Reference Leg Temperature Considerations:

While the level to be measured is in a pressurized tank at elevated temperatures, a number of further consequences have to be considered. As the temperature of the fluid in the tank is rise, the density of the fluid decreases. As the fluid's density decreases and the fluid expands, occupying more volume.  Even through the density is less, a mass of the fluid within the tank is the similar.  The problem encountered is which, since the fluid in the tank is cooled and heated, the density of the fluid changes, but the reference leg density remains associatively constant, that causes the denoted level to remain constant.   The density of the fluid in the reference leg is dependent upon the ambient temperature of the room in which the tank is located; thus, it is relatively constant and independent of tank temperature.  Some means of density compensation must be incorporated in order to have an accurate indication of tank level if the fluid in the tank changes temperature, and thus density. This is the problem encountered while measuring pressurizer water level or steam generator water level in pressurized water reactors, and while measuring reactor vessel water level in boiling water reactors.