After leaving a prototype bubble display running for a week (in 80-100 degree weather) I returned to find the following clear case of evaporation:
The two tubes on the outside have glycerin, while the two tubes in the center (which are several inches lower) contain water. I didn’t leave a marker of what level the tubes were filled to at the beginning of the week, but the water tubes have obviously lost several inches due to evaporation. (The continuous bubbling action helps evaporation along quite a bit…) It looks like the glycerin suffers less from evaporation, but I expect that even a glycerin filled bubble display will need periodic refills. Note that the 80-100 degree (and dry) weather may have made this problem more pronounced.
You might try adding a small layer of oil at the top of the water / glycerin columns. That should help stem evaporation.
Good tip, I’ll have to try it. The two issues I anticipate is that the oil layer may not be 100% acceptable aesthetically, and that the bubbles that push up through the water may require that I have a very thick (1″) level of oil.