Abstract: An apparatus and method for measuring high temperatures in a boiler transmits pulses of acoustic waves, from one side wall of the boiler to an opposite side wall thereof. Acoustic noise within the boiler, as well as the transmitted pulses of acoustic waves are received at the opposite side of the boiler. The received signal is digitized and compared to a digitized sample of the pulses during a time period which is more than the maximum transit time for the pulses between the side walls. A point of maximum correlation between the sample and the signal is taken as the arrival time for the pulse and is used to calculate the transit time of the pulse across the boiler. This transit time is used in turn to calculate the velocity of the pulses. The temperature can then be calculated as a function of the velocity, the molecular weight of the medium and the specific heat ratio of the medium. Each pulse has a modulated frequency between 500 and 3,000 Hz.