Based on the principles of thermodynamics and hydromechanics, an equation was deduced to predict the vertical position of the interface formed by an inertial gas gap, by which the static pressure transducer is protected from contacting with viscous and corrosive slurries inside a special tube for on-line measurement of the density and level of liquid surface for the flowing system. The interface fluctuation due to the change of temperature and pressure inside the measuring tube was calculated for water and air system within the range of 160 ℃ and 0.7 MPa. An analytic criterion for gas escape which eventually leads to invalidation of measurement was obtained. Furthermore a mass transfer model was proposed to analyze the negative effect of mass transfer of gas molecules through the interface to the flowing liquid, which may cause invalidation of measurement if the majority of gas loses from the measuring tube. It was demonstrated by the calculation that this process of mass transfer is very slow for the slightly dissolved gas liquid system. Taking air water as an trial system, the experiments were made to verify the model. Under a constant pressure of 0.2 MPa and the temperature of 50 ℃ and 80 ℃ respectively, the change of interface position inside the measuring tube due to mass transfer was less than 5% during a period of 90 days, well consistent with the model prediction. It was indicated that the method was fitted well with the engineering requirement for reliable application.
