heat transfer fluid mixtures and methods of making and using same are presented . the inventive heat transfer fluid mixtures consist essentially of at least one light gas , such as hydrogen , and at least one heavy fluid , such as helium wherein the mixture will not be capable of mixing with air in any proportions to produce a self - sustaining flammable or combustible mixture , or wherein a concentration of hydrogen is sufficient to reduce cost of the mixture to an amount substantially less than cost of pure helium .