Steam drum level is both a critical and difficult measurement to take as control of the water level in the drum must be precise.
A water level that is too high can result in water carryover into the steam piping. A level that is too low can expose the generating tubes (down comers), preventing the water in the drum from cooling the furnace tubes properly and possibly damaging them. Several factors make this measurement difficult to obtain. The steam drum itself may not be perfectly level, and even at steady state conditions, considerable turbulence in the drum can cause the level to fluctuate. Constantly changing rates of water inflow and steam outflow add to the potential for measurement error.