Pressure common units

The dimensions of permeabiUty become clear after rearranging equation 1 to solve for P. The permeabiUty must have dimensions of quantity of permeant (either mass or molar) times thickness ia the numerator with area times a time iaterval times pressure ia the denomiaator. Table 1 contains conversion factors for several common unit sets with the permeant quantity ia molar units. The unit nmol/(m-s-GPa) is used hereia for the permeabiUty of small molecules because this unit is SI, which is preferred ia current technical encyclopedias, and it is only a factor of 2, different from the commercial permeabihty unit, (cc(STP)-mil)/(100 in. datm). The molar character is useful for oxygen permeation, which could ultimately involve a chemical reaction, or carbon dioxide permeation, which is often related to the pressure in a beverage botde. 

The water-vapor transmission rate (WVTR) is another descriptor of barrier polymers. Strictly, it is not a permeabihty coefficient. The dimensions are quantity times thickness in the numerator and area times a time interval in the denominator. These dimensions do not have a pressure dimension in the denominator as does the permeabihty. Common commercial units for WVTR are (gmil)/(100 in. d). Table 2 contains conversion factors for several common units for WVTR. This text uses the preferred nmol/(m-s). The WVTR describes the rate that water molecules move through a film when one side has a humid environment and the other side is dry. The WVTR is a strong function of temperature because both the water content of the air and the permeabihty are direcdy related to temperature. Eor the WVTR to be useful, the water-vapor pressure difference for the value must be reported. Both these facts are recognized by specifying the relative humidity and temperature for the WVTR value. This enables the user to calculate the water-vapor pressure difference. Eor example, the common conditions are 90% relative humidity (rh) at 37.8°C, which means the pressure difference is 5.89 kPa (44 mm Hg). 

The Iduematic viscosity (v) is defined as the ratio of the absolute viscosity to density at the same temperature and pressure. The most common unit corresponding to the poise is the stoke (1 cmVsec). The SI unit would be mVsec. 

The quantity of a solute adsorbed can be given conveniently in terms of moles or volume (for adsorption) or ion-equivalents (for ion exchange) per unit mass or volume (dry or wet) of sorbent. Common units for adsorption are moV(m of fluid) for the fluid-phase concentration Cj and moV(kg of clean adsorbent) for adsorbed-phase concentration /ij. For gases, partial pressure may replace concentration. 

The term g in Equation 2-56 is the dynamic viscosity of the fluid. The dynamic viscosity is the ratio of the shear stress to the shear rate. It has units of (force x time)/(area). The most common unit of viscosity is the centipoise (1 centipoise = 0.01 g/cm - s). Dynamic viscosity may be a function of temperature, pressure, and shear rate. 

This equation defines the permeability (K) and is known as Darcy s law. The most common unit for the permeability is the darcy, which is defined as the flow rate in cm3/s that results when a pressure drop of 1 atm is applied to a porous medium that is 1 cm2 in cross-sectional area and 1 cm long, for a fluid with viscosity of 1 cP. It should be evident that the dimensions of the darcy are L2, and the conversion factors are (approximately) 10 x cm2/darcy C5 10-11 ft2/darcy. The flow properties of tight, crude oil bearing, rock formations are often described in permeability units of millidarcies. 

As these conversion equations show, in addition to the molecular weight of a measured gaseous pollutant, the temperature and pressure at the time of the measurement must be known. Because this information is often not given in the literature, no attempt has been made in this report to convert to a common unit. 

Many units are used to express pressure. Because pressure is defined as force per unit area, a common unit used in the United States is pounds per square inch. This unit is commonly used for tire inflation pressure. The atmospheric pressure at sea level is about 14.7 pounds per square inch. In the metric system, the basic unit for force is the newton, abbreviated N, and area is mea-... 

The most common unit of <2rel is bed volumes per hour (BV/li). Space velocity is also used in catalytic reactors, especially in three-phase fixed-beds, and is referred to as liquid hourly space velocity (LHS V) for the liquid phase, and gas hourly space velocity (GHSV) for gas phase. As mentioned above, space-time and space velocity are measured under the entrance conditions. However, for space velocity, other conditions are frequently used (Fogler, 1999). For example, the LHSV is measured at 60 to 75 °F, and GHSV at standard temperature and pressure. 

Microbar A unit of pressure, commonly used in acoustics which equals 1 dyne per square centimeter or one newton. A reference point for tire decibel, is 0.0002 dyne per square centimeter, or 20 Newtons/M2. 

Several methods are commonly used to specify the abundance of substances in the atmosphere. For gaseous constituents common practice is to specify abundances as mixing ratios, or equivalently as mole fractions of the species in air. This quantity is simply the ratio of the partial pressure of a substance to the total pressure. The advantage of this unit is that it is independent of pressure and temperature, and for an atmospheric component that is well mixed, the mixing ratio will be constant as the pressure or temperature changes. Common units for specifying mixing ratios are parts... 

