Sight Glass

A sight glass or water gauge is a type of level sensor, a transparent tube through which the operator of a tank or boiler can observe the level of liquid contained within.
Simple sight glasses may be just a plastic or glass tube connected to the bottom of the tank at one end and the top of the tank at the other. The level of liquid in the sight glass will be the same as the level of liquid in the tank. Today, however, sophisticated float switches have replaced sight glasses in many such applications.