Ultrasonic measurement of gaseous air in pulp suspensions

This paper is based on a discovery made in the laboratory during experiments involving ultrasonic interaction in a pulp suspension. Large air bubbles were observed to have only a modest effect on the attenuation of ultrasonic transmissions, while entrained air produced a far more dramatic response. In the case of entrained air, the water-air interface scatters sound strongly. This occurs because of the considerable difference between the acoustic impedances of water (1. 5 multiplied by 10**6 kg/m**2s) and air (4. 3 multiplied by 10**2 kg/m**2s). The entrained air attaches to the fiber flocs in the suspension and scatters sound effectively. The method has been tested in the laboratory and in several paper mills. Tests prove that a commercial meter can be developed to control the feed of chemical defoamer in paper machines.