In closed tanks, you cannot see the level in the tank from the outside. However, with a level indicator, also called a gauge glass, you can see the level in the tank from the outside. Gauge glasses work on the principle of the law of commuting vessels. A gauge glass is a vertical tube connected at the bottom and top to a tank. The tube then creates the same level as in the tank. Inside the tube of the gauge glass is a float with a magnet on it, which causes the red and white level indicator to change color.
