Capacitance sensor design for reducing errors in phase concentration measurements

Abstract

A capacitance sensor configuration which can measure the water fraction and undissolved gas in crude oils with higher accuracy than currently available non-intrusive capacitance sensors has been studied. In this sensor the surface electrodes are helical so that the electrical field is twisted 180° or 360° 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 model is based on the “Finite Element Method” (FEM). This model enabled us to simulate the capacitance of the helical sensors as a function of changes in flow parameters, including water fraction, void fraction, permittivity and distribution of the flow components and changes in sensor geometry and design such as length of the sensor, type of guard electrode, thickness and permittivity of the electrode insulation layer (liner), diameter of sensor screen, etc. The model was verified against 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 helical surface plate capacitance sensor with respect to sensitivity, accuracy and flow regime independency. Our simulations show that by using helical surface electrodes the overall accuracy is estimated to be within ±0.4% for gas/oil mixtures and ±5% for oil–continuous water/oil mixtures of full scale.