J. Tollefsen et Ea. Hammer, CAPACITANCE SENSOR DESIGN FOR REDUCING ERRORS IN PHASE CONCENTRATION MEASUREMENTS, FLOW MEASUREMENT AND INSTRUMENTATION, 9(1), 1998, pp. 25-32
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.