Sight Glass

A device with a glass tube or window used to determine the level in a tank or reservoir. One side of the device connects to the bottom of the vessel and the other end is connected to the top of the vessel. If the connecting lines are fully open, the level of liquid in the sight glass will be the same as the level of liquid in the tank. Around rotating machinery, sight glasses are used on suction drums, oil reservoirs, seal pots, seal tanks, etc.