The valve on the top of the scuba cylinder is the weakest
link in the system. Most tank "incidents" resulting in leaks of air, damage,
tank launches, injuries, and death are the result of tank valve misuse. Letting
the tank fall on the valve, unscrewing a valve from a tank that is under
pressure, not turning the valve on all the way prior to the dive, and allowing
the tank to roll around while being transported are but a few of the reported
The valve consists of a knob to turn the tank on and
off, a thread of sufficient length to safely keep it in the pressurized tank,
either teflon tape or an O ring to keep the air from leaking around the threads,
a dip tube, an over-pressure plug, and a hole surrounded by another O ring
to make a seal with the regulator. Some valves have reserves on the side
opposite the on-off knob.
The dip tube is there to prevent any particles in the
tank from getting into the regulator when the diver is head down. The snorkel
sticks into the tank about 2" allowing some room for avoiding things that
should not be in the tank in the first place!
Prior to putting the regulator on the tank it is a good
idea to let a small amount of air escape through the valve. This insures
no foreign particles get into the intake of the regulator. The regulator
is then put on the valve so the holes line up and a seal is made with the
O ring. The valve is turned on all the way prior to diving.
Do not turn it back a quarter after opening the valve!
In the past, it was thought valves could get stuck
in the on position if they were not turned back a quarter of a turn. This
does not happen. But, a dangerous thing has happened. When a buddy check
was done sometimes the buddy would turn a valve the wrong way (to off) and
then back a quarter. This would allow the air to enter the regulator and
give the false impression it was fully on. While the tank was full, the high
pressure could run by the small opening to supply the diver's needs. When
the pressure dropped in the tank, not enough air could pass through the opening,
especially at depth where more air is needed, and the diver would "run out
keep aware of the amount of air in their tank with the submersible pressure
gauge (spg). In the early days of scuba there were no spg's. Divers either
judged how much air was left in the tank by experience and guessing or went
by time under. There were reserve valves to give a warning when the tank
pressure was getting low. A reserve valve restricts the air supply when the
tank pressure was 300-500 psi. When the restriction was felt a lever was
pulled that moved the reserve valve into the second position. Then the air
flowed easily and the diver was warned to begin to surface. The spg is safer.
Sometimes the dive took place with the reserve valve already pulled so there
was no extra air after the restriction was felt. Surprise! Sometimes they
malfunctioned because of improper tank-filling procedures.
Today we dive with the reserve lever down so there
is no reserve!
The early US Divers catalog listed tank valves for purchase.
Everything was in alphabetical order. "A." might have been a regulator, "B."
might have been a face mask, etc. The letter "J." was followed by the reserve
valve name and picture. The letter "K." was followed by the valve that had
no reserve. Today the reserve valve is called, "the J Valve," and the
non-reserve valve is called, "the K Valve." It's
Prior to removing a gauge, regulator, etc. from the tank valve
it is very important to be sure all the air has been removed after turning
the tank off. Gauges have a knurled nut to do this. Regulators may be bled
of air using the purge button on the front of the 2nd stage. Failure to let
all the air out will make turning the knob for removal next to impossible.
If the knob is forcefully turned, eventually the O ring making the seal will
blow out creating a very loud pop and probably tearing the O
When cylinders are rated above 3000-3300 psi the standard tank
valve may be inadequate because of distortion due to the high pressure. Another
type of valve, the DIN valve, is used instead. The regulator is actually
screwed into the valve. There is an o ring that makes contact with the valve
when it is seated. Because there are threads holding the regulator into the
valve there is a stronger