THE TANK VALVE
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 "incidents."
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. Turning the valve back a quarter turn almost caused a fatality with one of my students. A long time ago it was thought that if a large storage cylinder, which might have the valve on for a long period of time, was not tuned back a quarter of a turn the valve might get stuck in the on position. In all my years of teaching scuba that has never happened with a scuba tank valve. However, some people do not know which way to turn a valve to the on position. If a person (diver, buddy, Divemaster, etc.) unwittingly turns a valve that is already on to the off position and then back a quarter the diver will be unaware the tank is almost off. The pressure gauge will read the correct tank pressure. The breathing at the start of the dive when the tank is at high pressure will seem fine. Off the diver goes into the depths. When the tank pressure drops to about half and/or the diver hits a deep portion of the dive the air will feel restricted. It really feels like you are running out of air. The near fatality was caused by the wife on a dive boat. She wanted to make sure her husband had turned on his air which he had. She turned it off and back on a quarter. At 90' he thought he was out of air. He went for her alternate air but didn't make it. He raced for the surface and was pulled from the water unconscious! The near drowning that followed (with no DCI or embolism) put him in the emergency room for 36 hours. If it is recommended to turn the valve on all the way, and a person accidentally turns it off all the way, there will be no way a diver will be able to dive without there being an immediate warning.
Divers today 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. What a 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 that stupid.
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 ring.
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. These are very common in foreign countries. 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 the O-ring cannot sneak out..