top priority, is that your pony regulator can be very quickly, reliably, and easily accessed in any emergency. Take a few breaths from your pony every dive. Practice switching regulators on the shore if you haven't done it in a while (I was guilty of this), and remember to purge or exhale when switching. Switch regulators underwater too, at least once per dive. Also ensure the pony reg can be found, by feel (no looking!) almost instantly. Stuff the hose under some bungies, use a retainer, or whatever you need to do, to ensure the pony-reg is always in the same place.

In SideMount, often one regulator (left/main tank) is on a necklace or long-hose properly wrapped. The other (right/secondary) is usually clipped to a d-ring on the chest, about in front of your collar-bone. You don't have to do that configuration, but it is an example configuration that works.

Personally, lately, I've been getting "lazy" and instead of clipping the pony, I leave the hose stuffed in bungies on the tank. The tank is slung, the reg is easy for me to access there, and it's one less step when donning my gear. And of course, I practice using it every dive.

thank you all do pretty much all this.

 

[Scubagermany]

Sorry I did not have time to read the whole thread.
When diving in the red sea I use an 80 cft aluminiun bottle. With that I can make a nice dive of 45 to 50 min. Even if m deepest was 35 m ...
I woul presume your pony in that size would be enough to make a safe ascent in any way.

 

[Capt Jim Wyatt]

Lets assume you have a SAC rate of 1 cfm and are at 4 ata. Your breathing rate at 4 ata would be 4 cfm.
At 20 fsw your breathing rate would be 1.6 cfm.


From 100 fsw (4 ata) to your safety stop at 20 fsw your ascent should take about 3 minutes with an ascent rate of 30 fpm. Then add another 3-5 minutes at 20 fsw for a safety stop.

SO: You use 3 X 4 = 12 cubic feet on the initial descent and another 8 cubic feet on the safety stop. A Grand Total of 20 cubic feet in order to get to the surface.

Al 40 is sufficient IMO assuming you stay within the NDL.

 

[Gandalf-the-Diver]

You GUE guys need to come to terms that people are going to continue to use pony bottles for redundancy. It is a perfectly valid configuration for recreational diving. Just let it go.

The Kool-Aid is strong in that one.

 

[34109411]

The Kool-Aid is strong in that one.

 

[34109411]

At what PSI do you normally start heading for your safety stop, on the deepest dive you intend to do with the pony-bottle?

As generic advice: If you're unsure, bigger is usually the safer route, assuming you consistently bring it with you on every dive. However, if it's big enough you sometimes leave it behind, then even a small one is better than the one you left at home.

I had a good deal on a brand new 30 cf Catalina. bought it this morning. Thank you for your help.

 

[34109411]

It's pretty easy to find a lower bound by just noting the time and pressure before ascent (on your normal tank), or check your log if you use AI. You will need more when stressed, but if that easy scenario consumes more than 19 cf, that size is clearly not feasible. I'd suggest that if you consumed more than 11.5 cf (which is 430 psi in an AL80 cylinder), then the 19 cf is not appropriate. (That allows for 50% increase in consumption due to stress and a couple hundred psi to keep the reg happy.)

Thank you a bought a 30 cf this morning.