Back mount pony bottle

[Ross19966]

A 30cf pony saved my ass in a 1st stage failure at 90 feet. Short of using independent doubles, I think I'll keep my lame looking pony.

 

[johndiver999]

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.

If there were, all the world’s energy problems would be instantly resolved.

 

[SlugLife]

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 and gas will go from high pressure to low pressure. So, air would flow from your 2000 psi pony tank into the 500 psi AL80. To increase the pressure in the pony bottle, you'll need another bottle with a higher pressure than the pony bottle. Volume is irrelevant to the direction air flows through the whip.

In other words, I'd expect the equalized pressure to be roughly (500psi * 80cu + 2000psi * 19cu) / (80cu + 19cu) = 787.8 psi in both tanks once they equalize.

There are some slight nuances like temperature, or a small volume of gas-loss in the whip, etc which might make it not exactly that number, but it'll be close enough.

----

edit: If you had a completely empty AL19, and a 500psi AL80, you could equalize both tanks to about (500 * 80 + 0 * 19) / (80 + 19) = 404psi. From there you could continue to top up the pony from any tank that has more than 404 psi, like a 1000psi tank, then a 1500psi tank, etc. I have used partially full tanks to partially top up nearly empty pony bottles.

 

[SlugLife]

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?

F*** that organizer. Don't redesign how you dive around some annoying micromanager.

Pony rentals are not common, but they do exist. You'll have to call around wherever you're traveling to.

 

[SlugLife]

Maybe I'm just vain but I wouldn't be caught dead carrying a pony.

I'm not one to tell other people how to dive, but....

Some people are caught dead because they didn't carry a pony.

...exactly this.

I have several extra loaner pony bottles, including 6cu and 19cu, and am always a little surprised how rare it is for someone to take me up on the offer. I don't really understand why people are embarassed to carry one. Maybe some see it as a crutch or training wheels?

After having an equipment incident (loose regulator hose), I like having the insurance that should nearly anything go wrong with my primarily air supply, it'll only ever be a minor annoyance.

As a bonus, it even made me more confident with servicing my own regulators. Not that I think I'd make a mistake after getting some experience, but if I did, I'd have a completely redundant system with me.

 

[happy-diver]

You wish!

I wouldn't be caught dead carrying a pony.

 

[Angelo Farina]

Pressure and gas will go from high pressure to low pressure. So, air would flow from your 2000 psi pony tank into the 500 psi AL80. To increase the pressure in the pony bottle, you'll need another bottle with a higher pressure than the pony bottle. Volume is irrelevant to the direction air flows through the whip.

In other words, I'd expect the equalized pressure to be roughly (500psi * 80cu + 2000psi * 19cu) / (80cu + 19cu) = 787.8 psi in both tanks once they equalize.

There are some slight nuances like temperature, or a small volume of gas-loss in the whip, etc which might make it not exactly that number, but it'll be close enough.

----

edit: If you had a completely empty AL19, and a 500psi AL80, you could equalize both tanks to about (500 * 80 + 0 * 19) / (80 + 19) = 404psi. From there you could continue to top up the pony from any tank that has more than 404 psi, like a 1000psi tank, then a 1500psi tank, etc. I have used partially full tanks to partially top up nearly empty pony bottles.

Although you are roughly correct, in reality these calculation can be slightly wrong, because you are factoring the "total filled capacity" of each tank (80 cu.feet), whilst you should use the REAL capacity (12 liters).
The total capacity is the real capacity times the nominal filling pressure.
So your math works only if both tanks are rated to the same nominal filling pressure.
Using the real capacity makes the calculation correct wathever the nominal filling pressures.
That is the reason for which here in Europe we are used to name the tanks by their real capacity in liters, not by their nominal full capacity.

 

[SlugLife]

Although you are roughly correct, in reality these calculation can be slightly wrong, because you are factoring the "total filled capacity" of each tank (80 cu.feet), whilst you should use the REAL capacity (12 liters).
The total capacity is the real capacity times the nominal filling pressure.
So your math works only if both tanks are rated to the same nominal filling pressure.
Using the real capacity makes the calculation correct wathever the nominal filling pressures.
That is the reason for which here in Europe we are used to name the tanks by their real capacity in liters, not by their nominal full capacity.

Correct. For example, the above calculations would be off on an HP100 (3500psi) steel tank or LP80 (2400psi?), because the "CU" (Cubic feet) rating would be taking the air inside at max-pressure, and converting it to the volume it would fill at 1ATM. So, I should have used liters instead of CU.

 

[Angelo Farina]

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.

I see you your point and I agree on the last part.
But a reserve double valve tank on a reasonably sized cylider (15 liters steel, in my case), still makes perfect sense to me.
First of all, this is my setup since 1985, so I am well used to it, and changing anything will need adaptation.
My preferred reserve valve is the Technisub one. Truly simple, never had a problem in almost 40 years, and very safe, as you cannot pull it by error, thanks to its spring-loaded mechanism.
People criticising reserve valves usually never used a Technisub one. Most deprecation comes from people who employed only the crap J-valves of the sixties and seventies. The Technisub reserve arrived later, and possibly never reached US. So please, do not criticise a system which you have never seen in action...
The tank has two posts and two valves, so I can use two fully independent regs. A first stage or O-ring failure is not a problem, I just close that valve and use the other reg.
15 liters at 200 bar provide a decent amount of air, so reserving 1/4 for the reserve mechanism still gives me enough "normal" air for planning the dive not accounting for the reserve.
Regarding the SPG; it failed me at least 4 or 5 times in the last 45 years, so it is not 100% fail safe. And there are situations in which you cannot materially look at it very often.
So, having the tank with double valves and the reserve, I really do not see any reason for switching it for a smaller tank, with a single valve and no reserve, this will reduce redundancy with no benefit.
I understand that finding such tanks nowadays can be difficult in some areas (not here, where they are still common).
But if one finds such a tank, I think it is way better than using a pony tank.
Regarding doubles: I have a nice Aralu twin tank, but the manifold does NOT separate the two tanks. Instead the Technisub reserve DOES separate the two tanks.
I think that this kind of valve is better than the "modern" manual separation manifold, but discussing this here would be slightly off topic.
And yes, for deep dives I always used my Aralu twin set. Although the air capacity is substantially the same as my 15-liters steel, the twin tank provides some slightly better redundancy.

 

[inquis]

Although you are roughly correct, in reality these calculation can be slightly wrong, because you are factoring the "total filled capacity" of each tank (80 cu.feet), whilst you should use the REAL capacity (12 liters).

A minor nit, an AL80 is 11 liters water volume (11.1 actually) rather than 12.

FWIW, an equivalent "imperial" approach would use the tank factors (rated cuft capacity per 100 psi} in the weighted average. I have these memorized for my normal tanks, much as metric folks memorize the water volume.