water Level gauges in super critical boiler

In plant where steam is used primarily for generating power, water and steam cycle becomes more complicated. Some of these are reffered to supercritical boiler where steam is produced in the pressure exceeding 220 bar. At this pressure the density of water and steam are same. Because there is no difference in density these boilers does not require water level  guages because there is no water level. the supercritical boiler is also called once through boiler as it does not have drum.  After the water is converted into steam in the boiler it passes through the superheater coil, more tubes are exposed to the flue gases to increase the temperature of steam. As steam passes through these tubes the temperature of steam further increases and forms superheated steam. (If the saturated steam which is heated again is converted into supersatured steam.)