What Happened to My Gas?
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.