Pressure, barometric

The liquid in B rapidly volatilises at the bottom of the tube T, the stopper being thrown off, and bubbles of air escape from D into the tube C. Continue boiling the liquid in J steadily until no more bubbles escape into C. Then carefully slip the end of D from under the tube C, close the end of C securely with the finger, and then transfer the tube to a gas-jar of water, so that the level of the water inside and outside C can be equalised. Measure the volume of air in C, and note the room temperature and the barometric pressure. The vapour density can now be calculated (see p. 428). 

For distillations conducted at atmospheric pressure, the barometric pressures are rarely exactly 760 mm. and deviations may be as high as 20 mm. To correct the observed boiling point to normal pressiu e (760 mm.), the following approximate expression may be used ... 

In order to carry out a distillation, the apparatus is completely assembled, the water pump turned on to its maximum capacity, and the screw clip on the capillary tube in the Claisen flask adjusted so that a gentle stream of air bubbles through the liquid (see Section 11,19 for details of the preparation of the capillary tube). The barometric pressure is read, and if the resulting vacuum deter mined from the reading on the mano meter is satisfactory (as estimated from the temperature of the tap water), the flask may be heated in an air (Fig. II,... 

The density of moist air depends upon the temperature, the humidity, and the barometric pressure. It is expressed by the equation... 

Psychrometrics. Psychrometrics is the branch of thermodynamics that deals specifically with moist air, a biaary mixture of dry air and water vapor. The properties of moist air are frequentiy presented on psychrometric charts such as that shown ia Figure 2 for the normal air conditioning range at atmospheric pressure. Similar charts exist for temperatures below 0°C and above 50°C as well as for other barometric pressures. AH mass properties ate related to the mass of the dry air. 

Relative Humidity (rh). Relative humidity is the ratio of the mole fraction of water vapor present in the air to the mole fraction of water vapor present in saturated air at the same temperature and barometric pressure it approximately equals the ratio of the partial pressure (or density) of the water vapor in the air to the saturation pressure (or density) of water vapor at the same temperature. 

Pump Suction. The net positive suction head required (NPSHR) affects the resistance on the suction side of the pump. If it drops to or near the vapor pressure of the fluid being handled, cavitation and loss of performance occurs (13). The NPSHR is affected by temperature and barometric pressure and is of most concern on evaporator CIP units where high cleaning temperatures might be used. A centrifugal booster pump may be installed on a homogenizer or on the intake of a timing pump to prevent low suction pressures. 

There are problems to be considered and avoided when using Hquid-in-glass thermometers. One type of these is pressure errors. The change in height of the mercury column is a function of the volume of the bulb compared to the volume of the capillary. An external pressure (positive or negative) which tends to alter the bulb volume causes an error of indication, which may be small for normal barometric pressure variations but large when, for example, using the thermometer in an autoclave or pressure vessel. 

Fig. 9. Ethylene oxide sterilisation 4-h cycle, 12/88 12 wt % ethylene oxide/88 wt % chlorofluorocarbon (Freon 12), where the horizontal line represents standard barometric pressure A, preconditioning and humidification at 87.8—94.55 kPa, 30—40 min B, exposure for 3/4 h at 55.17—68.95 kPa above standard barometric pressure C, air washes at 81.04—87.8 kPa, 20 min. To convert kPa to psi, multiply by 0.145.

The mercury barometer (Fig. 10-11) indicates directly the absolute pressure of the atmosphere in terms of height of the mercuiy column. Normal (standard) barometric pressure is 101.325 kPa by definition. Equivalents of this pressure in other units are 760 mm mercury (at 0°C), 29.921 iuHg (at 0°C), 14.696 IbFin, and 1 atm. For cases in which barometer readings, when expressed by the height of a mercuiy column, must be corrected to standard temperature (usually 0°C), appropriate temperature correction factors are given in ASME PTC, op. cit., pp. 23-26 and Weast, Handbook of Chemistty and Physics, 59th ed., Chemical Rubber, Cleveland, 1978-1979, pp. E39-E41. 

FIG. 12-1 Psychro metric chart—low temperatures. Barometric pressure, 29.92 inHg. To convert British thermal units per pound to joules per Idlogram, multiply by 2.326 to convert British thermal units per pound dry air-degree Fahrenheit to joules per Idlogram-kelvin, multiply by 4186.8 and to convert cubic feet per pound to cubic meters per Idlogram, multiply by 0.0624. 

TABLE 12-2 Additive Corrections for Hf h, and v When Barometric Pressure Differs from Standard Barometer... 

AH = moisture-content correction of air saturated at wet-bulb temperature when barometric pressure differs from standard barometer, gr/lb dry air NOTE To obtain AH reduce value of AH by 1 percent where t — t, = 24 F and correct proportionally when t — is not 24 F h = enthalpy of moist air, Btu/lb dry air... 

Example At a barometric pressure of 25.92 with 220 F dry-bulb and 100 F wet-bulb temperatures, determine H, h, andi . Ap = —4, and from table AH = 50.4. From note. 

Example 8 Determination of Air Properties For a barometric pressure of 25.92 inHg (Ap = —4), a dry-bulb temperature of 90 F, and a wet-bulb temperature of 70 F determine the following absolute humidity, enthalpy, dew point, relative humidity, and specific volume. 

If pressure drop through system = 1.0 in Hg and barometric pressure =... 

Properties of water I - Atmospheric and barometric pressure readings at different altitudes... 

Barometric pressure should be measured at the test site at 30-minute intervals during the test. 

Nozzle arrangements for various applications vary considerably. For subcritical flow measurement at the outlet end, where nozzle differential pressure p is less than the barometric pressure, flow should be measured with impact tubes and manometers as shown in Figure 20-3. 

Pb Barometric pressure, mmHg Pc Critical pressure of water, 166,818mmHg Pd Saturation pressure of w-ater at the dry-bulb temperature, mm Fig... 

The absolute, barometric pressure is not normally required in ventilation measurements. The air density determination is based on barometric pressure, but other applications are sufficiently rare. On the other hand, the measurement of pressure difference is a frequent requirement, as so many other quantities are based on pressure difference. In mass flow or volume flow measurement using orifice, nozzle, and venturi, the measured quantity is the pressure difference. Also, velocity measurement with the Pitot-static tube is basically a pressure difference measurement. Other applications for pressure difference measurement are the determination of the performance of fans and air and gas supply and e. -haust devices, the measurement of ductwork tightness or building envelope leakage rate, as well as different types of ventilation control applications. 

By connecting manometer hoses to both output pressures given by the tube, the pressure difference Ap can be measured directly. The barometric pressure and the fluid temperature are required for the determination of the fluid density. The Pitot-static tube is not a suitable instrument for measuring low velocities. It can be applied in cases where the flow velocity is high... 

Barometric pressure at standard temperature and pressure = 101.325 kPa Standard temperature and pressure (STP) = 0 °C at 101.325 kPa (also known as normal temperature and pressure)... 

Barometer An instrument used to measure the barometric pressure at a given location in the earth s atmosphere. 

Available horsepower from a gas turbine is a function of air compressor pressure ratio, combustor temperature, air compressor and turbine efficiencies, ambient temperature, and barometric pressure. High ambient temperatures and/or low barometric pressure will reduce available horsepower while low ambient temperatures and/or high barometric pressure will increase available horsepower. All industrial turbines will have high-temperature protection, but in areas subject to very low ambient temperatures horsepower limiting may be required. 

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