Why there is two water level indicator in boiler?

Any commercial or domestic boiler uses a heating element to provide heat to the water which is governed by heat transfer laws or thermodynamics. Since cold water is heavier than hot water, it remains at bottom and hot water occupies top portion of the storage tank. Thus heating elements are placed at the bottom. They are usually designed to heat water and do not provide good resistance to air. Oxygen oxidizes these elements and the rate of reaction increases with temperature. So, in order to avoid damage two water level indicators are used to govern the water in it. One to prevent overflow and other to prevent heating elements getting in contact with air, especially at high temperatures.