There have been several different configurations of valves. The "J" valve, the
"K" valve and the "R" valve are the more common. Although the name "K"-valve
comes from Walter Kidde Co., the manufacturer, the designations "J", "K" and "R"
were the item numbers in one of the early U.S. Divers catalogs. They all sit on
straight regulator attachment risers, except for a Demone Manufacturing Company
valve. (Introduced in 1961.) Their valve had a 30 degrees angled stem. This puts
the regulator further away from the head and slanted. Quite comfortable. I can only
imagine that it was cost that made them discontinue it. Demone Mfg. Co. closed its
operation shortly thereafter. Lately the valve has made a comeback, particularly in
Europe with their DIN valves.
"J" valves; That is a constant reserve valve that has a lever activated by a
pull rod that you set in the up position. That will give the diver a 300 - 500 psi
reserve where he can pull down the lever on your left side and get the last few psi
of air out of the cylinder. However remember that you have only 300 psi left when it
operates, no matter if you are at 30 or 300 feet. You pull the rod on the cylinder
and come up. It had its problems though. Suppose you brushed against something,
or inadvertently pulled down the lever, then when you ran out of air, you were
really out of air. With no reserve as expected. Now you had to buddy breathe with
somebody, or do an emergency swimming ascent exhaling all the way.
"K" valves; This was the Walter Kidde Co. standard On-Off valve no frills,
and the only one that has survived the passing of time.
"R" valves; The positive or automatic reserve valve differs from the constant
"J" reserve in that when the reserve is in effect, the diver must rise in the water to
get sufficient air to be comfortable. That is unless he reaches over his shoulder and
completely opens the valve to "fill position." This was a difficult maneuver, and the
valve did not survive
