From "The Skin Diver's Bible" by Owen Lee published in 1963 and then again in 68. I will be posting some excerpts from this here. Just to give a taste of what diving and dive training and advice used to be. BTW 160 pages with a cover price of 1.95.
"Today only two kinds of air valves are used on tanks. They are the straight K valve and the constant-reserve J valve. The K valve is a simple open-and-close valve mechanism that provides air to the breathing regulator straight from the cylinder. There is no built-in air-reserve mechanism. When the air pressure in your cylinder becomes low, it gradually becomes harder for you to breathe. When at last the air pressure in the cylinder becomes almost equal with the water pressure surrounding it, it becomes impossible to breathe. But the moment the diver begins his ascent towards the surface, the air in the tank expands and provides him with a few more breaths, which are adequate to get him safely to the surface. He cannot however go any deeper before starting his ascent. In fact the only advantage to a K valve is the relatively small price of $8.00. Because of it's lack of a constant-reserve device, I do not recommend it for depths beyond 30 feet."
" If an air tank is equipped with a J valve, a spring loaded shut off device stops the air flow automatically when the air pressure drops to about 300 pounds per square inch. At that point the diver pulls a lever that releases the remaining 300 pounds per square inch of air pressure for breathing, and the diver knows he has from five to fifteen minutes (depending on his depth) of air remaining in order to reach the surface. If necessary he can swim deeper to free himself from entanglements or to leave the interior of a sunken wreck or cave before heading toward the surface. The straight K valve does not permit going deeper once the air flow stops. Thus the J valve is highly recommended. Although it costs more than three times as much as the straight K valve, it is well worth the price."
There are a lot of these entries that have me going hmmm. This one stood out the most so far.
"Today only two kinds of air valves are used on tanks. They are the straight K valve and the constant-reserve J valve. The K valve is a simple open-and-close valve mechanism that provides air to the breathing regulator straight from the cylinder. There is no built-in air-reserve mechanism. When the air pressure in your cylinder becomes low, it gradually becomes harder for you to breathe. When at last the air pressure in the cylinder becomes almost equal with the water pressure surrounding it, it becomes impossible to breathe. But the moment the diver begins his ascent towards the surface, the air in the tank expands and provides him with a few more breaths, which are adequate to get him safely to the surface. He cannot however go any deeper before starting his ascent. In fact the only advantage to a K valve is the relatively small price of $8.00. Because of it's lack of a constant-reserve device, I do not recommend it for depths beyond 30 feet."
" If an air tank is equipped with a J valve, a spring loaded shut off device stops the air flow automatically when the air pressure drops to about 300 pounds per square inch. At that point the diver pulls a lever that releases the remaining 300 pounds per square inch of air pressure for breathing, and the diver knows he has from five to fifteen minutes (depending on his depth) of air remaining in order to reach the surface. If necessary he can swim deeper to free himself from entanglements or to leave the interior of a sunken wreck or cave before heading toward the surface. The straight K valve does not permit going deeper once the air flow stops. Thus the J valve is highly recommended. Although it costs more than three times as much as the straight K valve, it is well worth the price."
There are a lot of these entries that have me going hmmm. This one stood out the most so far.
