Pascal, measurement unit

The measuring unit of pressure in SI system is called pascal (Pa) = newton/m2. One Pa corresponds to a very small pressure. Also for the latter reason, several other units are commonly used in vacuum practice and instrumentation. In Table 1.1, the conversion among the most frequently used pressure units is reported. 

Finally, the world literature on energy production and consumption is plagued by a proliferation of measurement units. Variously, data are presented in terms of the International System of Units (SI, e.g., metres, pascals, joules), traditional industry-based units e.g., barrels of oil, kilowatt hours of electricity, million tonnes of oil equivalent) and, especially in the USA, Imperial units e.g., miles, British thermal units of heat, quads of energy, cubic feet of natural gas, bars of pressure). For the expression of time, however, units of days and years are generally more appropriate than the SI unit (seconds) in this field. In order to assist readers in translating units into those with which they are familiar, a set of conversion factors has been included. 

According to the International System of Units (SI), the dynamic viscosity measurement unit is the Pascal-second (Pa-s). This unit is the fundamental measurement unit of viscosity. Respectively, in the CGS system, the viscosity measurement unit is in dyne-s/cm (=1 g-s/cm). This unit is known as poise (P). The relation between Pa s and poise is as follows 1 Pa-s = 10 P. Sometimes, centipoise (cP) is also used, where 1 cP = 0.01 P. Centipoise may have been adopted due to the fact that water s viscosity at 30°C is 1 cP. 

A pressure of 1 mm Hg is also called 1 torr to honor Torricelli for his invention of the barometer. The average atmospheric pressure at sea level at 0°C is 760 mm Hg. Pressures are often measured in units of atmospheres. One atmosphere of pressure (atm) is defined as being exactly equivaient to 760 mm Hg. In SI, pressure is expressed in derived units called pascals. The unit is named for Blaise Pascal, a French mathematician and philosopher who studied pressure during the seventeenth century. One pascai (Pa) is defined as the pressure exerted by a force of one newton (1 N) acting on an area of one square meter. In many cases, it is more convenient to express pressure in kilopascals (kPa). The standard atmosphere (1 atm) is equal to 1.013 25 x 10 Pa, or 101.325 kPa. Several pressure units and common uses for them are summarized in Figure 1.5. 

Pressure is defined as force per unit of area. The International System of Units (SI) pressure unit is the pascal (Pa), defined as 1.0 N /m. Conversion factors from non-SI units to pascal are given in Table 1 (see also Units and conversion factors front matter). An asterisk after the sixth decimal place indicates that the conversion factor is exact and all subsequent digits are 2ero. Relationships that are not followed by an asterisk are either the results of physical measurements or are only approximate. The factors are written as numbers greater than 1 and less than 10, with 6 or fewer decimal places (1). 

The important point is that the magnitude of a stress is always equal to the magnitude of a force divided by the area of the face on which it acts. Forces are measured in newtons, so stresses are measured in units of newtons per metre squared (N m" ). For many engineering applications, this is inconveniently small, and the normal unit of stress is the mega newton per metre squared or mega(lO ) pascal (MN m or MPa) or even the giga(10 )newtons per metre squared or gigapascal (GN m or GPa). 

It is conventional to take as the activation volume the value of AV when P = 0, namely —bRT. (This is essentially equal to the value at atmospheric pressure.) Pressure has usually been measured in kilobars (kbar), or 10 dyn cm 1 kbar = 986.92 atm. The currently preferred unit is the pascal (Pa), which is 1 N m 1 kbar = 0.1 GPa. Measurements of AV usually require pressures in the range 0-10 kbar. The units of AV are cubic centimeters per mole most AV values are in the range —30 to +30 cm moP, and the typical uncertainty is 1 cm moP. Rate constant measurements should be in pressure-independent units (mole fraction or molality), not molarity. ... 

In the metric system, pressure has a unit of newtons per square meter, which is called a pascal (Pa). Although the pascal is the scientific unit and is preferred, pounds per square inch (Ibs/iif) is comnion in the United States. For example, in most of Europe, tire pressure is recorded in pascals (typically 220,000 Pa), whereas tire pressure in American cars is measured in pounds per square inch (typically 32 Ibs/in ). As a point of reference, the pressure that the earth s... 