Subatmospheric pressure usually is expressed in reference to perfect vacuum or absolute zero pressure, lake absolute zero temperature (the concept is analogous), absolute zero pressure cannot be achieved, but it does provide a convenient reference datum. Standard atmospheric pressure is 14.695 psi absolute, 30 inches of mercury absolute, or 760 mmHg of density 13.595 g/cm3 where acceleration due to gravity is g = 980.665 emir. ] mmHg. which equals 1 torr. is the most commonly used unit of absolute pressure. Derived units, the million or micrometer, representing 1/1000 of 1 mmHg or 1 torr, are also used for subtorr pressures. 

The most common unit of pressure in the United States is pounds per square inch (psi). The pound is a unit of weight, and is the same as the force exerted by gravity on a 0.454 kg mass. One inch is equal to. 0254 meters. Convert a pressure of 1 atm into psi. 

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). 

The units of all terms of this equation must be the same. The product PV has the units of energy, as does U therefore H also has units of energy. In the SI system the basic unit of pressure is the pascal or N m"2 and, for volume, the m3. Thus the PV product has the unit N m or joule. In the English engineering system a common unit for the PV product is the (ft lbr), which arises when pressure is in (lbr)(ft) 2 with volume in (ft)3. This result is usually converted to (Btu) through division by 778.16 for use in Eq. (2.6), because the common English engineering unit for U and H is the (Btu). 

For many years, atmospheric pressure was measured in millimetres of mercury (mm Hg). In the British Commonwealth and the United States, inches of mercury were used. Standard atmospheric pressure, the pressure of the atmosphere at sea level and 0°C, is 760 mm Hg. More recently, in honour of the work of Torricelli, standard atmospheric pressure has been defined as 760 torr. 1 torr represents a column of mercury 1 mm in height at 0°C. Another common unit for measuring pressure is atmospheres (atm), where 1 atm is equivalent to 760 torr. While mm Hg, torr, and atm are still used to measure pressure, especially in technological and medical applications, the SI units are pascals (Pa) or kilopascals (kPa). 

The common unit of measurement for natural gas is the standard cubic foot in the English system and the standard cubic meter in the metric system. Each of these standards is expressed at pressures and temperatures commonly used as standard to the system in the geographical area of concern. In the United States, where standards frequently vary from state to state, the cubic foot is frequently expressed in the English system at standard conditions of 14.73 pounds per square inch absolute (psia) and 60 degrees Fahrenheit (60°F), although there are a number... 

There are a number of units that can be used to measure barometric pressure. For example, millimeters can be converted into inches thus, a standard pressure of 760 mm Hg can also be recorded as 30.0 inches of mercury. Inches of mercury are the units used for weather reports in the United States. Three other very common units that correspond to 760 mm Hg are ... 

Because gases are compressible, they exert pressure on their surroundings. Pressure is the force that is exerted over a unit area. For example, the atmosphere exerts a pressure known as atmospheric pressure. The Earth s atmosphere is a function of the location and the weather conditions, and it decreases with a higher altitude. The unit of pressure commonly used in chemistry is the atmosphere (atm). The standard atmosphere is 1 atm or a measurement of 760 millimeters of mercury (mm Hg or torr) on a manometer. 

Pressure is expressed in terms of newtons per square meter, and is given the name, pascal. The pascal is a very small unit, and the kilopascal is suggested as the most common unit for pressure. 

There is often confusion about different units of pressure measurement, but the way they relate to one another is quite simple. The most common units are mm Hg, and 1mm Hg is the same as ITorr. The other common scale is mbar which relates to atmospheric pressure, 1 bar = 1 atmosphere = 760mm. Thus ... 

The common unit of P, is Barrer (IBarrer = 10 10cm3 (STP)-cm/ cm2scmHg = 0.76-10 17m3 (STP) m/m2 s Pa). The ratio P,/l is referred to as the permeance, and its common unit is the gas permeation unit (GPU), which is 10 scm3 (STP)/(cm2 s cmHg). If the downstream pressure is negligible compared with the upstream pressure, the selectivity can be expressed as... 

Pressure is force per unit area and has dimensions of [M/LT2]. Pressure is an important measurement in many fields of science, and each held has different traditional units. The SI unit, the pascal [N/m2 or kg/(m sec2)], is commonly used, along with the dyn/cm2. Other pressure units frequently encountered include the millimeter of mercury (mm Hg), the atmosphere (atm), the bar (106 dyn/cm2), and the pound per square inch (psi). The origin of some of these units is implicit in their names the millimeter of mercury (also called a torr) is the amount of pressure that causes the mercury in a manometer to rise by 1 mm—an easy unit of measure for the laboratory experimentalist to use. Many of the common units of pressure and their interconversion factors are shown in Table A-7. 

The reader should note that absolute and gauge pressures are usually expressed with units of atm, psi, or mm Hg. This statement also applies to partial pressures. One of the most common units employed to describe pressure drop is inches of H2O, with the notation in. H2O or IWC (inches of water column). 

Psi is a common unit of pressure measure, but other units are used frequently, some of which are listed in the table on the next page. 

---