Pony Bottle: Valve On or Off while diving

[doctormike]

@doctormike hopefully they read the whole post where I tried to lead off with "if you're going from 30ft" though who knows. Frankly though this may be a legitimate use case for a Spare Air if we don't want to advocate for CESA's anymore though I do think from 30ft it's a perfectly reasonable solution to a dire emergency since you have an unltd NDL at that depth. We use the spare air's in aquaria where many of the big tanks are in the 25-30ft range.

Right, I understand, but the problem is that nuance and specifics are often overlooked or soon forgotten, while the central message of "this experienced tech diver said that CESA was a good option" is much more likely to be retained.

Decompression stress is not only about NDLs. Of course, there is a reductio ad absurdum case to disprove anything, but my point is that many "undeserved hits" are from decompression stress within the confines of an algorithm. Human physiology is not standardized, there is a bell curve for everything. So all things being equal, if you think that there is benefit from a slow ascent and a safety stop, then you should set yourself up so that you are almost always able to do it.

 

[Moerae]

@Moerae

Fraction of the cost? If they own a pair of tanks anyway...

Most people don't own tanks. For those that do, doubling them up is likely not be a good solution for 90% of their diving.

@Moerae

... On the rock bottom thing, if the pony can't serve for rock bottom what is the point of bringing it in the first place?

Yes agreed. I stated the reason I even brought that up and included the caveat that it might be obvious.

 

[Lorenzoid]

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,

Holy tossing of SB hand grenade in here! Did you miss this recent 1000-post thread?

I know nothing of Solo procedures, so I'm staying out of here. Bye!

 

[johndiver999]

If you keep the pony bottle second stage on a necklace and around your neck, then you should notice a free flow, so it is not a huge problem to leave the tank valve on for the whole dive.

If you back mount it upside down, then I guess you can turn it off, but you increase the chance of flooding the regulator and it really does not provide a whole lot of benefit in my opinion and there is more task loading to actually use it.

If you are going to run a long spg hose for the pony, then you can easily check the pressure when you want. If you are going to complicate your rig with a second pressure gage - then I see VERY little potential benefit to diving with the back mounted bottle off.

I prefer a simpler rig, just a button gauge on my pony- no long spg hose- so I can only check the pressure before the dive, yet I leave the pony on and trust that I will hear, feel or detect a significant leak from my pony. Of course having the second stage on a necklace should make failing to notice a free flow almost impossible. In most types of situations, I would be comfortable with no pony and solo in 30 feet, but if you want a pony for that application, then even a 6 cuft tank is enough.

 

[SurteesDiver]

An option no-one has discussed... when I did my solo or intro to tech I was trained to dive with the pony slung and turned on 1/2 a turn. I believe that the rationale was that the reg was charged (and therefore would not flood), there was a mouthful of air in the reg for an emergency (giving time to sort yourself and turn the tank fully on), and any freeflow would be relatively slow. With the pony slung, the valve is right there and easy to turn on quickly, also both 1st & 2nd stages are easily seen so you will detect a freeflow or leak immediately.
I looked at smaller ponies and bought a 40cuft. Even if a 13 will do your current use case, a 40 is no more hassle but a far more useful cylinder for a range of diving that you may find yourself doing in the future. Cost is little different, I'd buy a 40.

 

[davehicks]

Dive with your slung 30-40cf pony bottle turned on at all times. If it's leaking or loose, then get it tuned up so your safety equipment is actually useful and safe. A charged but off reg will inevitably get drained and result in flooding your 1st stage. This can damage the reg to the point it needs a full overhaul ahead of schedule.

Get rid of your 13cf bottle, it's not appropriate for this purpose. An al 40cf bottle is bigger, but it's actually the easiest size to use as it will be neutrally buoyant when you sling it. You really won't even notice that it's there.

 

[Nemrod]

I would say that turning a valve on partly is an incredibly unsmart thing to do. N

 

[davehicks]

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.

If you start solo diving it is unrealistic to believe that you will only go to 30 feet, unless it is not possible to go deeper. Why limit yourself so arbitrarily? And it is true that a 40 is lighter underwater than the 13.

 

[uncfnp]

Get rid of your 13cf bottle, it's not appropriate for this purpose.

I get so tired of seeing absolute posts such as this.

 

[SurteesDiver]

I would say that turning a valve on partly is an incredibly unsmart thing to do. N

Could you please expand on your statement Nemrod. I detailed why I believed that it was an option... are they not valid thoughts?