Now, it is obvious that if the amount of steam leaving the boiler was always the same as
the amount of water entering, the water level would remain the same. This state of affairs
seldom transpires. Variations in load cause variations in steam flow; variations in the fuel
supply and air supply cause variations in the rate of combustion, which in turn results in
variations in the rate of evaporation, and all of these cause change s in the water level.
This makes it necessary for the operator to maintain a continual watch on the water level.
If it drops, he increases the water supply; if it rises, he decreases it.