Abstract: A control apparatus and method is provided for controlling temperatures, temperature differences, or, respectively, liquid levels in refrigeration cycles used for example in heat pumps or refrigeration installations, which is not dependent on the refrigerating medium employed or on the field of application. The flow of the liquid is controlled by two position relay control valves which are electromagnetically actuated and operated by fluid pressure, are suited for large numbers of cycles and are wear resistant. Instabilities in the refrigeration medium cycle are avoided and variations in the end point of the evaporation no longer have a negative influence on the functioning of the control system. The conventionally employed thermostatic expansion valves are replaced by wear resistant two position valves with high frequencies of switching.