In any case, after I built that homemade backpressure trigger I set about working on a tank. Seoulman got one working but it had sealing problems, so I took a novel approach. I worked with the same types of materials as I used on my tank.
Here's what I had:
There's a 3/4" CPVC endcap and coupler, some 1/2" PVC, some 1/4" OD vinyl tubing, a bit of brass, a rubber stopper, and a metal washer.
Here is the base of the tank. It is simple to see how it works: pressure comes in through the tubing, pushes the brass piece forwards, the rubber seals against the washer in the front, and the pressure builds up inside the tank. Pressing the backpressure trigger will release pressure in the tubing sucking out the stopper. That is where problems occurred, which I will get to later.
That's the front of the mechanism, where the rubber seals.
Considering the problems Seoulman encountered, the first time I pressurized this thing I was really pleased. It was leaky on the first pump but after two it was sealed perfectly, and definitely building up pressure. I pressed the button to fire and... Nothing. The pressure had all come out of the trigger; the seal between the brass and PVC has to be ideal to let air into the tank but suck the stopper back. Mine was too loose, and I haven't been motivated to get back to this project until lately.
It should be a pretty easy fix, and if I could get some larger piping parts the tanks can easily be made to have more appreciable volume. This is what I have so far and it's certainly a buildable design with some tweaking; good luck.
Zorn and I talked for a bit and he made the good point that there needs to be a vertical force acting on the poppet. The stoppers with trapezoidal cross-sections are better here but I need to find a way to make them and use them.
Oh, thought of a potential solution.
Since the rubber stopper have a trapezoidal cross-section, they should be usable when they're cut down (say, in half). There is also too much space behind the poppet. This can be fixed using two small pieces of brass: ideally one could drill a hole through an endcap and use 1/4" ID brass tubing, which would fit the vinyl tubing to the tank, and attach the poppet to 7/32" brass inside the other brass. From my sleep-addled mind and the busted BS tanks I have, this should work.
Edited by Ice Nine, 05 May 2009 - 07:46 PM.