Picture explains much better than words can (see attachment) the idea is to monitor the level between the two phases. the problem with this is you need to be constantly pumping out of that tank, or it will overflow, and the person who designs it gets chewed out. The guy before me wanted to get sight glasses installed. But I suspect the site glasses would have a high rate of fouling, and maintenance around here is pretty tied up. would like to keep it as simple as possible. I was thinking maybe some pneumatic fail/close check valves at the outlet, since the pump is pneumatic. but I don't know if those exist. even then, it'll be hard to keep the level constant without some type of level control.. meaning everything needs to be enclosed so is sight glass the best option then?