The constant R is called the gas constant and has the same value for all gases because R is independent of the identity of the gas, we say that it is a universal constant. The value of the gas constant can be found by measuring P, V, n, and T and substituting their values into R = PV/nT. When we use SI units (pressure in pascals, volume in meters cubed, temperature in kelvins, and amount in moles),... 

The accepted SI unit for pressure is the pascal (Pa). Pressure is defined as force per unit area, P/, so the pascai can be expressed by combining the SI units for these two variabies. The SI unit of force is the newton (N), and area is measured in square meters (m ). Thus, the pascai is 1 N/m. Expressed in pascais, the numericai vaiue of atmospheric pressure is quite iarge. By intemationai agreement, 1 atm is defined exactiy in terms of pascais ... 

The SI unit of viscosity is pascal-second (Pa s) or newton-second per meter squared (N s m-2). Values tabulated are mN s m-2 (= centipoise, cP). The temperature in degrees Celsius at which the viscosity of a substance was measured is shown in parentheses after the value. 

There are two variables used in the description of fluid flow shear stress and shear strain. Stress is measured in units of Pascals and the strain is dimensionless. 

The tensile test is performed by placing a specially shaped specimen in the heads of the testing machine. The specimen is pulled apart through a hydraulic or mechanical loading system (Figure 15.33). Most ordinary tensile tests are conducted at room temperature and the tensile load is applied slowly. The unit measure of tensile strength is the pascal (Pa), or newtons per square meter (N-nf2), and is defined by the following equation ... 

As mentioned above, gases in solution are often measured in partial pressures. Recall that Dalton s Law of Partial Pressures states that the total pressure is the sum of the pressures partially exerted by all of the component, noninteracting gases. For one gaseous component in equilibrium with the same component dissolved in a liquid, the partial pressure of that gas in solution is the pressure that gas would exert in the atmosphere. It is symbolized by either p or P followed by the specific gas (eg, either or pO ). For example, typical blood P02 levels range between 75 and 100 torr (or, 10.0 to 13.3 kPa since the pascal (Pa) is an SI unit). 

Viscosity is then a measure of the resistance of a material to flow. In fact, the inverse of viscosity is given the name fluidicity. A material s resistance to flow increases with its viscosity. Viscosity has been reported using a number of different names. The centimeter-gram-second (CGS) unit of viscosity is called the poise, which is a dyne seconds per square centimeter. Another widely used unit is the pascal (or Pa), which is Newton seconds per square centimeter. In fact, 1 Pa= 10 poise. 

The practical chemist working with vacuum systems has, in the past, used practical units, such as millimetres of mercury and atmospheres, for measuring the quality of the vacua which he has produced. However, for the modern chemist it is important to have a coherent system of units in which no numerical factors are inherent. Within the SI system, the unit of pressure is the pascal, which has the units and dimensions given below ... 

Table 1-2 lists some quantities that are defined in terms of the fundamental quantities. For example, force is measured in newtons (N), pressure is measured in pascals (Pa), and energy is measured in joules (J), each of which can be expressed in terms of the more fundamental units of length, time, and mass. 

Mpa - The abbreviation for mega-pascal, a metric unit of measurement. 

PRESSURE. If a body of fluid is at rest, the forces are in equilibrium or the fluid is in static equilibrium. The types of force that may aci on a body are shear or tangential force, tensile force, and compressive force. Fluids move continuously under the action of shear or tangential forces. Thus, a fluid at rest is free in each part from shear forces one fluid layer does not slide relative to an adjacent layer. Fluids can be subjected to a compressive stress, which is commonly called pressure. The term may be defined as force per unit area. The pressure units may be dynes per square centimeter, pounds per square foot, torr. mega-Pascals, etc. Atmospheric pressure is the force acting upon a unit area due to the weight of the atmosphere. Gage pressure is the difference between the pressure of the fluid measured (at some point) and atmospheric pressure. Absolute pressure, which can be measured by a mercury barometer, is the sum of gage pressure plus atmospheric pressure. 

