Units, pressure

The simplest unit of pressure is the atmosphere (atm), the average pressure at sea level. The SI unit of pressure is the pascal (Pa), defined as 1 newton (N) per square meter. 

The pascal is a much smaller unit of pressure 1 atm is equal to 101,325 Pa. 

The pressure in a fully inflated mountain bike tire is about 6 atm, and the pressure on top of Mount Everest is about 0.31 atm. 

Since mercury is 13.5 times as dense as water, it is pushed up 1/13.5 times as high as water by atmospheric pressure. 

In a baromefer, fhe mercury column rises or falls with changes in atmospheric pressure. If fhe pressure increases, the level of mercury within the column rises. If fhe pressure decreases, fhe level of mercury wifhin the column falls. Since 1 atm of pressure pushes a column of mercury to a height of 760 mm, 1 afm and 760 mm Hg are equal. 

In a barometer, when the atmospheric pressure rises, the height of the mercury column rises as well. Similarly, when atmospheric pressure falls, the height of the column faUs. The unit millimeter of mercury is often called a torr, after the Italian physicist Evangelista Torricelli (1608-1647) who invented the barometer. 

There is a difference between psia (= psi absolute) and psig (= psi gauge) (manometer), the latter meaning psi in excess of atmospheric. 

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The ratio n/c2 is called the reduced osmotic pressure-and can be plotted with or without the RT-and the zero-intercept value (subscript 0) is the limiting value of the reduced osmotic pressure. Quite an assortment of different pressure units are used in the literature in reporting n values, and the units of R in Eq. (8.88) must be reconciled with these pressure (as well as concentration) units. 

The basic form of the equation is normally modified so that the differential is expressed in pressure units and the flow coefficient is divided into the product of an experimentally deterrnined discharge coefficient, iC, and a series of calculated coefficients. In this form, for concentric restrictions ... 

Thermal Dispersion. Thermal dispersion level switches are used on appHcations where multiple shifts inhquid characteristics are present. The unit is responsive only to a change in the thermal conductivity of the Hquid and ignores shifts in specific gravity, dielectric, density, temperature, and pressure. Units are used for alarm signal however, pump control maybe obtained using two units with a latching relay. 

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). 

Three pressure units are commonly in use in shock-wave research kilobar (kbar), gigapascal (GPa), and megabar (Mbar). These are equal to 10 , 10 , and 10 dyne/cm, respectively. The shock pressure range of primary interest in this review article is 100- 4000 kbar. 

Pressures Turboexpanders ean be designed to operate at up to 3,000 psi and higher inlet pressures as required by eonditions. Expansion pressure ratios ean also be adjusted for eaeh proeess over a wide range. A majority of effieient expansion ratios are below 5 1, although pressure ratios up to 10 1 ean be aeeommodated with reasonable effieieney. Smaller, lower pressure units are popular for air separation and helium liquefaetion. Intermediate pressure (100-1,000 psi) and high pressure expanders (1,000-3,000 psi) are widely used in natural gas proeessing and industrial gas liquefaetion. 

Both the high- and low-pressure units faee the same issues regarding eatalyst reeovery and expander design. However, eontrols may differ in those designs that produee more horsepower than ean be used by the primary driven maehine. 

Note When P is expressed in absolute pressure units, hswill be in absolute units. If P Is less than atmospheric pressure P is (-) If expressed as a gauge reading and will be a negative feet of liquid. P is (-(-) if expressed in absolute units. The friction loss hs Includes any entrance or exit losses and other such fittings in the system. 

The pressure units (gauge or absolute) must be consistent for all components used in determining both suction side and discharge side conditions. Most designers use gauge as a reference, but this is not necessary. 

The dimensionless ratio R is the ratio of the maximum deflagration pressure, in absolute pressure units, to the maximum initial pressure, in consistent absolute pressure units. For gas/air mixtures, R shall be taken as 9.0 for dust/air mixtures, R shall be taken as 10.0. 

Total shell side Ap (refer to the pressure drop section of this chapter) can be neglected for pressure units unless an unusual condition or design exists. To check, follow procedure for unbaffled shell pressure drop. 

The pressure drop due to condensing is usually negligible in a unit of this type. As a maximum, it may be taken as one half of the gas flow drop calculated for one baffle. This would be 1.16/8 = 0.145 psi for the condensing portion. Note that this does not recognize tube supports at 50% cut area, but for pressure units, this pressure drop will be nil. 

The effect of density of the air or gas entering the suction of the second (and/or third, etc.) fan is significant for high-pressure units operating individually above about 15 in. of water static. The static pressure rise in the second fan corrected for suction pressure is as follows, neglecting losses. ... 

The quantity consumed or produced is conveniently expressed in partial pressure units if the substance is a gas. Concentration units are convenient if the reactant or product is in solution. The time measurement is also expressed in whatever units fit the reaction microseconds for the explosion of household gas and oxygen, seconds or minutes for the burning of a candle, days for the rusting of iron, months for the rotting of wood. 

The data have been expressed in concentrations, although pressure units are more usual for a reaction involving gases. 

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