Pressure Levels and Terminology Atmospheric Pressure— The atmosphere that surrounds the earth can be considered a reservoir of low-pressure air. Its weight exerts a pressure that varies with temperature, humidity, and altitude. For thousands of years, air was considered weightless. This is understandable, since the net atmospheric pressure exerted on us is zero. The air in our lungs and the blood in our cardiovascular system has an outward pressure equal to (or perhaps slightly greater than) the inward pressure of the outside air. Since we feel no pressure, we are unaware of the air's weight. The weight of the earth's atmosphere pressing on each unit of surface constitutes atmospheric pressure, which is 14.7 psi (101,300 Pa or 0.1013 MPa) at sea level. This pressure is called one atmosphere. In other commonly used units, one atmosphere equals 29.92 inches of mercury (in. Hg), 760 mm Hg (or 760 torr), and 1.013 bar(1 bar=0.1 MPa). Since atmospheric pressure results from the weight of the overlying air, it is less at higher altitudes. As Fig. 3 shows, atmospheric pressure in Denver, Colorado (altitude 5,280 feet), is only 12.2psi. And in Mexico City, Mexico (altitude 7,800 feet), it is 11.1 psi. On top of Mount Everest, the pressure has fallen to one-third of an atmosphere. Atmospheric pressure also varies from time to time at a single location, due to the movement of weather patterns. While these changes in barometric pressure are usually less than one-half inch of mercury, they need to be taken into account when precise measurements are required. Gauge Pressure—Atmospheric pressure serves as a reference level for other types of pressure measurements. One of these is "gauge pressure." Gauge pressure is either positive or negative, depending on its level above or below the atmospheric pressure reference. For example, an ordinary tire gauge showing 30 pounds (actually, 30 psi) is showing the excess pressure above atmospheric. In other words, what the gauge shows is the difference between atmospheric pressure and the pressure of the air pumped into the tire. Gauge pressures can be either positive (above atmospheric) or negative (below atmospheric). Atmospheric pressure represents zero gauge pressure. Absolute Pressure—A different reference level is used to obtain a value for "absolute pressure." This is pressure measured above a perfect vacuum. It is composed of the sum of the gauge pressure (positive or negative) and the atmospheric pressure. Where there might be confusion, gauge and absolute pressures are distinguished by adding the letter "g" or "a," respectively, to the abbreviation for the units ("psig" or "psia").To obtain the'absolute pressure, simply add the value of atmospheric pressure (which averages 14.7 psi at sea level) to the gauge pressure reading. To find the current value of atmospheric pressure in psia at a given location, multiply the barometer reading in in. Hg by 0.491. This conversion factor arises from the fact that a cube of mercury with one inch sides weighs 0.491 pound and thus exerts a pressure of 0.491 psi. Using the simple tire pressure example, the absolute pressure including the atmospheric pressure—exerted by the air within the tire is 44.7 psia (30 psig plus 14.7 psi). Thus, the absolute pressure is 14.7 psi more than would be read on a tire-pressure gauge. Absolute pressure must be used in virtually all calculations involving pressure ratios. Vacuum—Vacuum is a pressure lower than atmospheric. Except in outer space, vacuums occur only in closed systems. In the simplest terms, any reduction in atmospheric pressure in a closed system may be called a partial vacuum. In effect, vacuum is the pressure differential produced by evacuating air from the system. In a vacuum system more sophisticated than a suction cup, the enclosed space would be a valve actuator or some appropriate work device. A vacuum pump would be used to reduce atmospheric pressure in the closed space. The same principle would apply, however. By removing air from one side of an air-tight barrier of some sort, atmospheric pressure can act against the other side. Just as with the suction cup, this action creates a pressure differential between the closed system and the open atmosphere. The pressure differential can be used to do work. For example, in liquid packaging (bottling), reducing the pressure in a bottle (the enclosed space) makes the filling operation go much faster because the liquid or other material is literally pulled into the bottle, rather than simply falling by gravity. Vacuum is usually divided into four levels: Low vacuum