Steam Generator Level Instrument:

Figure illustrates a typical steam generator level detection arrangement. The P detector measures actual differential pressure. A divided pressure detector measures the pressure of the saturated steam. Because saturation pressure is proportional to saturation temperature, a pressure signal could be used to correct the differential pressure for density. The electronic circuit uses the pressure signal to compensate for the difference in density among the reference leg water and the steam generator fluid.

Figure: Steam Generator Level System

As the saturation temperature and pressure increase, a density of the steam generator water will reduce.  The ?P detector should now indicate a higher level, even though the actual ?P has not changed.  An increase in pressure is used to increase the outcome of the ?P level detector in proportion to saturation pressure to reflect the change in fact level.