Experimental Technique - Pressure Measurement

SME1912 EXPERIMENTAL TECHNIQUES Chapter 3 – Part 2 Measurement of Pressure SOME DEFINITIONS Force applied uniformly over a surface, measured as force per unit of area Pressure plays a number of important roles in daily life, among them its function in the operation of pumps and hydraulic presses. The maintenance of ordinary air pressure is essential to human health and well-being: SOME DEFINITIONS When a force is applied perpendicular to a surface area, it exerts pressure on that surface equal to the ratio of F to A, where F is the force and A the surface area. Hence, the formula for pressure (p) is p = F/A. UNITS FOR PRESSURE Pressure is measured by a number of units in the English and metric The principle SI unit is called a pascal (Pa), or 1 N/m2. A newton (N), the SI unit of force, is equal to the force required to accelerate 1 kilogram of mass at a rate of 1 meter per second squared. Thus, a Pascal is equal to the pressure of 1 newton over a surface area of 1 square meter. UNITS FOR PRESSURE In the British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs./in2. This is equal to 6.89 · 103 Pa, or 6,890 Pa. British unit of pressure is a familiar part of Malaysian driver's daily life, because tire pressure is usually reckoned in terms of pounds per square inch. (The recommended tire pressure for a mid-sized car is typically 30-35 lb/in2.) UNITS FOR PRESSURE There are also two other specialized units of pressure measurement in the SI system: the bar, equal to 105 Pa, and the torr, equal to 133 Pa. The torr, once known as the "millimeter of mercury," is equal to the pressure required to raise a column of mercury (chemical symbol Hg) 1 mm. It is named for the Italian physicist Evangelista Torricelli (16081647), who invented the barometer. PRESSURE UNITS MORE DEFINITIONS Atmospheric Pressure Atmospheric pressure is pressure caused by the weight of the atmosphere. At sea level it has a mean value of one atmosphere (1atm) = 760 mm of mercury = 14.70 lbs per square in. = 101.35 kilopascals, Pressure reduces with increasing altitude PRESSURE CHANGE This plastic bottle was closed at approximately 2000m altitude, then brought back to sea level. As a result, air pressure crushes it. MORE DEFINITION Vacuum A space in which there is no matter or in which the pressure is so low that any particles in the space do not affect any processes being carried on there. It is a condition well below normal atmospheric pressure The most nearly perfect vacuum exists in intergalactic space, where it is estimated that on the average there is less than one molecule per cubic meter MORE DEFINITION – Absolute, Gauge and Differential Pressure Absolute pressure of a fluid is referenced against a perfect vacuum Gauge pressure (eg. as read by barometer) is referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Differential pressure is the difference in pressure between two points. PRESSURE DEFINITION DIAGRAM METHOD TO MEASURE PRESSURE • By elastic deformation – Pressure create force which in turn deform the elastic material • By liquid column – Hydrostatic pressure is the pressure due to the weight of a fluid. – p = ρgh ELASTIC DEFORMATION – Bourdon Tube ELASTIC DEFORMATION – Bourdon Tube A Bourdon gauge uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm connected to the tube. Coiled tube made of copper Mechanical movement linked by rack and pinion Range = 100kN/m2 – 300MN/m2 ELASTIC DEFORMATION – Diaphragm Based LVDT = linear variable displacement transducer Spring can be used instead of LVDT and attached to force (F) measuring device p = kx/A INDUSTRIAL PRESSURE DIAPHRAGM ASSEMBLY DIAPHRAGM SHAPES & MATERIALS Shape Üflat for low pressure measurement or ÜCorrugated for high pressure measurement Material = Copper-beryllium ELASTIC DEFORMATION – Bellows Based For extremely low pressure measurement (0.5 – 75psig to a 1000psigmax) psig = pound per square inch gauge ANEROID GAUGE FITTED WITH BELLOW ANEROID CONCEPT an enclosed and sealed bellows chamber, called an aneroid, which means "without liquid". Early barometers used a column of liquid such as water or the liquid metal mercury suspended by a vacuum. It uses the sealed chamber as a reference pressure and expand when driven by the external pressure PRESSURE MEASUREMENT – Liquid Column Concept Liquid column gauges consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube HYDROSTATIC PRESSURE ATMOSPHERIC PRESSURE COMPONENT U-TUBE MANOMETER U-TUBE MANOMETER Equating the pressure at the level X'(pressure at the same level in a continuous body of fluid is equal) LHS; Px = P1 + ρg(a+h) RHS; Px' = P2 + ρga + ρmgh Since Px = Px' P1 + ρg(a+h) = P2 + ρga + ρmgh P1 - P2 = ρmgh - ρgh i.e. P1 - P2 = (ρm - ρ)gh. WELL-TYPE MANOMETER WELL-TYPE MANOMETER p2 p1 hc hw WELL-TYPE MANOMETER LHS: p1 – ρghw p2 RHS: p2 + ρghc p1 hc hw LHS = RHS p1 – ρghw = p2 + ρghc p1 – p2 = ρghc + ρghw By constant volume, hwAw = hcAc ⇒ hw/hc = Ac/Aw Therefore p1 – p2 = ρghc (1+Ac/Aw) = ρghc when Aw>>Ac WELL-TYPE MANOMETER Measuring range = 1bar – 1.5bar INCLINED TUBE MANOMETER INCLINED MANOMETER INCLINED MANOMETER INCLINED MANOMETER The inclined version is used for better sensitivity. Measuring range = 0.1bar – 30mbar PIEZOMETER MERCURY BAROMETER A barometer is an instrument used to measure atmospheric pressure. A standard mercury barometer has a glass column of about 30 inches (about 76 cm) in height, closed at one end, with an open mercury-filled reservoir at the base. MERCURY BAROMETER Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. MERCURY BAROMETER MANOMETER LIQUID SELECTION • • • • • Low viscosity Low coefficient of expansion Low cohesiveness Low vapour pressure Non corrosive MANOMETER LIQUID IN USE • • • • • • Mercury Water Kerosene Alcohol Ethyl Benzene