This unit describes a method for measuring the viscosity (r ) of Newtonian fluids. For a Newtonian fluid, viscosity is a constant at a given temperature and pressure, as defined in unit hi. i common liquids under ordinary circumstances behave in this way. Examples include pure fluids and solutions. Liquids which have suspended matter of sufficient size and concentration may deviate from Newtonian behavior. Examples of liquids exhibiting non-Newtonian behavior (unit hi. i) include polymer suspensions, emulsions, and fruit juices. Glass capillary viscometers are useful for the measurement of fluids, with the appropriate choice of capillary dimensions, for Newtonian fluids of viscosity up to 10 Pascals (Newtons m/sec 2) or 100 Poise (dynes cm/sec 2). Traditionally, these viscometers have been used in the oil industry. However, they have been adapted for use in the food industry and are commonly used for molecular weight prediction of food polymers in very dilute solutions (Daubert and Foegeding, 1998). There are three common types of capillary viscometers including Ubelohde, Ostwald, and Cannon-Fenske. These viscometers are often referred to as U-tube viscometers because they resemble the letter U (see Fig. HI.3.1). 

As is frequently the case with SI units, which must serve many scientific disciplines, the pascal is an inconvenient size for most chemical measurements. Thus, the alternative pressure units millimeter of mercury (mm Hg) and atmosphere (atm) are more frequently used. 

This scheme differs from the various systems in use in industry and academia in that it uses the mole instead of the cc(STP) to express the quantity of matter being transported, the pascal rather than the atmosphere or the cm. Hg. to express pressure, the meter rather than the mil, the inch, or the centimeter to express length, and the second rather than the day to express time. Our experience indicates that the existing variety of unit systems leads to confusion and that calculations of related physical properties such as permeabilities, diffusion coefficients, and solubilities are easier using the SI units. More modern measurement systems which detect permeants by means of the electrical currents generated by individual atoms are easier to analyze when one uses moles rather than cc(STP) to express the amount of matter undergoing transport. Applications involving the transport of mixed permeant species are also easier to deal with on a molar basis. Conversion tables between the SI units and customary units are provided on the SRM certificate and in the appropriate standards documents (4, 5). ... 

In a rheomety experiment the two plates or cylinders are moved back and forth relative to one another in an oscillating fashion. The elastic storage modulus (G - The contribution of elastic, i.e. solid-like behaviour to the complex dynamic modulus) and elastic loss modulus (G" - The contribution of viscous, i.e. liquid-like behaviour to the complex modulus) which have units of Pascals are measured as a function of applied stress or oscillation frequency. For purely elastic materials the stress and strain are in phase and hence there is an immediate stress response to the applied strain. In contrast, for purely viscous materials, the strain follows stress by a 90 degree phase lag. For viscoelastic materials the behaviour is somewhere in between and the strain lag is not zero but less than 90 degrees. The complex dynamic modulus ( ) is used to describe the stress-strain relationship (equation 14.1 i is the imaginary number square root of-1). 

We first convert the measured quantities to SI units and compute the osmotic pressure in pascals ... 

The shear stress results are expressed in Pascals (N/m2). The calculation is obtained by dividing the applied force indicated by the tensile machine by twice the actual bonded area of one block in square meters (nominally 20 x 25 x 10 The shear strain is obtained by dividing half the actual deformation by the actual thickness of one block. All measurements must be in the same units. 

This relationship has four quantities. To exactly solve a problem, you need three of the four variables. Boyle s Law is not sensitive to units of pressure or volume. You can measure pressure in units of pascals, psi, atm, or whatever. Likewise, you can measure volume in units of cubic meters (m3), milliliters, or gallons. However, once you have selected a set of units for one side of the equation, you have to use the same units on the other side. 

A variety of pressure units are in use. These units are related to the column of mercury shown in Figure 6.2. The height of the mercury column in the tube is commonly measured in inches or in millimeters. As you listen to the weather report on the evening news, notice that the atmospheric pressure is given in inches. Where the metric system is in use, millimeters are favored. Millimeters are synonymous with torr, a unit named for the Italian scientist Evangalista Torricelli. Less frequently used units are atmospheres and pascals. The following equation shows the relationship among these various units of pressure. 

The pressure P of a fluid on a surface is defined as the normal force exerted by the fluid per unit area of the surface. If force is measured in N and area in m2, the unit is the newton per square meter or N nTJ, called the pascal, symbol Pa, the basic SI unit of pressure. In the English engineering system the most common unit is the pound force per square inch (psi). 

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