Gases have negative effects on the production efficiency and the product quality. Both entrained and dissolved gases can be found from virtually all pulp flows, but fast and accurate inline measurement technique for gas content measurement has yet to been established. In these experiments new sonar technique for gas content measurement was tested.

Experiments where done in pilot-scale pumping system where pulp suspension was circulated with centrifugal pump in closed loop. Measurements where located in the vertical outlet pipe of the pump. Air content of the system could be adjusted with two different principles. Macro bubbles (∅>100μm) were fed to the system through membrane tube controlling volumetric air flow with rotameters. Micro bubbles (∅=20-100μm) were created precipitating bubbles from dissolved form controlling the level of dissolved oxygen and volumetric flow of the supersaturated suspension. Measurement accuracy of the sonar method was tested with four different air content levels and two different bubble types. Furthermore the effect pulp grade, consistency, flow velocity and process pressure were tested. Microwave and radiometric methods were used as reference measurements.

Results show that with sonar method accurate gas content result can be obtained independently on the pulp grade, process pressure, flow velocity or bubble size.