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Electrical Pressure Elements - Variable Capacitance Transducer |
Consists of
two metal plates; one of which is linked to a movable mechanical
pressure element ie. diaphragm and the other is a static plate
Both plates are separated from each other by dielectric material
When distance between plates changes, capacitance changes resulting in
changes in opposition to current flow in the AC circuit
Thus, by measuring the current flow, changes in pressure can be detected
Ideal for measuring low input levels
Capacitance
Capacitance pressure transducers were originally developed for use in
low vacuum research. This capacitance change results from the movement
of a diaphragm element (Figure). The diaphragm is usually metal or
metal-coated quartz and is exposed to the process pressure on one side
and to the reference pressure on the other. Depending on the type of
pressure, the capacitive transducer can be either an absolute, gauge, or
differential pressure transducer.
Stainless steel is the most common diaphragm material used, but for
corrosive service, high-nickel steel alloys, such as Inconel or
Hastelloy, give better performance. Tantalum also is used for highly
corrosive, high temperature applications. As a special case, silver
diaphragms can be used to measure the pressure of chlorine, fluorine,
and other halogens in their elemental state.
In a capacitance-type pressure sensor, a high-frequency, high-voltage
oscillator is used to charge the sensing electrode elements. In a
two-plate capacitor sensor design, the movement of the diaphragm between
the plates is detected as an indication of the changes in process
pressure.
 Figure Capacitance-Based Pressure Cell
As shown in Figure, the deflection of the diaphragm causes a change in
capacitance that is detected by a bridge circuit. This circuit can be
operated in either a balanced or unbalanced mode. In balanced mode, the
output voltage is fed to a null detector and the capacitor arms are
varied to maintain the bridge at null. Therefore, in the balanced mode,
the null setting itself is a measure of process pressure. When operated
in unbalanced mode, the process pressure measurement is related to the
ratio between the output voltage and the excitation voltage.
Single-plate capacitor designs are also common. In this design, the
plate is located on the back side of the diaphragm and the variable
capacitance is a function of deflection of the diaphragm. Therefore, the
detected capacitance is an indication of the process pressure. The
capacitance is converted into either a direct current or a voltage
signal that can be read directly by panel meters or microprocessor-based
input/output boards.
Capacitance pressure transducers are widespread in part because of their
wide rangeability, from high vacuums in the micron range to 10,000 psig
(70 MPa). Differential pressures as low as 0.01 inches of water can
readily be measured. And, compared with strain gage transducers, they do
not drift much. Better designs are available that are accurate to within
0.1% of reading or 0.01% of full scale. A typical temperature effect is
0.25% of full scale per 1000¡ F.
Capacitance-type sensors are often used as secondary standards,
especially in low-differential and low-absolute pressure applications.
They also are quite responsive, because the distance the diaphragm must
physically travel is only a few microns. Newer capacitance pressure
transducers are more resistant to corrosion and are less sensitive to
stray capacitance and vibration effects that used to cause "reading
jitters" in older designs.
Advantages
- Low hysteresis, Good linearity, stability and repeatability, Fast
response
Disadvantages - High impedance output, Complex electronics
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