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The History of Pressure GaugesToday we take it for granted that we have pressure gauges with us at all times when we’re diving. They make it easy to judge when it’s time to surface. All you have to do is look down and read your gauge. The story starts with the Aqua-Lung.
For the first time, divers could go underwater without having to be fed air from the surface. Fortunately for the divers of old, it was fairly easy to tell when you were running out of air.The regulator on the Aqua-Lung wasn’t as advanced as our regulators are today and as the amount of pressure in the tank got lower, it became harder and harder for the diver to breathe. So with some experience, the diver could gauge roughly how much air they had left in the tank and when they needed to surface. They weren’t always right. As time went on regulators began to improve and as regulators got better, they began to breathe easier even at lower tank pressures. This created a bit of a problem for early divers because with an easy-breathing regulator, it became harder to tell when the tank pressure was low. They needed a way to bail themselves out in case they misjudged the amount of air they had left. Enter the J-valve. (All this was prior to pressure gauges). 
The J-valve was actually a neat little piece of equipment. It screwed into the scuba tank and had one lever and one knob.The knob turned the air on and off and the lever would have a bar attached to it that ran down one side of the tank. If the lever was in the up position, it would hold about 500 psi (34 bar) of air in reserve. Once the lever was pulled down, the air reserves would become available to the diver and they could surface. It sounded good in theory, but in practice there where a couple of problems. The first problem was that the lever could get knocked down without the diver realizing it. The diver could be reaching under a ledge for a lobster or wiggling through a hole in a shipwreck and the lever would get accidentally pushed into the down position. The diver would go through their dive oblivious to what had happened until they ran out of air. The diver would reach back to pull the bar only to discover that the lever was already down. Not a lot of happy thoughts went through that diver’s mind. The second problem was that 500 psi (34 bar) is fine if you’re surfacing from 60 feet (18m), but what if you’re surfacing from a 120 foot (36m) wall dive. At that depth, 500 psi (34 bar) just ain’t gonna cut it! So pressure gauges came along and there was peace in diving land. Now you can look down and see exactly how much air you have left. You have the freedom to surface at whatever pressure your little heart desires. Return to Underwater Diving Equipment Home Page.
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