I can't figure out if I have a unique and good idea or if I just missed the bus.  My guess is I missed the bus a long time ago, but that won't stop me from throwing this nugget out there.  I finally replaced the mercury bubblers in my research lab.  It was time.  I have had people say they use oil bubblers.  I tried that for about a day and really hated the lack of pressure that I was so accustom to.  I had to find another solution.  That's when I found Inert Relief Valves (20 PSI) from Cole-Palmer.  I am sure there are others out there, and I get no money from Cole-Palmer, they just happen to be the one I found.  Anyway, the valve has a threaded input and output that pretty easily can be set for standard tubing sizes.  There is a set screw that controls the pressure at which the valve will open.  I have found opening the screw almost all the way and then setting it in place with a glue gun works well.  On the outlet I hook up a normal oil bubbler.  It will bubble pretty fast when the line is in operation, but you do get a bit of a pressure boost without the Hg.  I have been using these for about a year and they seem to hold up pretty well.  And they aren't too expensive ($64).