Absolute Pressure and Gauge Pressure
Absolute pressure is the total pressure exerted by a fluid, including atmospheric pressure. It is measured relative to a perfect vacuum. Gauge pressure, on the other hand, measures pressure relative to atmospheric pressure alone. It does not take into account the atmospheric pressure itself.
8 Key excerpts on "Absolute Pressure and Gauge Pressure"
eBook - ePub- P.K. Jayasree, K Balan, V Rani(Authors)
- 2021(Publication Date)
- CRC Press(Publisher)
...12 Fluid Mechanics 12.1 Pressure and Its Measurement A fluid is a substance which deforms continuously under the action of shearing forces. That is, if a fluid is at rest, there can be no shearing forces acting, and therefore, all forces in the fluid must be perpendicular to the planes upon which they act. Pressure is the measure of a force on a specified area. However, depending on the application, there can be many different ways of interpreting pressure. 12.1.1 Pressure Terminology Absolute pressure (Figure 12.1) is measured relative to absolute zero on the pressure scale, which is a perfect vacuum. (Absolute pressure can never be negative.) Absolute pressure is indicated by p and is identical to the familiar thermodynamic pressure. FIGURE 12.1 Pressure terminology. Gauge pressure (Figure 12.1) is measured relative to the local atmospheric pressure. Gauge pressure is thus zero when the pressure is the same as atmospheric pressure. (It is possible to have negative gauge pressure.) Gauge pressure is indicated by p g and is related to absolute pressure as follows: p g = p − p a, where p a is the local atmospheric pressure. Vacuum pressure (Figure 12.1) is also measured relative to the local atmospheric pressure, but is used when the gauge pressure is negative, i.e., when the absolute pressure falls below the local atmospheric pressure. (Positive vacuum pressure means that the gauge pressure is negative.) Vacuum pressure is indicated by p vacuum and is related to absolute pressure as follows: p vacuum = p a − p, where p a is the local atmospheric pressure. Differential pressure is the pressure difference between two media. Although most gauge pressures are technically a differential pressure sensor—measuring the difference between the medium and atmospheric pressure. A true differential pressure sensor is used to identify the difference between the two separate physical areas...
eBook - ePub- G. M. S. de Silva(Author)
- 2012(Publication Date)
- Routledge(Publisher)
...Pressures measured in reference to the atmospheric pressure are known as gauge pressures. The difference between a pressure higher than atmospheric and atmospheric pressure is a positive gauge pressure, while the difference between atmospheric pressure and a pressure lower than atmospheric is referred to as negative gauge pressure or vacuum. Gauge pressure values being dependent on atmospheric pressure change slightly as the ambient pressure changes. The relationship between absolute and gauge pressure is given below: Absolute pressure = gauge pressure + atmospheric pressure (5.1) A differential pressure is the difference of pressure values at two distinct points in a system. For example, the flow of a fluid across a restriction in a pipe causes a pressure differential and this is used to determine the flow of the gas or liquid. This is the principle of the orifice plate as shown in Fig. 5.2. 5.4 Primary standards A number of different physical standards are used for the realization of pressure values at the primary level. The two most common instruments for the positive gauge pressure range are the mercury manometer and dead weight pressure tester. The spinning ball gauge standard is used in the negative gauge pressure (vacuum) range. Figure 5.1 Pressure modes and their relationships Figure 5.2 Differential pressure in an orifice plate 5.4.1 Mercury manometer The basic principle of the mercury manometer is illustrated in Fig. 5.3. Vessels A and B are connected using a flexible tube. Vessel A is at a fixed level while vessel B can be moved up and down using a lead screw mechanism. The output pressure is obtained from vessel A. A vacuum pump is sometimes used to evacuate the air above the meniscus of the moving vessel...
eBook - ePub- John Bird(Author)
- 2019(Publication Date)
- Routledge(Publisher)
...Thus practical instruments actually determine the difference between the pressure being measured and atmospheric pressure. The pressure that the instrument is measuring is then termed the gauge pressure. In Figure 29.6, the line EF represents an absolute pressure which has a value greater than atmospheric pressure, i.e. the ‘gauge’ pressure is positive. Figure 29.6 Thus, absolute pressure = gauge pressure + atmospheric pressure. Hence a gauge pressure of, say, 60 kPa recorded on an indicating instrument when the atmospheric pressure is 101 kPa is equivalent to an absolute pressure of 60 kPa + 101 kPa, or 161 kPa. Pressure-measuring indicating instruments are referred to generally as pressure gauges (which acts as a reminder that they measure ‘gauge’ pressure). It is possible, of course, for the pressure indicated on a pressure gauge to be below atmospheric pressure, i.e. the gauge pressure is negative. Such a gauge pressure is often referred to as a vacuum, even though it does not necessarily represent a complete vacuum at absolute zero pressure. Such a pressure is shown by the line GH in Figure 29.6. An indicating instrument used for measuring such pressures is called a vacuum gauge. A vacuum gauge indication of, say, 0.4 bar means that the pressure is 0.4 bar less than atmospheric pressure. If atmospheric pressure is 1 bar, then the absolute pressure is 1 – 0.4 or 0.6 bar. 29.8 The manometer A manometer is a device for measuring or comparing fluid pressures, and is the simplest method of indicating such pressures. 29.8.1 U-tube manometer A U-tube manometer consists of a glass tube bent into a U shape and containing a liquid such as mercury. A U-tube manometer is shown in Figure 29.7(a). If limb A is connected to a container of gas whose pressure is above atmospheric, then the pressure of the gas will cause the levels of mercury to move as shown in Figure 29.7(b), such that the difference in height is h 1...
