13-06-2012, 01:58 PM
Project on Electric Pressure Transducers
Electric Pressure Transducers.pdf (Size: 926.67 KB / Downloads: 104)
Abstract
In any process industry there are various process variables, which we have to measure for safe run of plant. One of them an important process variable is pressure. Pressure is defined as force per unit area. In any chemical industry pressure is the main process variable which must be monitored. Two main types for pressure transducers are Electric pressure transducers and Elastic pressure transducers. This report is mainly concerned with electric pressure transducers.
INTRODUCTION
Pressure:
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. Pressure is an effect which occurs when a force is applied on a surface. Pressure is the amount of force acting on a unit area. The symbol of pressure is P. [5].
Electric Pressure Transducers:
In general “a transducer is a device which converts one form of energy into another form of energy.” However, in the field of electrical instrumentation, a transducer is a tool that converts a physical quantity, a physical condition or mechanical output to an electrical signal. Most of method converting mechanical output into an electrical signal works equally well for the bellows, the diaphragm, the bourdon tube. In this conversion a mechanical motion is first converted into a change in electrical resistance and then change in resistance is transformed into change in electrical current or voltage.
Working Principle:
The strain gauge is a fine wire which changes its resistance when mechanically strained, due to physical effects. A strain gauge may be attached to the diaphragm so that when the diaphragm flexes due to the process pressure applied on it, the strain gauge stretches or compresses. This deformation of the strain gauge causes the dissimilarity in its length and cross sectional area due to which its resistance also changes, as shown in fig (a).The resistance change of a strain gauge is usually converted into voltage by linking one, two, or four similar gauges as of Wheatstone bridge (known as strain gauge bridge), and applying excitation to the bridge. The bridge output voltage is then a measure of the pressure sensed by strain gauges.
Potentiometric Pressure Transducers:
In this type of pressure transducer, there is a potentiometer (basically a variable resistance) which is made by winding resistance wire around an insulated cylinder. A moveable electrical contact, called a wiper, slides along the cylinder, touching the wire at one point on each turn. The position of the wiper determines how much wire, and therefore, how much resistance, is between the end of the wire and the wiper. A mechanical linkage from the pressure sensing element (such as bellows, a diaphragm, and the bourdon tube) controls the position of the wiper on the potentiometer. Some potentiometer is made curved so that the wiper can pivot in a circular motion rather than moving along a straight line. The location of wiper determines the confrontation of the potentiometer which in turn determines the pressure.