![]() The complete measuring device includes a sensing element ( bourdon tube, bellows or diaphragm ), a strain gauge attached to the element, a stable power source and a read out device.Ī strain gauge element and a typical transducer is shown in the figure below This is measured by a Wheatstone bridge circuit, preferably of the null balance type, so that the strain gauge carries no current. The change in resistance is a measure of the pressure producing the mechanical distortion. ![]() ![]() The strain gauge changes a mechanical motion into an electrical signal when a wire length is changed by tension or compression, altering the wire diameter and, hence, changing the electrical resistance. Strain is defined as a deformation or change in the shape of a material as a consequence of applied forces.Ī strain gauge is a device which uses the change of electrical resistance of a wire under strain to measure pressure. When the two input pressure are equal, the diaphragm is positioned centrally and the capacitances are equal.Ī difference in the two input pressures causes displacement of the sensing diaphragm and is sensed as a difference between the two capacitances. The sensing diaphragm and capacitor thus form a differential variable separation capacitor. The pressure acting on the isolating diaphragms set up similar pressures in the silicone oil filling the space between them.Ī net force proportional to the difference between the two pressures acts upon the metal sensing diaphragm and deflects it to one side or the other, depending on which input pressure is the greater.Įach plate forms a capacitor with the sensing diaphragm, which is connected electrically to the metallic body transducer. This principle can be applied to differential pressure measurement, as shown in Figure. The capacitance of this type just like the fiat-plate type is proportional to the area. A secondary isolating diaphragm is used to protect the sensing diaphragm.Īnother type of capacitor uses concentric hollow metal cylinders. This is shown in Figure, where the movement of a flexible diaphragm relative to a fixed plate is sensed by the capacitance change. This imbalance is detected in a capacitance bridge circuit and converted to a D.C. The capacitance pressure sensor operates on the principle that, if the sensing diaphragm between two capacitor plates is deformed by a differential pressure, an imbalance of capacitance will occur between itself and the two plates. The reduced movement very nearly eliminates drift, friction and hysteresis common to bellows, diaphragm and bourdon elements that require relatively large movements. This is due in part to the accuracy of their electronic circuitry and in part to the extremely small movement required to the elastic elements in order to obtain the needed electrical change. Generally, electrical pressure detectors are more accurate and have much faster response times. ![]() Most electronic pressure sensors incorporate different elements as the primary pressure detector, and it is used to vary a measurable electrical quantity to produce a proportionately variable electronic signal.īecause the energy form is transferred from a mechanical to an electrical nature, these devices are often classified as transducer. Electronic Pressure Transmitters / Sensors Principle ![]()
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