Determination of the optimal amount of refrigerant in field assembled refrigeration systems by use of a sight glass installed in liquid line is often misleading and unreliable. This is attributed to interferences to laminar flow of the refrigerant in the sight glass by ice formation in the expansion valve, vaporization of the liquid in pipes and foreign particles. Due to these interferences refrigeration systems may be overcharged or undercharged with the refrigerant. Measurement of temperatures and pressures at the inlet and exit of an evaporator may provide an accurate and reliable criterion of establishing the optimal amount of refrigerant in a refrigeration system. Using Rayleigh''s method of dimensional analysis, a mathematical model that expresses the evaporator cooling rate for field assembled refrigeration systems in terms of physical properties of the evaporator and the thermodynamic properties of refrigerant R134 a was developed. The model expresses the optimal amount of refrigerant in a refrigeration system corresponding to the maximum cooling rate.