in a calibration apparatus and method for a resistance thermometer a reference thermometer is located in thermal contact with a gas composition to generate , during a calibration period , a first reference temperature value for the gas composition at a first temperature , and an acoustic meter is located in acoustic contact with the gas composition and , during the calibration period , makes an acoustic velocity - related measurement for use in generating a second reference temperature value for the gas composition at a second temperature . a calibration unit coordinates the first and second reference temperature values with first and second measurement temperature values provided by the resistance thermometer at the first and the second temperatures , respectively , and establishes a calibration relationship therefrom .