2. How would you define pressure? What do you know about pressure measuring devices?
Read and translate the text A.
Pressure is defined as the normal force per unit area exerted by a fluid (liquid or gas) on any surface. The surface can be either a solid boundary in contact with the fluid or, for purposes of analysis, an imaginary plane drawn through the fluid. Only the component of the force normal to the surface needs to be considered for the determination of pressure. In the limit that the surface area approaches zero, the ratio of the differential normal force to the differential area represents the pressure at a point on the surface. Furthermore, if there is no shear in the fluid, the pressure at any point can be shown to be independent of the orientation of the imaginary surface under consideration. Finally, it should be noted that pressure is not defined as a vector quantity and is therefore nondirectional. Three types of pressure measurements are commonly performed:
Absolute pressure is the same as the pressure defined above. It represents the pressure difference between the point of measurement and a perfect vacuum where pressure is zero.
Gage pressure is the pressure difference between the point of measurement and the ambient. In reality, the ambient (atmospheric) pressure can vary, but only the pressure difference is of interest in gage pressure measurements.
Differential pressure is the pressure difference between two points, one of which is chosen to be the reference. In reality, both pressures can vary, but only the pressure difference is of interest here.
Since pressure is defined as the force per unit area, the most direct way of measuring pressure is to isolate an area on an elastic mechanical element for the force to act on. The deformation of the sensing element produces displacements and strains that can be precisely sensed to give a calibrated measurement of the pressure. This forms the basis for essentially all commercially available pressure sensors today. Specifically, the basic requirements for a pressure-sensing element are a means to isolate two fluidic pressures (one to be measured and the other one as the reference) and an elastic portion to convert the pressure difference into a deformation of the sensing element. Many types of pressure-sensing elements are currently in use. A special form of tube, known as the Bourdon tube , is curved or twisted along its length and has an oval cross-section. The tube is sealed at one end and tends to unwind or straighten when it is subjected to a pressure applied to the inside. In general, Bourdon tubes are designed for measuring high pressures, while capsules and bellows are usually for measuring low pressures.
A detection means is required to convert the deformation of the sensing element into a pressure readout. In the simplest approach, the displacements of a sensing element can be amplified mechanically by lever and flexure linkages to drive a pointer over a graduated scale, for example, in the moving pointer barometers. Some of the earliest pressure sensors employed a Bourdon tube to drive the wiper arm over a potentiometric resistance element. In linear-variable differential-transformer (LVDT) pressure sensors, the displacement of a Bourdon tube or capsule is used to move a magnetic core inside a coil assembly to vary its inductance. In piezoelectric pressure sensors, the strains associated with the deformation of a sensing element are converted into an electrical charge output by a piezoelectric crystal. Piezoelectric pressure sensors are useful for measuring high-pressure transient events, for example, explosive pressures.