CAPACITANCE SENSOR DESIGN FOR REDUCING ERRORS IN PHASE CONCENTRATION MEASUREMENTS

A capacitance sensor configuration which can measure the water fractio n and undissolved gas in crude oils with higher accuracy than currentl y available non-intrusive capacitance sensors has been studied. In thi s sensor the surface electrodes are helical so that the electrical fie ld is twisted 180 degrees or 360 degrees from one end of the sensor to the other. This sensor has been investigated in detail by means of a three-dimensional capacitance model based on Poisson's equation. The m odel is based on the ''Finite Element Method'' (FEM). This model enabl ed us to simulate the capacitance of the helical sensors as a function of changes in flow parameters, including water fraction, void fractio n, permittivity and distribution of the flow components and changes in sensor geometry and design such as length of the sensor, type of guar d electrode, thickness and permittivity of the electrode insulation la yer (liner), diameter of sensor screen, etc. The model was verified ag ainst measurements on different types of sensors and flow regimes, and the maximum difference between simulated and measured results was +/- 5%. The mathematical model was used to optimize the design of the hel ical surface plate capacitance sensor with respect to sensitivity, acc uracy and flow regime independency. Our simulations show that by using helical surface electrodes the overall accuracy is estimated to be wi thin +/- 0.4% for gas/oil mixtures and +/- 5% for oil-continuous water /oil mixtures of full scale. (C) 1998 Elsevier Science Ltd. All rights reserved.