Missing a dive because of lost gas is a very annoying experience, and
it's often blamed on a short fill. The more likely reason is a gas leak
caused by carrying a cylinder with the handwheel and allowing the valve
to be inadvertently opened slightly. This typically occurs when the
cylinder is being lifted up into a vehicle or tank rack, or when the
cylinder shifts during transport and the handwheel is rubbed open. We
also regularly see leaking neck O-rings and even leaking burst disk
assemblies. Leaks through the valve seat are rare but do happen with
very old and worn valves. Sometimes leaks are so slow that the hiss of
gas escaping can't even be heard. Often with such tiny leaks the
pressure will drop a few hundred PSI and the leak will stop, but the
cylinder almost never leaks to completely empty; thus the appearance of a
short fill.
Leaks from the valve outlet can be prevented by meticulously double
checking the valve is closed whenever you move a cylinder. Leaks from
the neck or burst disk assembly can only be prevented by regular
maintenance of your valve. When your cylinder and valve receive their
annual visual inspection make sure the O-ring groove in the neck is
cleaned and a new clean and unlubricated neck O-ring installed. Dive
Gear Express recommends that the burst disk be replaced whenever the
cylinder is hydro tested (i.e., every five years) and annually if the
cylinder is known to have been overfilled.
