Backflow prevention and pressure indication -- Please help

[blakej]

Hey all,
I built a fixture for leak testing cylinder valves. The potential leak points are tested using very small ID stainless steel tubing which drop into a small container of water or leak detector solution. We currently monitor for bubbles and will fail the valve at very low leak rates (a few bubbles per min.) The problem I'm having is that when loading the valve, a vacuum is created inside the test fixture (which I measured to be approx. 5-6"H2O) which causes the ss tubing to suck up the water into the test fixture which contaminates the valves.
To prevent this, I’ve looked at a few options—
-Placing the water container lower/putting an arch in the ss tube, so the vacuum isn’t strong enough to pull the water into the fixture-- This will get in the way of testing.
- Putting in a manual valve for exposure to atmosphere while loading—this is ineffective if the tester forgets to open or close the valve when they’re suppose to.
-Putting in an electronic valve – this is still an option.
-Using a check valve to either allow flow into the fixture when loading the valve, or to prevent the backflow of the water—possible options, but I’m worried about finding a check valve with a reliably low enough crack point.
-Using a pressure indicator instead of water-- haven't found one yet which would be good for detecting tiny leaks and that won't get destroyed if over pressurized.
-Using alcohol (which won't contaminate the valves) instead of water or leak detector -- this is what we're doing now, bubbles don't appear as well as with water and the testers don't like the fumes... I'd like a better option.
If anyone has any ideas or knows of any products that may help solve this issue, I would love to hear them!
Thanks!
Blake

 

[Greg Bernhardt]

I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?

 

[blakej]

Thanks for the follow-up. I experimented with a very small funnel shaped extension on the end of the bubble test tubes with a small floating ball inside. When the vacuum inside the fixture starts to suck the water up the funnel, the floating ball floats up with the rising water level and plugs the small end of the funnel preventing back flow. It worked, which was nice because a traditional check valve needs some cracking pressure in the direction that allows flow and this solution is always open when checking for leaks because the valve relies on buoyancy and not a spring. It's some type of float valve I guess... anyway, Id consider this resolved but I appreciate the reply.