Time dependence of dielectric properties of tissue-equivalent dielectric liquid materials

Standard measurement methods of the specific absorption rate including recipes of tissue-equivalent dielectric liquids have been discussed by international organisations in order to evaluate mobile telecommunication equipment. Specific absorption rate is generally determined by measuring the electric field distribution in a phantom made of a shell filled with tissue-equivalent dielectric liquid. Based on the dielectric properties of various tissues, recipes of the liquids are proposed in international standards. We measured the time dependence of permittivities and conductivities of tissue-equivalent dielectric liquids made of deionised water and polyhydric alcohol, and discussed its influence on specific absorption rate values. The results suggest that permittivities decreased with time due to evaporation of water. The specific absorption rate, however, was not affected significantly by the change of permittivity.