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> >As an unmanaged gas source the gas volume is checked prior to the dive and
> >assumed to be available if needed during the dive.  To insure this the
> >valve is normally (or should be) closed during the dive and opened if the
> >gas source is needed.

My philosophy has always been that, if a pony bottle is going to be used 
only as a backup, then you will either use all of it, or you won't (i.e., 
it will be enough or it won't).  Knowing how much is left shouldn't have 
any affect on your ascent pattern. Also the gauge could potentially serve 
to confuse a stressed diver who momentarily forgets which gauge goes with 
which cylinder.

However, as my perceived value of deep decompression stops increases, 
I've begun to wonder if it would be a good idea to know what the pressure 
is halfway through the bailout. If there is more than enough to make 
it to a stage bottle, then it might be worth incorporating an extra stop 
or two during the ascent to reduce DCS probability.  On the rebreather, 
all my gas supplies are in ponies, and they all have gauges (they are my 
diluent sources, so I need to monitor their usage).  Most serve as OC 
bailout supplies.

> I've had a competing recomendation made here.  My instructor recommends
> the method you outline (gas off on pony until needed), since it eliminates
> the risk of an unnoticed freeflow on the pony reg.  Another diver suggested
> the problem with this approach was the posibility of drawing water into the
> unpresurized low pressure hose.  I don't have enough experience to counter
> this, do you have any thoughts?

If you leave the valve off, then be sure to pressurize the hose(s) first,
then turn the valve off.  This will reduce the chances of water getting
into the hoses from backpressure.  However, if you go below a depth where
the ambient pressure exceeds the interstage pressure of the regulator, or
if the gas pressure in the hose drops due to leakage or bumped purge
botton, then you will almost certainly get water inside. (I've seen it
happen more than once). I don't know if this has any significant effect on
the probability of a failure within a single dive, but if left unchecked,
corrosion can render your pony regulator unreliable. 

Aloha,
Rich

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