The vertical U-tube is the simplest type of pressure gage and consists either of a single U-shaped glass tube having a uniform bore with vertical arms or two separate glass tubes 3466 connected to a cistern. The level of the liquid in the vertical U-gage can be read easily to 0.1 in (2.5 mm) and in well-made instruments to 0.05 in (1.3 mm) water gage.