How is water level inside the pressurizer of a Pressurized Water Reactor is measured?

The level in the pressurizer is determined by measuring the difference in pressure between an external column of water of a known height (reference leg) and a variable column of unknown height inside the pressurizer (variable leg). The differential pressure between these two columns of water is converted into a pressurizer level signal. Four differential pressure transmitters (d/P cells) are mounted on the pressurizer. Each d/P cell converts the sensed d/P into an electrical current that corresponds to a level varying from 0 to 100%. Each transmitter has an external, bellows-type, sealed reference leg, with a condensate pot attached at the top of the leg, to generate the static pressure head of the reference leg. The actual water level inside the pressurizer constitutes the dynamic or variable leg. Since the density of water varies with temperature, any temperature change of the coolant inside the pressurizer affects its indicated level. Therefore, the pressurizer level instruments are calibrated based upon pressurizer temperature. Three of the four level transmitters are calibrated for normal operating temperatures and are used for indication, control and protection. The remaining level transmitter is calibrated for cold conditions and is used only for indication with the plant in cold shutdown or when a steam bubble is drawn in the pressurizer. The cold-calibrated transmitter does not provide an input to the pressurizer level circuitry or an input into the reactor protection system. The output of each level transmitter indicates a level of 0 to 100% for the water volume inside the pressurizer. A selector switch, located on the main control board, allows the control room operator to select two of the three transmitters used for control. One channel is used for level control, letdown isolation, and pressurizer heater cutoff, while the other channel is used for backup letdown isolation and heater cutoff. The third channel can be selected to replace either of the two controlling channels during testing or failures. An additional selector switch is provided on the main control board that allows the operator to select any one of the three transmitters for recording.

The water level in such components is measured using two sets of pressure sensors. a pressure sensor is placed at the bottom of the component (usually submerged section)  and the other in the upper part (usually a dry section).

delta_P=rho g h.  this system is calibrated on nominal operation conditions. to sum up, we make use of the linear relationship between Pressure and level.  

the water level inside the pressurizer is measured by coolant level monitoring system which is present on pressurizer. this coolant level monitoring system makes use of level sensors which continuously give feedback about the level of liquid coolant in the pressurizer to the control room. based on the feedback signal of coolant monitoring system, either heaters or the sprays in pressurizer are turned on as a corrective action.