Many years ago while building steam locos I utilized the redline water level sight glass system that greatly enhanced water level visibility. Currently using WL sight gages that are smaller in diameter than most redline gage tube material offered. To solve this problem you can easily make a gage redline on any tube diameter you want. My method consist of masking the portion of the tube length you want to have a red line on approximately 90-120 Deg. of tube OD and do a light blast with glass bead material for that masked off area and then apply a light coat of Dykem Red layout dye on the masked surface where you have lightly textured the tube surface, this will allow the dye coloration to stick to the tube exterior surface. If you over do the Dykem dye liquid application and the dye runs under the masked area you can easily wipe the excess from outside the masked are by wiping with a solvent wetted Q tip. I utilize the glass bead media as it cleans Aluminum parts with minimum metal alteration in preparation for painting, much better than the silica sand materials offered and also generates less dust inside the portable benchtop blast cabinet I utilize in my shop.