The bicolour level gauges allow a sure and clear reading of the level in case of the fluid is water steam. The operating principle is the different index refraction of the light in the water and steam. According the above, the special construction of the body and the special illuminator used, we can observe the red colour in correspondence of STEAM and green colour in correspondence of water. The bicolour level gauges are manufactured with large window for low and medium pressure and circular window (port hole) for high pressure.