Abstract: The invention comtemplates preparation of glycol/water de-icing mixtures in a demand-operated system wherein index of refraction and temperature are continuously monitored in a recirculating loop. The supply of heat and changes in mixing proportions of separate flows of water and of glycol are monitored until such time as predetermined values of mixed-fluid temperture and refractive index are achieved. A computer accepts monitoring-signal values for mixed-fluid temperature and refractive index, and further accepts ambient-condition signal values for set-point variation in mixing proportions, as necessary for economical use of glycol in the mixture, under each of a plurality of different ambient conditions.