DENSITY COMPENSATIONLevel DetectorsFigure 12 illustrates a vessel in which the water is at saturated boiling conditions.Figure 12 Effects of Fluid DensityA condensing pot at the top of the reference leg is incorporated to condense the steam andmaintain the reference leg filled. As previously stated, the effect of the steam vapor pressure iscancelled at the DP transmitter due to the fact that this pressure is equally applied to both the lowand high pressure sides of the transmitter. The differential pressure to the transmitter is due onlyto hydrostatic head pressure, as stated in Equation 3-3.Hydrostatic Head Pressure = Density x Height (3-3)ReferenceLegTemperatureConsiderationsWhen the level to be measured is in a pressurized tank at elevated temperatures, a number ofadditional consequences must be considered. As the temperature of the fluid in the tank isincreased, the density of the fluid decreases. As the fluid’s density decreases, the fluid expands,occupying more volume. Even though the density is less, the mass of the fluid in the tank is thesame. The problem encountered is that, as the fluid in the tank is heated and cooled, the densityof the fluid changes, but the reference leg density remains relatively constant, which causes theindicated level to remain constant. The density of the fluid in the reference leg is dependentupon the ambient temperature of the room in which the tank is located; therefore, it is relativelyconstant and independent of tank temperature. If the fluid in the tank changes temperature, andtherefore density, some means of density compensation must be incorporated in order to have anaccurate indication of tank level. This is the problem encountered when measuring pressurizerwater level or steam generator water level in pressurized water reactors, and when measuringreactor vessel water level in boiling water reactors.IC-03 Page 12 Rev. 0