represents pressures above one torr absolute. Flow in this range is viscous, as represented by most common fluids. Mechanical vacuum pumps are used for low vacuum, and represent the large majority of pumps in industrial practice. Medium vacuum represents pressures between 1 and 10-3 torr absolute. This is a transition range between viscous and molecular flow. Most pumps serving this range are also mechanical. High vacuum represents pressures between 10"3 and 10"6 ton absolute. Flow in this region is molecular or Newtonian, with very little interaction between individual molecules. A number of specialized industrial applications, such as ion implantation in the semiconductor industry, fall in this range. Nonmechanical ejector or cryogenic pumps (which are not discussed in this book) are usually used. Very high vacuum represents absolute pressures below 10-6 torr. This isprimarily for laboratory applications and space simulation, Keep in mind that a "perfect" vacuum—that is, a space with no molecules or atoms—is a purely theoretical condition. Only in interstellar space is this condition approached at all closely, and even there a few atoms per cubic meter will be found. In practice, all vacuums are partial. Pressure Pressure The normal stress on any plane through a ﬂuid element at rest is equal to a unique value called the ﬂuid pressure, P, taken positive for compression by common convention. The dimension of pressure is derived from the fundamental dimensions of mass, length and time In the English Engineering Units system, pressure is commonly expressed in units of pounds per square inch (lb f ⁄in2 )or (psi). For instance, automobile manuals state that “tires must be inﬂated to 26 psi front and 24 psi rear.” in the SI system, pressure is expressed in Pascals where The problem with the Pascal is that it is a very small unit, so the pressure is more often expressed in kPa or MPa. For instance, atmospheric pressure is about100,000 Pa (at sea level -standardvalue is 101,350 Pa); it is much better represented as 100 kPa. 1 psi = 6.9684 kPa .  Hydrostatic Pressure in Liquids Hydrostatic pressure in liquids is expressed in terms of the height of a liquid column or “head” (i.e., in of Hg, m of H2O, etc.). Atmospheric (standard) pressure is 760 mmHg. If the variations in ρand g are negligible, the pressure difference between any two arbitrary points in the ﬂuid Figure 1 Hydrostatic Pressure 1 Gage Pressure and Vacuum Pressure If pressure is measured relative to the surrounding atmospheric pressure, then the pressure is referred to as gauge pressure. Common notation is Pgauge (psig). If the pressure is measured relative to an absolute vacuum, then the pressure is referred to as the absolute pressure. Common notation is Pa or (psia). Figure 2 Absolute, atmospheric, gage, and vacuum pressures Pressure Measurements There are many devices designed to measure pressure. Gravitational Devices The basic relation for the measurement of pressure by the use of manometers and barometers is derived from the consideration of a ﬂuid at rest having a density ρ acted upon by the earth’s gravitational acceleration g. U-Tube Manometer The U-Tube Manometer is usually made of glass or other transparent material in the shape of “U”. Both ends of the tube are open for the connection to the equipment, and is usually ﬁlled half way with manometer ﬂuid. Figure 3 U-Tube Manometer For the U-tube manometer, the pressure difference between the tubes is easily obtained from the fundamental relation derived earlier. Inclined Tube Manometer The inclined tube manometer is similar to manometer except that the tube has been tipped to a known angle for greater accuracy. This manometer is very convenient for measuring small pressure differences. From the fundamental relation, the pressure is Figure 4 Inclined Tube Manometer Barometer One of the devices which is used to determine the absolute atmospheric pressure is a mercury in glass barometer. A sketch of such an instrument is shown below. Figure 5 Mercury-in-Glass Barometer If the value of the gravitational acceleration at the given location, and if the mercury density at the given temperature is also known, then Where po is the vapor pressure of the mercury at the surrounding temperature. If the gravitational acceleration and/or mercury density is unknown, standard values for these may be used as follows: where ∆h is a correction to compensate for differences of gravitational acceleration. This correc tion is based on the latitude. gstd = 32.174 ft ⁄sec 2 or 9.80665 m ⁄sec at 45.5° latitude. Houghton is at47°7.5′ latitude. The