Many years ago while building steam locomotives I utilized the Redline type water level sight glass system that greatly enhances water level visibility. Currently using WL sight gage glass that are smaller in dia. than most redline gage glass material offered. To solve the problem you can easily make a gage redline on any tube diameter you would like to use. My method consist of masking the portion of the gage glass you want to have the red line on, I use somewhere between 90 & 120 degrees of the tube OD of tube and do a light glass bead blast of that masked off area, then apply a coat or two of Dykem red layout dye to the textured surface, the dye application adheres to the lightly textured surface quite well. If you over apply the Dykem and experience some run under the masked surface the excess can be easily removed from the non textured tube surface wit a solvent wetted Q tip. Have tried this method on tubes as small as 3/16" in diameter and as large as 12mm.