bubble counter water evaporating

Glycerin is sometimes used, as is mineral oil. My experience with my Milwaukee regulator bubble counter was that it wasn't evaporation that was removing the water in my bubble counter, but leakage from the bottom of the counter. When I used a bubble counter that didn't leak I never had to replace the water in it.

 

If you have the Milwaukee regulator bubble counter, it is sealed at the bottom with a very thin O-ring, so teflon tape may not work. I couldn't stop the leak no matter what I tried, so I just removed the bubble counter and attached the CO2 tubing directly to where the bubble counter was screwed on - it is an identical fitting. Then I made my own bubble counter and used it instead.