Spirit Level: Moulded Glass Vials, Filled and Trued

A spirit level's vial is a glass tube almost filled with liquid, leaving a bubble. Alcohols such as ethanol are used rather than water, often coloured with fluorescein; a level that must work either way up uses a barrel-shaped vial, slightly wider in the middle. A 1985 patent makes vials by heating a glass tube, putting it in a mould with a row of vial-shaped cavities and blowing it out to fill them, then cutting the vials apart, sealing their ends with heat, drilling a hole, filling through it and closing it, for example with solder. The vial is set in the body and trued: read the level, turn it end for end, and move the vial to take up about half the difference, until the reading is the same both ways.