If a pressure, P, acting on a diaphragm compresses a spring until an equilibrium is produced, the pressure can be represented as:
In this equation, F represents the force of the spring and A represents a surface area of the diaphragm. The movement of the spring is transferred via a system of levers to a pointer whose deflection is a direct indication of the pressure (Fig. 6.10). If the measured value of the pressure must be transmitted across a long distance, the mechanical movement of the pointer can be connected to a variable electrical resistance (potentiometer). A change in the resistance results in a change in the measured voltage, which can then easily be evaluated by an electronic circuit or further processed. This example illustrates the fact that a physical quantity is often subject to many transformations before it is finally evaluated.
Piezoelectric crystals may be utilized to measure pressure. Electrical charges are produced on the opposite surfaces of some crystals when they are mechanically loaded by deflection, pressure, or tension. The electrical charge produced in the process is proportional to the effective force. This change in the charge is very small. Therefore, electrical amplifiers are used to make it possible to process the signals (Fig. 6.11).
Pressure in this situation is measured by transforming it into a force. If the force produced by pressure on a diaphragm acts on a piezoelectric crystal, a signal that is proportional to the pressure measured can be produced by using suitable amplifiers.
Strain gauges can also measure pressure. The electrical resistance of a wire-type conductor is dependent, to a certain extent, on its cross-sectional area. The smaller the cross section (i.e., the thinner the wire), the greater the resistance of the wire. A strain gauge is a wire that conducts electricity and stretches as a result of the mechanical influence (tension, pressure, or torsion) and thus changes its resistance in a manner that is detectable. The wire is attached to a carrier, which in turn is attached to the object to be measured. Conversely, for linear compression, which enlarges the cross-sectional area of a strain gauge, resistance is reduced. If a strain gauge is attached to a diaphragm (Fig. 6.12), it will follow the movement of the diaphragm. It is either pulled or compressed, depending on the flexure of the diaphragm.
FIBER-OPTIC PRESSURE SENSORS
A Y-guide probe can be used as a pressure sensor in process control if a reflective diaphragm, moving in response to pressure, is attached to the end of the fiber (Fig. 6.13). This type of pressure sensor has a significant advantage over piezoelectric transducers since it works as a noncontact sensor and has a high frequency response. The pressure signal is transferred from the sealed diaphragm to the sensing diaphragm, which is attached to the end of the fiber. With a stainless-steel diaphragm about 100 ?m thick, hysteresis of less than 0.5 percent and linearity within ±0.5 percent are obtained up to the pressure level of 3 ? 105 kg/m2 (2.94 MPa) in the temperature range of ?10 to +60°C.
The material selection and structural design of the diaphragm are important to minimize drift. Optical-fiber pressure sensors are expected to be used under severe environments in process control. For example, process slurries are frequently highly corrosive, and the temperature may be as high as 500°C in coal plants. The conventional metal diaphragm exhibits creep at these high temperatures. In order to eliminate such problems, an all- fused-silica pressure sensor based on the micro-bending effect in optical fiber has been developed (Fig. 6.14). This sensor converts the pressure applied to the fused silica diaphragm into an optical intensity modulation in the fiber.
A pressure sensor based on the wavelength filtering method has been developed. The sensor employs a zone plate consisting of a reflective surface, with a series of concentric grooves at a predetermined spacing. This zone plate works as a spherical concave mirror whose effective radius of curvature is inversely proportional to the wavelength. At the focal point of the concave mirror, a second fiber is placed that transmits the returned light to two photodiodes with different wavelength sensitivities. When broadband light is emitted from the first fiber to the zone plate, and the zone plate moves back and forth relative to the optical fibers in response to the applied pressure, the wavelength of the light received by the second fiber is varied, causing a change in the ratio of outputs from the two photodiodes. The ratio is then converted into an electrical signal that is relatively unaffected by any variations in parasitic losses.
About the Author
Sabrie Soloman, Ph.D, is the Founder, Chairman and CEO of American SensoRx, Inc.
Excerpted from Sensors Handbook, 2nd Edition by Sabrie Soloman (McGraw-Hill; 2010) with permission by McGraw-Hill.
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