This paper describes the ultrasonic attenuations in gas mixtures of CO2 and air to investigate the possibility of a CO2 gas concentration monitor using with the ultrasonic attenuations. Three kinds of ultrasonic transducers (40, 215 and 500 kHz) were used for the measurements. Measurements of the attenuation per unit distance of each ultrasonic wave were carried out by varying the length of a test cell of 26 mm in diameter from 30mm to 150mm. From these fundamental results, the practical gas detection cells were designed and the detection method in order to monitor the CO2 gas concentration was developed. Two 215 kHz ultrasonic transducers were installed on the both sides of the gas detection cell of 4 mm in diameter and 70 mm or 40mm long. The attenuation of 215 kHz ultrasonic is the largest of the three ultrasonics. The ultrasonic waves are sent intermittently from the transducer and are propagated through the cell. The sound pressure of the ultrasonic waves are detected by another transducer on the other side. The peak detected value of the sound pressure in the first receiving waves is decayed sharply by introducing CO2 and air mixtures into the cell compared with air. The relationship between the peak detected value;P1 and CO2 concentration; C (%) is presented.
The summary of the measurements is as follows:
a) The peak detected value of the sound pressure;P1 extremely decreases with increasing CO2 concentration in a range of C=0-20%(Fig. 11).
b) The ratio between the peak detected value of gas mixtures and air P1 may be treated to be independent of temperature (Fig. 13).
c) The peak detected value is comparatively influenced by adding water vapor into the gas mixtures (Fig. 14).
