Humidity can be calculated by measuring the speed of sound and the temperature in air. A new type of hygrometer has been constructed for industrial applications and therefore special attention has been paid to easy maintenance. The measurement of the speed of sound is effected in a steel pipe through which the air can flow freely. A 2-22 kHz wide-band frequency sweep is used as the sound signal, the transit time of which is measured with a fast fully parallel polarity correlator. The temperature measurement method is aimed at giving the real mean temperature of the air in the steel pipe. The use of a multiplexer enables the measurement of humidity at several places simultaneously. The results obtained in a test chamber have shown that good accuracy can be achieved with the hygrometer.
Hiismäki, P., & Kämäräinen, V. (1987). A hygrometer based on measuring the speed of sound and the temperature. Journal of Physics E: Scientific Instruments, 20(12), 1465-1468. https://doi.org/10.1088/0022-3735/20/12/006