temperature correction ∆hT corrects hobs to both the standard scale temperature of62°F(16.7°C) and the standard density of mercury at32°F(0°C) is ρHg0°C = 848.43 lb ⁄ f t3 13595.5 kg ⁄m 3. Elastic Transducers The basic principle involved is, that a conﬁned ﬂuid at some pressure different from surroundings will exert forces on the material conﬁning the ﬂuid. If the forces are sufﬁcient to cause detectable stresses and resulting strain is in the elastic region of the material, such strains can be used to indicate the pressure acting on the conﬁned ﬂuid. Test Gauge Bourdon-Gauges Tube An example of this type of transducer is the Bourdon Tube Pressure Gauge, which is widely used to measure pressure differences. The essential features of this gauge is shown on the sketch below. Figure 6 Test Gauge-Bourdon Tube The pressure sensing element is a tube of oval cross-section bent to a circular shape. One end of the tube is ﬁxed to the gauge case and is connected to the ﬂuid whose pressure is to be measured. The other end is closed and is free to move as it is connected via mechanical linkage and gear sector to a pointer. As measured ﬂuid pressure increases above that of surroundings, the tube cross-section tends to become circular and causes the tube to deﬂect at this second end. This motion is transmitted via linkage to the pointer, which would directly indicate on the calibrated scale or dial the gauge pressure. Deadweight Tester Deadweight Testers supply highly accurate pressures for calibrating other, less accurate pressure measuring devices, such as transducers, bottom-hole and bourdon-tube gauges. A tester may be used as a Deadweight Gauge to determine unknown pressures by connecting the gas pressure to the gauge connector through suitable tubing and a valve. Exact regulation of the supply pressure to the tester is obtained by balancing the force exerted by the oil pressure on a piston of known area against weights of known mass. A tester’s weights and piston are calibrated in sets to give an accuracy of 0.1%, which is 1 pound in 1000 pounds. The most accurate instruments available for measurement of pressures above the range where manometers may be used is the Deadweight Tester. This type of tester operates on the principle of balancing a known mass against the force exerted by an unknown pressure on a piston of a known area. When an exact balance is achieved, the unknown pressure P is equal to mass M of the weights divided by the area A of the piston, according to the formula P = F ⁄A .  Figure 7 Deadweight Tester Strain Gage-Diaphragm This type of pressure transducer consists of a unbounded strain gauge element mounted with two of its wires on a ﬁxed frame and the other two wires mounted on a moveable armature. The moveable armature is fastened to an elastic diaphragm, which displaces the armature causing two of the wires to elongate while reducing tension in the other two wires. This causes a bridge imbalance proportional to the applied pressure, which can be related to the applied pressure by calibration. This type of pressure transducer can generally be used in ranges up to 10,000 psig. Figure 8 Strain Gage-Diaphragm Semiconductor-on Bending Beam This type of pressure transducer consists of four strain gauge elements bonded to a beam to measure the bending strain. Two of the strain gauges are mounted on the bottom side of the beam to measure the positive (tensile) strain, and the other two are mounted on the top side to measure the negative (compressive) strain. The diaphragm in this case supplies the force to the beam and isolates it from the process. This type of pressure transducer can generally be used in ranges up to 30,000 psig. Solid State-Piezo Resistive Piezo Resistive pressure sensors operate based on the resistive dependence of silicon under stress. Similar to a strain gauge, a piezoresistive sensor consists of a diaphragm onto which four pairs of silicon resistors are bonded. Unlike the construction of a strain gauge sensor, here the diaphragm itself is made of silicon and the resistors are diffused into the silicon during the manufacturing process. Bonding the diaphgram to an unprocessed wafer of silicon completes the diaphragm. This type of pressure transducer can generally be used in ranges up to 150 psig. Figure 9 Solid State-Piezo Resistive The 5 Pressure Types Gauge, Absolute, Differential, Compound, and Vacuum It is important to understand the "type" of pressure that is required for an application. The terms "vacuum," "absolute," and "compound" are usually the basis for this misunderstanding; often demonstrated by incorrectly combining terms such as "absolute vacuum" or "compound vacuum." Let's identify and define the 5 basic pressure types. Dial gauge configurations will be referenced for each pressure type because they serve as the best means of illustration. 