Decanting a Twin Tank Reserve
- I was told that Cousteau's first triple tanks used the 3rd tank as a "Decanting" reserve. They'd be diving off the other two tanks until empty, then open the valve from the the reserve tank to "decant" more air into the other two... until all three were the same pressure. Then close the valve on the third tank. Again, they'd keep diving the other two tanks until empty, then open the reserve until the tanks balanced again and close the valve on the third tank. This system allowed them to know how much they had left toward the end of a dive without having a pressure gauge.
You can do it too, with TWIN Tanks and a J-Valve!
- After a really good re-build of the reserve mechanism on my Twin 38s, I was able to "Decant" from one tank to the other. I did it while diving at Sea Hunt Forever but only for 2 pulls and I didn't track how much time I got from each pull.
- Last week I re-built the reserve on ANOTHER set of twins (my "Bill's 38s"). Did it the same way and yesterday we tested it at the pool. The first pull gave me 8 minutes. Second pull gave me 3-4 minutes. 3rd pull 1-2 minutes. And 4th pull about 30 seconds. But even that last pull went from sucked dry to 30 seconds more of normal good breathing pressure.
J-Valve Re-Build Tips
1. The J-valve orifice and seat must be clean and smooth. I used 0000 Fine steel wool on the end of a pencil eraser to polish the orifice. Then, I blew out any dust or debris with HP air. THEN, I used a Q-tip and paper towel moistened with food grade silicon spray to wipe any remaining particles from the orifice.
2. I replaced the seat with a NOS seat I was fortunate to acquire from Captain Tom.
3. The spring may or may not need replacing but take note of any little washer/spacers. These are used to tighten/adjust the spring tension if you need it. You can test this now: Blow with just your lung pressure into the tank and flip the lever closed. Listen to see if it leaks. Flip the lever open to see that the reserve releases properly. If it leaks when the lever is closed, you may need a better seat, orifice, spring, or another spacer to tighten the spring.