I was getting tired of the tank siphoning back to 50%, and decided that it had to be fixed. I was debating between using an aspirator to aerate the feed, adding an aftermarket vacuum breaker/anti siphon device, or building an air gap.
After crawling around underneath, I found the vent line is merely a straight drop of 1" pex, and that the tank itself is plastic and virtually inaccessible. While the vent comes out behind the propane tank, there is barely enough room to get at it where it penetrates the bulkhead. With this in mind, it looked like the air gap would be the simplest.
My first thought was to simply cut the vent a couple of inches below the penetration, and fab a larger pipe that would fit over the stub and drop back down to the same cross member. Couldn't quite get anything up there to cut it. Finally decided to drill a hole through the Pex. A 1/2 inch hole going through both sides would give an open area equal to the ID of the vent and should be adequate to break the vacuum. Got the cordless up there as far as possible, and it wasn't to tough to make the holes. Then used some 1 1/2" PVC pipe with a bell on one end, cut to the proper length,, and put it over the 1" pex as a secondary containment and fastened it to the cross member with a hose clamp drilled to accept the same screw that held the vent.
Filled the tank until it was overflowing vigorously, shut the water off and viola, within a couple of minutes the water stopped and the tank still reads 100%.
Love it when a plan comes together.
