Optimal Pony Bottle Size for Failure at 100ft?

[OMyMyOHellYes]

really struggling also to decide if i go with a 19 or a 30 even if i do really shallow dive i am afraid that a 19 might be not enough i know i take a lot of air while diving.

From 130' you are literally two minutes and ten seconds from the surface, if necessary. An al 80, at just over 77.5 cu ft you get what? An hour? Maybe if you're an air hog 40 minutes. A 19 is 25% of that. So you've got roughly 10 minutes, worst scenario if you're the 40 minute air hog. If your normal dive profile is an hour, then 15 minutes. If your average dive depth is 60-80 feet, little scenario that would call for more than a 19, except maybe entanglement.

 

[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 i was thinking of that also considering the stress factor.

 

[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.

thank you

 

[34109411]

Trom 130' you are literally two minutes and ten seconds from the surface, if necessary. An al 80, at just over 77.5 cu ft you get what? An hour? Maybe if you'r'e an air hog 40 minutes. a 19 is 25% of that. So you've got roughly 10 minutes, worst scenario if you're the 40 minute air hog. If your normal dive profile is an hour, then 15 minutes. If your average dive depth is 60-80 feet, little scenario that would call for more than a 19, except maybe entanglement.

oh i am not gonna say an air hog but thank you

 

[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.

usually turning at 1500 going up at 1200 honestly very shallow profile and the end of the dive less than 15 feet doing my safety stop up i end up never below 900 psi.

 

[34109411]

 

[34109411]

View attachment 817107

thank you all i post a map of the dive park plenty of informations as you see very shallow i don't venture out more than 30 feet. Picture this big as football field size. The dive shop rent me the 30 cf but iam thinking to buy my one next year this is why i was asking if the 19 cf could do the job. ( i like to point out i dive solo and i am looking for redundancy).

Thank you again helping me to choose. Using a 80 cf i turn around at 1500 start to go up at 1200 end i end up the dive at the surface with at least 900 to 1000 psi. ( my entry point is the first stairs on the left of the picture).

 

[SlugLife]

usually turning at 1500 going up at 1200 honestly very shallow profile and the end of the dive less than 15 feet doing my safety stop up i end up never below 900 psi.

I'd suggest just running the test I mentioned earlier with the 30cu pony, and see how much is left. No "casual surfacing", but rather surface at a safe rate, hit your safety stop, and then the surface.

thank you all i post a map of the dive park plenty of informations as you see very shallow i don't venture out more than 30 feet.

At 30ft deep, most divers could probably even manage with a 3cu, or even CESA. A 19cu should be more than enough, and even give you the opportunity to go deeper, even if you are an air-hog.

 

[34109411]

I'd suggest just running the test I mentioned earlier with the 30cu pony, and see how much is left. No "casual surfacing", but rather surface at a safe rate, hit your safety stop, and then the surface.


At 30ft deep, most divers could probably even manage with a 3cu, or even CESA. A 19cu should be more than enough, and even give you the opportunity to go deeper, even if you are an air-

 

[steinbil]

Trom 130' you are literally two minutes and ten seconds from the surface, if necessary.

60ft/min from 130ft? I'd rather not.