eBook - ePub- Gregory D. Wight(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
...2 BASIC GAS CONCEPTS 2.1 INTRODUCTION This chapter provides a brief review of some essential principles of physics, thermodynamics, and fluid mechanics. 2.2 PRESSURE MEASUREMENT Fluid pressure measurement is nearly always made relative to the surroundings or atmospheric pressure by a mercury or water manometer, or by a pressure transducer or mechanical gauge. Such relative pressure is called “gage” pressure (while “gauge” is the preferred spelling, “gage” is an accepted alternate) and is positive if higher than surroundings, negative if lower. Relative to atmospheric pressure, a negative gage pressure is called “vacuum”. The most commonly used units for gage pressure are pounds per square inch, gage (psig), inches or millimeters of mercury or water, or Pascals or kiloPascals (Pa, kPa; 1 Pa = 1 Newton per square meter.) See Table 2.1 for conversions. Table 2.1 Pressure Units 1 atm = 14.69 psi (lbf/in 2) = 760 mm Hg = 101.325 kPa = 1013.25 mB = 29.92″ Hg density ratio: ρ Hg = 13.6 ρ H 2 O 1″ Hg = 13.6″ H 2 O = 25.4 mm Hg = (13.6 × 25.4) = 345.44 mm H 2 O Gage pressure is used because it is easy to measure, record, and report; but for any computation involving fluid pressures, the actual or “absolute” pressure is needed, rather than the relative pressure. Since gage pressure is that amount above or below the surroundings, it is obvious that absolute pressure is the sum of surrounding and gage pressures. The surrounding pressure, in the case of air sampling measurements, is nearly always the local atmospheric or barometric (that which the barometer reports) pressure. Mercury or aneroid barometers are used to measure atmospheric pressures and commonly used units are pounds per square inch, absolute (psia), kPa, inches of mercury (″ Hg), millimeters of mercury (mm Hg), or millibars (mB). P abs = P bar + P gage or P atm + P gage in consistent units (2.1) Gage pressure, recall, may be positive or negative; thus, P abs may be greater or less than P atm...
eBook - ePub- Kaveh Azar(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...A total vacuum may be defined as a volume at zero pressure, i.e. there are no atoms present to exert a force against the walls of the container. Psia devices are most commonly used in pressure measurement where the atmospheric pressure will be changing during the measurement, e.g. rockets. Gage — psig in the English system where the g suffix denotes gage, which means that the reference pressure is local atmospheric pressure (gauge is a variation of gage). Since atmospheric pressure is a function of altitude, humidity, wind, etc., the absolute value of the pressure can only be determined if local atmospheric pressure is known. Sealed — psis in the English system where the s denotes sealed, which represents a device with the reference pressure cavity sealed at standard (or Normal) atmospheric pressure. Sealed pressure sensing devices are usually chosen to prevent the huge zero offset (essentially full scale) in the electrical output of a 103 kPa (15 psia) absolute transducer which occurs since barometric (atmospheric) pressure is about 101 kPa (14.7 psia). True psis devices should be avoided whenever possible because the trapped gas at 101 kPa (14.7 psia) makes an excellent thermometer via Charles' gas law. Simulated psis devices with a reference vacuum and an electrical offset are usable, as they are really psia devices. Line — this term commonly is defined as the pressure existing simultaneously on both sides of a sensing element, e.g. a diaphragm. Common mode pressure is now synonymous with line pressure. See Defined Terms in Section 6.8 for the rigorous definition of line pressure. Henri de Pitot developed the procedures for measuring fluid flow in 1732, and these basic procedures are still in use today. When measuring pressure under flow conditions, there are two complementary measurements which must be made to describe the (incompressible) flow: the static pressure and the total pressure...
