I have been working on putting together a test bed to play with different refrigerants and desiccant material. I have done at least 10 iterations on trying to hold the pressure low enough to get the action happening with water but alas it continues to leak the smallest about but it is sufficient to stop the evapouration process and the temperature of water very quickly starts to rise after the vacuum pump is switched off. The pressure only rises a few tenths of a psi but that is enough to stop the process.
I have been working through each connection point and valve to isolate the leak. A trick I happened upon is to systematically submerge the apparatus one joint or valve at a time. The viscosity of the water is sufficient to plug the hole temporarily and it helps isolate the leaks. I immediately found the largest leak which was a faulty solder joint on one of the corners and it seems that the gas shot off valves are just not up to the job of holding a vacuum. I'm guessing they are engineered for positive pressures to make the seal.
Reading around it seems that the best valves are Globe Valves which also seem to be called shut off valves but instead of the 1/4 turn of the gas shut off valves these taken multiple turns and inside there is an actuator that tightens against a seat to shut it off. It seems that it is much like a normal water tap except that there are o-ring seals on the shaft that turns to close the flow so that it doesn't leak when in the open position. Makes sense in hind sight.
So here is a close up of the appartus with the gas taps.
As you can see lots of teflon tape, buckets of silicone to try to seal it and i even resorted to tacky tape ( the black stuff) for sealing. Eventually I went back and resoldered all the joints, changed the olives in the compression fittings and re-tapped the copper capillary joints i am using to put the quick connects on with and it would hold a vacuum level that was about 0.2 psi higher than when the vacuum pump was running. This 0.2 psi seems crititcal. Unfortunately i can't tell the exact pressure because the vacuum pressure sensor I've got is only rated to 1-30psi.
As an a side here is evidence there is a lot of force exerted on the vacuum vessel. I got down to 1 psi using this fuel can as a vacuum vessel and the bang the fucker imploded in a spectacular way
So I replaced the fuel can with screw top jars, lashing of silicone, tacky tape etc to stop the screw thread from leaking around the jar top. No go, leaked like a bastard. In desperation (just before giving it away for the day ) I cut a seal out of the anti-fatigue mat i was standing on, screwed one of the plumbing fittings into a flat piece of perspex. The idea being that when the pressure falls the force on the perpex squeezes down on the seal and creates a tighter and tighter seal. Goddamn if it didn't work!!! At last a success! It does solve the issue of the low level leak which i am pretty sure will require the valves to be replaced.
So next step is to replace the valves. i might see if there is any way that I can reduce it down to one.
