Pony Bottle: Valve On or Off while diving

[Marie13]

Been musing around the idea of backup/redundant air in the form of a pony bottle for a while now and I've been wondering a few things.

Let's first start with a couple of givens:

1. 13 ft^3 bottle
2. Mounted on the main tank or on a Zeagle Zipper attachment on the BC (I have a ranger LTD)
3. Stage Kit type rigging
   1. SPG
   2. DIN first stage
   3. Second stage with long hose and retainer band

For some reason one potential and routine problem that comes to mind is sudden free flow of the pony second stage, whether at the surface or just because it doesn't like the position, or losing air through an incipient leak through a loose fitting or the like.

My gut says that the pony tank valve should remain closed until needed. Of course this would mean having immediate and ready access to the valve which in turn means the tank should likely be mounted valve down.

Am I barking at the moon or is there some merit to this thought?

An AL40 would be better than 13.

 

[rfwoodvt]

First. Don't use pony bottles, proper planning prevents the need for pony bottles. If proper planning can't prevent the need for a pony bottle then you need a fully redundant setup in the form of doubles or sidemount, both of which take up less space than a pair of tanks and a pony.

If you're going to use a pony bottle, please at least use one that is useful for rock bottom.
For 100ft, that is NO LESS than (4ata*1cfm*3min+2ata*1cfm*3min+1.5ata*1cfm*5min)/.95=26.8cf. That is what it takes to get one diver to the surface in an emergency. First value is conflict resolution, second is ascent, third is safety stop, the /.95 is because only 95% of a tanks pressure is considered usable since you need at least 150psi remaining for the regulator to work. 13cf is nowhere near enough.

Second-if you're going to use one anyway, it should replace one of the second stages on your tank, not be in addition to. I would recommend you replace the second stage around your neck with it and that way if you have a real emergency you pass your primary and switch to the secondary which is then attached to the pony. Leave the bottle on at all times.

Thanks for the thoughts but it is not relevant to the question of running with the redundant tank on or off during the dive.

I do like your thoughts on just one second stage per tank. but I'm not sure what you mean by passing the primary given this is a solo dive scenario.

Anyhoo, the diving scenario I was thinking of was pretty specific:

Solo dive, depth not to exceed 30 feet. Redundant air system only to be used in event of failure of the primary system, not as "additional" air in case some problem or other issue prevents me from surfacing. Automatic abort of the dive and safe ascent begun once the redundant system is activated.

As for the air volume required, I did, indeed, do some calculations in coming up with the 13 cuft for the scenario. But I agree, that more is likely better with a 19cuft in this scenario would provide more margin. But I went with the 13 for simplicity's sake.

To do the calculations I used a spreadsheet I found here on SB. Though I need to verify the calculations (I'm no math wizard) I ran the above scenario through that calculator and below are the results.

for the Variables I went at a conservative SAC of 1.0 (my own SAC is 0.5) For depth I went with 60 (double my self limited solo dive depth of 30. For ascent rate I again went conservative with 30 fpm (as opposed to 60 as I have sometimes seen suggested as an acceptable emergency ascent rate), and I went with the standard 3 minute safety stop at 15 feet.

SAC Rate (cu-ft/min)	1	ft^3/min
Ascent Rate	30	feet per minute
Safety Stop Depth	15	feet
Safety Stop Time	3	minutes
Max Depth of Dive	60	feet (in increments of 5 ft)
Results:
Gas used on Ascent	3.97	cu-ft
Gas used on Safety Stop	4.36	cu-ft
Total Gas Volume Used	8.33	cu-ft

Roughly speaking, the above gives me 30% consumed for ascent, 33% foor safety stop, and 37% for margin from a 13 cuft tank.

 

[Marie13]

Breathing gas is just like ammo - you can never have too much.

 

[rfwoodvt]

An AL40 would be better than 13.

More is always nicer, but a fully redundant (ie essential double tank) is not practical for what I intend...also doesn't answer the original question.

 

[rfwoodvt]

The few times I did carry a pony tank for the whole dive, it was attached in the mid of my ARALU twin tanks. So it was far in the back, its valve was hard to reach.
Hence I did open it at time of entering water.
Furthetmore I do not like the idea of having a regulator not pressurized underwater. It is quite easy to loose some air from a secondary reg. If its valve is closed, after loosing the gas trapped in the hose, water can easily flow in...

I wonder if there is any type of isolation valve that could go inline with the lp hose to prevent such a back flow of water into the first stage?

 

[rfwoodvt]

Breathing gas is just like ammo - you can never have too much.

True but you can't carry all the ammo you want, there are some practical limitations.

 

[Marie13]

More is always nicer, but a fully redundant (ie essential double tank) is not practical for what I intend...also doesn't answer the original question.

I answered the original question. 13s are damn near useless IMO.

 

[rfwoodvt]

I answered the original question. 13s are damn near useless IMO.

The question wasn't about the 13. It was about leaving the valve open or closed until needed.

 

[Marie13]

Off but charged.

 

[tbone1004]

@rfwoodvt at 30ft I would just CESA, you're nowhere near deep enough to need a safety stop, and you're well within the realm of an easy CESA if it really hit the fan to just make a direct ascent.

Either way, the pony should replace your secondary around your neck with the valve on IMO