Units of measurement for pressure

Pascal the SI unit of measurement for pressure equal to one newton per square meter. 

As the reference state for all components in the reactions is defined as an ideal gas at 25°C (298.15K) and 1.01325 bar, the value that must be used for f° is 1.01325 bar. If another unit of measurement for pressure is used, the reference pressure must be changed correspondingly so that the activity becomes independent of pressure. If a component is a solid, such as carbon in carbon formation, the activity, a, is unity, since the reference state for the Gibbs free energy of carbon is also defined as the solid state. 

In the area of vacuum process engineering, frequently not Pascal is used as the unit of measurement for pressures but instead also the allowed unit bar or mbar . 

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... 

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. 

Permeance - A unit of measurement for the ability of a material to retard the diffusion of water vapor at 73.4 F (23 C). A perm, short for permeance, is the number of grains of water vapor that pass through a square foot of material per hour at a differential vapor pressure equal to one inch of mercury. 

Units of Measure Units of measure for dilution ventilation are volmne units of air flow. One measure is cubic feet per minute. Air changes per hour are not a good measure, because those reflect room volume only. This does not reflect the rate of contamination reduction. Dilution ventilation requirements are the amount of clean air required relative to the amount and rate of contaminant generated. Computation of ventilation required should adjust air data to standard temperatore (0 °C) and pressure (460 mm Hg). The volume of air varies somewhat with atmospheric conditions. 

This is referred to as Fanning s formula. The unit of measurement for Ff is [J/kg]. The pressure loss due to friction APf is then given by... 

The roentgen is the international unit of quantity for both x-rays and y-rays. It is the quantity of this radiation which will produce, as a, result of ionization, one electrostatic unit of charge, of either sign, in one cubic centimeter of dry air (as measured at 0° C and standard atmospheric pressure). 

An effective HE or cost-effectiveness analysis is designed to answer certain questions, such as Is the treatment effective What will it cost and How do the gains compare with the costs By combining answers to all of these questions, the technique helps decision makers weigh the factors, compare alternative treatments, and decide which treatments are most appropriate for specific situations. Typically, one chooses the option with the least cost per unit of measure gained the results are represented by the ratio of cost to effectiveness (C E). With this type of analysis, called a cost-effectiveness analysis (CEA), various disease end points that are affected by therapy (risk markers, disease severity, death) can be assessed by corresponding indexes of therapeutic outcome (mmHg blood pressure reduction, hospitalizations averted, life years saved, respectively). It is beyond the scope of this chapter to elaborate further on principles of cost-effectiveness analyses. A number of references are available for this purpose [11-13]. 

Retention is usually measured in units of time for convenience. Voliime units are more exact. Table 1.1, after suitable corrections have been applied (26). Under average chromatographic conditions liquids can be considered incompressible, but not so for gases, and in gas chromatography elution volumes are corrected to a mean column pressure by multiplying them by the gas compressibility factor, j, equation (1.2)... 

Litre—is defined as the volume occupied hy one kilogram of water at its temperature of maximum density (4°C) and subjected to normal atmospheric pressure . The litre is considered as the standard unit of volume for all volumetric measurements. 

Most explosive detection equipments do not truly detect explosive vapour, rather they key on minute particles of the explosive [11]. The reason for this is that most explosives have very low vapour pressure, and low vapour pressures are rather difficult to measure. Methods based on mass loss or the direct measurement of tiny pressures are particularly prone to the influence of trace impurities of more volatile substances. Consequendy, the values reported in the literature exhibit a high degree of scatter. To add to the confusion, difierent units of measurement are used. In general, measurements involving chemical determination of the amount of the specific compound in the vapour phase are to be preferred. If several difierent values are reported, and there is no better criterion for selection, it is probably best to take the lowest value. 

Table 2.4 Physical and crystalline properties of naturally occurring elemental arsenic polymorphs at25°C and 1 bar pressure (Baur and Onishi, 1978 fide, 2007 Matsubara et al., 2001 O Neil et al., 2001). See the text and Appendix B for explanations of the terms and units of measure.

Quantitative tests of hardness involve indenting a material with a ball, cone, or wedge of hardened steel or other metal under a pre-determined pressure for a specific amount of time. The size and depth of the cavity left in the material being tested is then measured and a formula applied. The resulting value is the hardness, which, like Moh s hardness, has no units of measure. 

Section 12.1 introduces the concept of pressure and describes a simple way of measuring gas pressures, as well as the customary units used for pressure. Section 12.2 discusses Boyle s law, which describes the effect of the pressure of a gas on its volume. Section 12.3 examines the effect of temperature on volume and introduces a new temperature scale that makes the effect easy to understand. Section 12.4 covers the combined gas law, which describes the effect of changes in both temperature and pressure on the volume of a gas. The ideal gas law, introduced in Section 12.5, describes how to calculate the number of moles in a sample of gas from its temperature, volume, and pressure. Dalton s law, presented in Section 12.6, enables the calculation of the pressure of an individual gas—for example, water vapor— in a mixture of gases. The number of moles present in any gas can be used in related calculations—for example, to obtain the molar mass of the gas (Section 12.7). Section 12.8 extends the concept of the number of moles of a gas to the stoichiometry of reactions in which at least one gas is involved. Section 12.9 enables us to calculate the volume of any gas in a chemical reaction from the volume of any other separate gas (not in a mixture of gases) in the reaction if their temperatures as well as their pressures are the same. Section 12.10 presents the kinetic molecular theory of gases, the accepted explanation of why gases behave as they do, which is based on the behavior of their individual molecules. 

The air pressure of car tires should be checked regularly for safety reasons and to prevent uneven tire wear. Find out the units of measurement on a typical tire gauge, and determine how gauge pressure relates to atmospheric pressure. 

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