Figure 14 illustrates a typical steam generator level detection arrangement. The DP detector measures actual differential pressure. A separate pressure detector measures the pressure of the saturated steam. Since saturation pressure is proportional to saturation temperature, a pressure signal can be used to correct the differential pressure for density. An electronic circuit uses the pressure signal to compensate for the difference in density between the reference leg water and the steam generator fluid.
As the saturation temperature and pressure increase, the density of the steam generator water will decrease. The DP detector should now indicate a higher level, even though the actual DP has not changed. The increase in pressure is used to increase the output of the DP level detector in proportion to saturation pressure to reflect the change in actual level.