The Horror of J-Valves

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Remember J-valves? Physics professor Ellen Domb (Riviera Beach, FL) does. For younger readers: J-valves were used before pressure gauges were popular. When it got hard to breathe, you pulled a long lever down the side of the tank to open the valve and release another 300-500 psi of air to get you home. Before you pulled the lever, the tank seemed empty, but it wasn't.

Inside the J-valve was a spring-loaded shutoff valve that restricted air flow at the neck of the tank when pressure dropped to around 300-500 psi, making it difficult to breathe -- this is the warning. The diver then pulled the lever down, which opened the valve fully and released the remaining reserve air. Too often, divers forgot to set the lever up before the dive, so the reserve would not engage, and they got no warning -- making it critical to check before descent....

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