Back mount pony bottle

[Rob9876]

Topping up is just a matter of using a transfill-whip, and is a minimal amount of air. I don't pay to have my pony tanks filled (much less VIP-tax).

I have a dumb question about transfill-whips: Does, for instance, an AL80 with 500psi left at the end of the dive, have enough pressure to top off a 19cf pony that still has 2,000 or 2,500 psi left? Or does the pressure of the donor tank have to be greater than the pressure of the pony tank?

 

[Marie13]

J valve? Really?

 

[Ross19966]

J valve? Really?

Don't know if you can even buy them anymore. A quick google turned up nothing.

 

[Ross19966]

I have a dumb question about transfill-whips: Does, for instance, an AL80 with 500psi left at the end of the dive, have enough pressure to top off a 19cf pony that still has 2,000 or 2,500 psi left? Or does the pressure of the donor tank have to be greater than the pressure of the pony tank?

Pressure will equalize. You will lose a little in the pony and gain a little in the AL80. Pressure in the AL80 would have to be greater than the pony. But since they are both theoretically 3000psi, pressure will still equalize. you will end up with less than 3000 in the AL80 and the same in the pony. A topped off HP could get a AL pony to 3000.

 

[Rob9876]

Pressure will equalize. You will lose a little in the pony and gain a little in the AL80. Pressure in the AL80 would have to be greater than the pony. But since they are both theoretically 3000psi, pressure will still equalize. you will end up with less than 3000 in the AL80 and the same in the pony. A topped off HP could get a AL pony to 3000.

Thanks. Forgive me if I'm being dense, but my example was an AL80 with 500psi left at the end of a dive, asking if that could top off a 19cf that was still at 2,000 or 2,500psi. Are you saying that if I hooked up both tanks then that the 2,000 psi pony would actually send gas to the 500psi AL80? Thanks for bearing with me.

 

[grf88]

Thanks. Forgive me if I'm being dense, but my example was an AL80 with 500psi left at the end of a dive, asking if that could top off a 19cf that was still at 2,000 or 2,500psi. Are you saying that if I hooked up both tanks then that the 2,000 psi pony would actually send gas to the 500psi AL80? Thanks for bearing with me.

Yes, you would almost drain your pony.

 

[DAJ]

If you have 500 psi pushing one direction and 2,000 psi pushing in the opposite direction it should be rather apparent which way the air would flow until the pressure is equalized. You would have a bit more air in the 80 and almost empty pony.

 

[Rob9876]

If you have 500 psi pushing one direction and 2,000 psi pushing in the opposite direction it should be rather apparent which way the air would flow until the pressure is equalized. You would have a bit more air in the 80 and almost empty pony.

Well, that's what I thought, but wanted to confirm -- I said it was a dumb question! I didn't know if there was some type of one-way flow mechanism, etc.

 

[A Bajo]

I just came from a trip and gave up on the pony. The organizer thought I was crazy but I insisted I wanted to use it,. However in the end I just could manage the extra weight in my luggage. On the boat my first stage did spring a leak that needed to be fixed. If this had happened under water, especially at any real depth, it would not had been fun.

Are pony bottles generally available for rent at resort dive shops?

 

[Nick_Radov]

If this is your concern, a pony can be a solution, but I think it is simply "too much" for a recreational dive.
What I use is much simpler: a tank with a good old reserve valve.
If you forget to watch your SPG and end up breathing hard, you pull the reserve rod and you get back 50 bars, plenty enough for ascending and doing your safety stop.
If you want maximum safety, you need a Technisub spring-loaded reserve valve: this avoids the risk of pulling the rod in advance by error.
With a reserve valve your equipment remains lightweight and streamlined, avoiding all the complexity added by a pony tank.

That is a bad idea. Reserve valves have been obsolete for decades. They are mechanically complex and cause more problems than they prevent. Such valves are barely even available anymore and have to be special ordered if you can get them at all.
I don't understand why some divers are trying to over complicate a simple issue. Usually, this stems from being unclear on the basics, and attempting to compensate for doing one thing wrong by doing a bunch of other things wrong as well.
If you need extra gas when real redundancy isn't needed, like for a very long shallow dive, then sling a stage tank filled with bottom mix and use that first before your back tank(s). If you can't monitor your own tank pressure, then you shouldn't be diving at all, and no amount of extra equipment can possibly compensate for being mentally unfit. If you have a mechanical failure, then your buddy has your redundant gas. And if you need a higher level of redundancy than your buddy can provide then buy a real set of doubles with a proper isolation manifold and learn how to manage failures.
But setting up a ridiculous little "pony" tank as a mismatched set of independent doubles is the worst of all possible options. It's like showing up for a race wearing different sized shoes that you scrounged out of a dumpster. Maybe I'm just vain but I wouldn't be caught dead carrying a pony.