Capacitive Sensors for Flow Application

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Capacitive Sensors for Flow Application
Capacitive sensors due to their relatively low cost of production and a wide range of
applications have been used for several years. One of the applications of capacitive sensors is the
detection and measurement of fluid flow, based on capacitive coupling. They work on the
principle that they can measure and distinguish the conductivity of the fluid that has a different
dielectric constant from the electrodes used. Furthermore, it employs the theory of pressure drop
due to the reduction in diameter to measure the flow of fluids in pipes associated with the
geometry of electrodes and the direction of fluid flow (Mohamad, Marwah and Rahim, 1). The
fluid flowing in the pipe, in this case, acts as the dielectric whereby the two opposite electrodes
produces different capacitance. Permittivity in flow measurement will then become ๐œ€
and ๐œ€
determined as the fluid dielectric property across the sensing volume in the measuring boundary
of the sensor (Mohamad et al., 1). The measurement of the capacitance is then obtained as:
๐ถ =
In this case:
C= Capacitance in Farads
= The free space permittivity
= Dielectric permittivity
A= plate area
= plate distance
Capacitive sensing in fluid flow is a non-destructive type of measurement. This is
because the flow of the fluid is measured without causing any kind of destruction to the flow
canal (Al Shaqsi, 1). The capacitive sensor has one pair of electrodes that are semi-cylindrical in
shape whereby the materials are made from adhesive foils of soft copper metal. The electrodes
are mounted on opposite sides of the pipe thereby allowing non-destructive measurement of the
fluid flow inside the tube. The two electrodes are connected to an external circuit that generates
an electromagnetic field in the pipe cross section, and therefore, the flow is measured. When no
flow is detected on the tube, the capacitance generated will be less than the one in the conductive
fluid. On the contrary, when the fluid flows, through the electromagnetic field the semi-
cylindrical electrode will measure a capacitance value (Al Shaqsi, 17). One of the electrodes is
the negative terminal while the other is positive, but are made such that they are equal in size and
area. The surface of the pipe is insulated with a non-conductive layer of dielectric material like
wood or paper so that charge loss and outside, charge-interaction is prevented. This ensures the
eddy flow is not disturbed.
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Fig1. The capacitor sensing for fluid measurement illustration (Al Shaqsi, 18).
Since flow meters will cause the fluid to change its displacement so as to determine the
flow, the capacitive sensing does not require a change in the movement of the fluid to take in
measurements. The capacitance of the capacitive sensor will vary with the fluid flow dielectric
property between the two semi-cylindrical electrodes (Mohamad et al., 1). The flow rate is
determined through the measurement of the difference in capacitance between the conductive
fluid and the semi-cylindrical electrode, therefore; it is a good sensor, unlike other flow meters.
A typical manufacturer of capacitive sensors for flow measurements is MTI instruments.
The model number of the capacitive sensor is the AS-9000/1 system. The instrument has an
accuracy of 0.02% FSR Root Mean Squared with a minimum range of 25ฮผF for every 0.001
inches. It measures the fluid flow with a precision of 25mm, pipe diameter thickness. The
relevant specifications include:
1. Offset/gain control function
2. Integral summing amplifier for the pipe thickness output
3. Bar graph display
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Works Cited
Al Shaqsi, Harith Hilal. "Non-Destructive Flow Measurement." Engineering project. 2010.
Digital print.
Mohamad, Evans Johana, Faizan Olga Marwah and Abdul Ruzari Rahim. "Electronic Design for
Portable Electrical Capacitance Sensor: A Multiphase Flow Measurement." Institute of
Electrical and Electronics Engineers (2011): 1-8. Digital print.

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