1) Gauge Pressure Because gauge pressure gauges allow the surrounding ambient pressure to effect both sides of the sensing element, the effects of barometric pressure are essentially negated. Therefore, a "gauge pressure" gauge with an open inlet port will start with the pointer at zero, which means that the gauge is indicating "no pressure in excess of barometric." Gauge pressure dial gauges usually position the zero point at approximately 7:00 on the dial and rotate in the clockwise direction. 2) Vacuum Vacuum gauges measure negative pressures, i.e. the removal of atmospheric pressure. Using "gauge pressure zero" as the starting point, the gauge will indicate the "vacuum level" in positive numbers as more pressure is removed. Vacuum gauges usually position zero at 5:00 on the dial and rotate counter clockwise. Most vacuum gauges are rated to a full scale of 30 In Hg or 15 psi. Remember that since "vacuum" is simply the removal of atmospheric pressure, the highest level of vacuum that can be achieved on a given day is equal to the barometric pressure. (You can only remove what's there to remove in the first place!). Users often comment that they cannot get the gauge to read "all the way down to 30 In Hg, therefore the gauge is not working properly ." In most cases, the reason that they cannot achieve a 30 In Hg vacuum reading is that the existing barometric pressure is less than 30 In Hg, thus a 30 In Hg vacuum reading is unachievable. Vacuum gauges will not tell you how far you are from a complete vacuum; they will only tell you how far you are from “gauge pressure zero”. 3) Compound Pressure Starting at "gauge pressure zero," a compound gauge simply combines the vacuum indication of the straight vacuum gauge with the gauge pressure indication of a gauge pressure gauge. The position of the zero is dependent upon the full scale rating of the pressure side. The pointer will travel in the counterclockwise direction for vacuum indications, and clockwise for pressure indications. 4) Differential Pressure Starting at "gauge pressure zero," a differential pressure gauge simply measures the difference between 2 input pressures. A differential dial gauge looks like a gauge pressure gauge, except it has a second inlet port, with one port marked “hi” and the other marked “lo”. The line that connects to the higher pressure side of the application is always connected to the “hi” port, while the lower pressure line is connected to the “lo” port. 5) Absolute Pressure Unlike "gauge pressure," an absolute gauge does not allow ambient pressure to affect both sides of the sensing element. To achieve a true absolute reading, an absolute pressure gauge must have the atmosphere removed from around one side of the sensing element (referred to as an "evacuated reference"). The result is that an absolute gauge with an open inlet port will indicate the barometric pressure (which is usually between 14 and 15 psi). This will allow the user to either apply pressure causing the gauge to read above the barometric pressure value, or remove pressure (pull vacuum) causing the gauge to read below the barometric reading. "Absolute zero" should be achievable under full vacuum, since the starting point is the actual barometric reading, and a full vacuum will remove all of this existing pressure. Thus, unlike standard “vacuum” gauges, the absolute gauge will indicate how far the pressure is from a complete vacuum. On absolute dial gauges, the zero point will be positioned at 7:00 on the dial and the pointer will rotate clockwise when pressure is applied, and counterclockwise when pressure is withdrawn. Many users will refer to "absolute" gauges as "vacuum" gauges because they are using them in vacuum applications, so there is likely to be confusion regarding which pressure type is required. Also, please keep in mind that "Torr" is a unit of measure equal to millimeters of mercury (mm Hg) absolute. Please note that although all of the Heise electronic absolute pressure instruments have an evacuated reference for true absolute pressure indication, the Heise dial mechanical pressure gauge does not. Therefore, an absolute Heise dial gauge is not self barometrically compensating and must be manually adjusted to indicate the existing barometric pressure value prior to use.
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