Above the water - insoluble & specifically heavier indicating liquid in leg 1, which is connected with the water space of the boiler, stands the boiler water.  In leg 2 is the steam condensate from the steam space of the boiler.  By recondensing the steam the water level in leg 2 is continually kept constant.  Condensate in excess is continually flowing back through leg 3 into the boiler (in the case of cold pressure tanks a drip-water feed is fitted on the condenser.  Maximum pressure 100 kg/cm2 & available in standard lengths of 350 mm & above. 

Every displacement of the water level in the boiler, resp. in leg 3, produces on analogues equal displacement of indicating liquid in the inspection glass.  if e.g. the water in the boiler rises, the increasing water column in the legs 1-3 presses the red indicating liquid (sealing liquid) from leg 1 into leg 2 & indicates in the inspection glass of the indicator a rise of the water level.  The water displaced from leg 2 flows from the condenser into leg 3 resp.  Into the boiler.  If the water level in the drum falls, the indicating liquid in leg 2 resp.  in the inspection glass, falls correspondingly.  By condensation of the water vapour in the condenser the water level in leg 2 is continually filled up & is consequently continually kept constant.