I have used these for many, many years, and still occasionally use either my twin tanks or my single with a "J" reserve and a double hose regulator. Swimming "clean" is a much different feeling than today's diver, with all the hoses, BC, tank guage, etc. which create a lot of water drag.
To avoid the problem of an accidently tripped "J" reserve, I've mounted one of my sets of doubles with the reserve on the right side (opposite of where it would normally be located), and it's facing inward, not outward. This effectively prevents it from being inadvertently tripped by bumping up against an object while underwater.
I've used both K and J valves, and have three singles, two with J's and one with a K valve. The one with the K valve has an interchangable mechanism with my Sherwood duel manifold, which I elected to set up as a "J" valve configuration (the doubles mentioned above). When I can, I use a pull rod with my J valves and I do use them. I leave them in the "up" position (which I think of as the "on" position). I also check it during the dive, pushing up on the lever at periodic intervals.
I just tried to duplicate with a half-full steel tank (~1200 psi left in it), and could not reproduce the restriction problem that was noted in another post above. If the valve is working correctly, then having it bumped halfway down should not affect the air flow at all. I think something is wrong with the valve, and perhaps the valve seat for the J reserve is coming apart or something weird like that is happening. I wouldn't dive that valve again until it is disassembled and examined.
I have two very good sources for descriptions of the "J" valve. They are the 1970 US Navy Diving Manual, and Fred Roberts book,
Basic Scuba. The 1070 US Navy Manual states:
(a) Air-Reserve valve.--The air-reserve valve is built into an elbow in the cylinder-block manifold assembly that connects one of the air cylinders to the shutoff valve. THe air-reserve valve serves as a warning and a safety device for the diver. A reserve supply of air is conserved in one of the air cylinders by the air-reserve valve for use by the diver when the main supply of air is nearly exhausted. The pressure in the reserve air cylinder is held at 500 psi as the diver expends the air in the other air cylinder. when the pressure drops below 500 psi, the diver wille xperience difficulty in breathing; this signal warns the diver that only a fraction of the air supply remains. The diver can then open the air-reserve valve by uplling down on the pull rod (see fig. D-10) on the side of the left air cylinder.
The air-reserve valve is a flow-check valve with a manual override. As long as the absolute pressure in the air cylinder to which the air-reserve valve is attached remains above 500 psi, flowing air opens the air-reserve valve. When the air-cylinder pressure drops below 500 psi, a spring forces the flow check against a port orifice, shutting off the flow of air from the cylinder. The pressure in this cylinder will then remain at 500 psi, because no air is then being taken from that air cylinder, while the pressure in the other air cylinder continues to drop during diver breathing.
When the diver desires to use the reserve air supply, he opens the air-reserve valve by pulling on the rod described above. This action rotates the valve slide lever one-quarter of a turn. This rotation advances a plunger pin, and pushes the flow check off the orifice against the action of the 500 psi spring. THe reserve air from the air cylinder which has been held at 500 psi then flows into the other air cylinder until the pressure equalizes. Because the plunger pin continues to hold the flow check open, the entire reserve supply becomes available to the diver.
The Navy Diving Manual then has a warning that "...ascent to the surface should be started immediately because the reserve air supply is only a fraction of the original air supply."
Fred Roberts calls the J valve a "Constant Reserve 'J' Type" valve. He has a description of the valve operation, then states:
When cylinder pressure falls to a given design value, generally 300 psig plus or minus in single units, the reserve spring is able to hold the reserve seat assembly into the air stream and effectively choke off the flow of air. The air is never actually cut off short, but rather breathing resistance is caused to climb rapidly, jolting the preoccupied diver back into reality. The seat of this particular mechanism is thus made so that it can leak a little...All constant reserve valves are, in effect, depth compenasted because of the inherent chareacteristics of the demand regulator. Because the regulator supplies air at ambient pressure, the spirng setting will always be the reserve sping value plus ambient pressure. THe resulting pressure will be the cut off point. For example, at 100 feet the gauge ambient pressure is over 44 psig greater than it is at the surface; thus regulator cut off is about 44 psig higher and reserve will therefore occur at 344 psig rather than 300 psig."
Note the difference between the Navy's description of this valve and Mr. Roberts description. The edition of
Basic Scuba I have was published in 1963, while the Navy manual was from 1970. This sounds like it's way back there to today's divers, but even that short time span resulted in a technological difference in the functioning of the valve. While the "constant reserve" feature remained the same (and still is today), the valve by 1970 actually completely shut off the air, whereas before about 1960 it apparently did not.
SeaRat
