Valve Types Virtually all scuba tank valves are made from chrome-plated brass. Historically, divers identified tank valves as two basic types: the K-valve, which is a sim-ple on/off vaive, and the J-valve, which has a built-in mechanism that signals when you run low on air.The J-valve contains a spring-operated shutoff valve that is held open by tank pressure until the pressure drops to approximately 20-40 bar/300-500 psi. When the tank pressure drops below that point, the pressure no longer holds the shutoff open, causing breathing resistance to increase and warning that air is low. Pulling down the reserve lever releases the remaining "reserve" air.

Although valves were almost standard equipment in the 1960s before common use of submersible pressure gauges, today you see them much less frequently, and usually left in the nonreserve position. An exception is in areas where regulations require them. They're prone to accidental tripping (so they don't warn you), and they increase the cost and service requirements of the valve. The only reliable way to monitor your tank pressure is to use a submersible pressure gauge (SPG), which you'll practice using during your confined water dives.

Today, you can identify tank valves as yoke valves or DIN (Deutschees Institut fuer Normung) valves. By far the most common are yoke valves; as the name implies you attach the • regulator via a yoke assembly. With the DIN valve system, you screw the regulator into the valve. Although less common worldwide, the DIN valve system has the advantage of being rated to higher working pressures.

The DIN system is very common in central Europe.One thing to notice is that all tank valve connections with the regulator require an O-ring, which makes an air tight real. You find the O ring mounted in the valve with the yoke system, and mounted in the regulator with the DIN system. Either way, you can't dive without this O-ring - the regulator won't seal - so learn to check for it when seting up your gear.