Around 1962-63 came the Submersible Pressure Gauge into the mainstream dive stores, but it was not until
-68 or -69 that it became popular. Amazing, who would want to drive a car without
a fuel gauge. The "J" valve was no longer of interest to the recreational diver. You
can't have a "J" valve in the up position and use a pressure gauge at the same time.
You will not get an accurate reading. Every time you would take a breath, the
pressure would drop. So the only places you will still see a "J" valve is probably on
some far out island in the Caribbean, or possibly on a professional Search &
Recovery team member's cylinder. The reason for this, is the water they dive in may
be so murky, one cannot read a pressure gauge.
To alleviate the problem of the fluctuating pressure reading when breathing,
one manufacturer came out with a cylinder valve with a pressure port below the "J"
valve. Now every cylinder had to have a small pressure gauge mounted on the valve,
or a pressure gauge on a hose, which made the cylinders difficult to handle.
There were also some regulators with built in "J" valves downstream of the
submersible pressure gauge port. These did not prove popular. The idea was that
you could use lower cost "K" valves on your cylinders and still have the "J" valve
wherever the regulator went.