eBook - ePub- Anders Andersson(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
...In a precise measurement, for example, a gas metering station, compensation is required for these variations, or design the system so that ambient pressure will not affect the measurement. Difference due to height is proportional to density, and since water has a much higher density than air, the pressure will change faster in the sea. If you dive, the pressure will increase with around 10 kPa/m. In a closed tank (without any leakage), the pressure is the same everywhere, and it is not important where to measure the pressure as all locations will give the same result. The same is true for a pipe where the flow rate is zero. However, in a pipe where a gas or liquid is flowing, it is different. In this case, there is a pressure drop along the pipe and the pressure will decrease along the pipe. It is possible to calculate the pressure drop, at least approximately, using pipe size, pipe material (surface roughness), flow rate and media properties. Tools and on-line calculators for this are available on the Internet or (more detailed and precise) from professional pipe manufacturers and design/consulting firms. There are several basic equations available, where the Darcy–Weisbach and Colebrook–White equations are most commonly used. At low Reynolds numbers and at very low pressures near vacuum (below around 100 Pa), other methods and equations are needed. Pressure Measurement Units When stating a pressure measurement, there are three things to write down; the measured value, the unit and the base reference (the ‘zero level’). There are two common references: vacuum (0 Pa) and normal atmosphere (101 325 Pa). Absolute pressure is measured with vacuum as reference, and this is indicated with (a) after the unit. Gauge pressure is measured with atmospheric pressure as reference, and this is indicated with (g). Differential pressure (dP or ΔP) is the pressure difference between two locations, like before and after a pump or at the top and bottom of a tank...
- Martin Novák(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
...10 Pressure The definition of pressure can be made using two equivalent definitions. a) Force acting on a given area (in normal direction to the surface)—see figure 10.1, left. Pressure is then p = F A = m ⋅ g A [ P a ; k g ; m ⋅ s − 2 ; m 2 ] (10.1) b) Hydrostatic pressure See figure 10.1 right p = ρ ⋅ g ⋅ h [ P a ; k g ⋅ m 3 ; m ⋅ s − 2 ; m ] (10.2) FIGURE 10.1: Pressure definition: (left) force acting on area, in the normal direction; (right) hydrostatic pressure The pressure can be measured in several. ways: Absolute pressure —p a —measured from zero pressure (vacuum) Relative pressure —measured from a selected reference. The reference can be, for example, barometric pressure p b. Over- or under-pressure is measured. Differential pressure gauges are used. In flowing fluids, the pressure also depends on the flow velocity. Total pressure = static pressure (for flow velocity 0) + kinetic pressure. Kinetic pressure is p d = ρ ⋅ w 2 2 (10.3) where w is flow velocity. The unit of pressure in SI system is 1 pascal (1 Pa) = 1 [kg·m −1 ·s −2 ]. Other units are still in use. Some selected ones are shown in table 10.1. TABLE 10.1: Selected units of pressure Unit Value in [Pa] 1 bar 10 5 1 mbar 100 1 kp/m 2 9,80665 1 atm (physical atmosphere) 101325 1 at (technical atmosphere) 980665 1 Torr (1 mm Hg) 133,322 1 1 mm H2O 9,80665 1 PSI 6894,757293 10.1 Calibration pressure gauges Calibration pressure gauges use the definition of pressure as their principle. They measure either force on a given area or hydrostatic pressure. Calibration instruments are then used to calibrate all other pressure gauges, such as deformation gauges. 10.1.1 Bell-type pressure gauges The bell-type pressure gauge is used for very fine measurement of low pressures. It is used for example in labs for gauge calibration. Its principle is shown in figure 10.2. Its basic component is a tank filled with a liquid with known density, for example water. An inverted bell is partially submerged in the liquid...
eBook - ePub- Michael Cable(Author)
- 2005(Publication Date)
- International Society of Automation(Publisher)
...4 PRESSURE INSTRUMENT CALIBRATION After completing this chapter, you should be able to: Calibrate the following pressure instrument types (to ISA standards, where applicable) and determine acceptability: • Gauges • Transmitters • Switches Select proper calibration procedure and calibration data sheet. Select appropriate certified test equipment. Properly set up/connect test equipment to DUT for calibration. Properly isolate pressure devices and/or remove from service for field calibration. Return equipment to service following calibration. Complete and properly maintain calibration documentation. 4.1 WHAT IS PRESSURE? Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. Atmospheric pressure is approximately 14.7 pounds per square inch (psi) at sea level. This means that if we could put one square inch of air from the ground to the upper atmosphere on a scale, it would weigh 14.7 pounds. We do not feel this pressure because it also acts internally and is thus balanced. Since there's a lot of “empty” space between air molecules, air can be compressed to fit in a smaller volume. When it’s compressed, air is said to be “under high pressure.” There are two ways to look at pressure: (1) the small-scale action of individual air molecules, or (2) the large-scale action of a large number of molecules. Starting with the small-scale action, a gas is composed of a large number of molecules that are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of any container. The molecules possess the physical properties of mass, momentum, and energy. As the gas molecules collide with the walls of a container, the molecules impart momentum to the walls, producing a force perpendicular to the wall...
