Barometric pressure correction

The exposure limits or TLVs for extended shifts (>8 hours per workday and >40 hours per work week) require a barometric pressure correction for altitude to account for possible accumulation of toxic substances in the body as a result of increased exposure and reduction in detoxification time. For cumulative hazards where detoxification does not occur (e.g., silica), extended work shift correction should be proportional to the actual hours worked exceeding the usual 2,000 hours per year. 

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 ... 

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. 

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... 

If V is the volume, t the temperature, B the barometric pressure, and f the vapour tension of water at then the corrected volume is given by the formula... 

Barometric pressure for altitudes above standard" sea level are given in the appendix. These correct values must be used wherever the need for the local absolute barometric pressure is involved in pressure calculations. 

Performance tables are based on standard dry air at 70°F at sea level (barometric pressure 29.92 in. Hg) with a density of 0.075 lb per fp. When the fans are required to handle gases at other conditions at the inlet, corrections must be made for temperature, altitude, and air or gas density. 

Meters are accurate within close limits as legislation demands. However, gas is metered on a volume basis rather than a mass basis and is thus subject to variation with temperature and pressure. The Imperial Standard Conditions are 60°F, 30inHg, saturated (15.56°C, 1913.7405 mbar, saturated). Gas Tariff sales are not normally corrected, but sales on a contract basis are. Correction may be for pressure only on a fixed factor basis based on Boyle s Law or, for larger loads, over 190,000 therms per annum for both temperature and pressure using electronic (formerly mechanical) correctors. For high pressures, the compressibility factor Z may also be relevant. The current generation of correctors corrects for pressure on an absolute basis taking into account barometric pressure. 

It may not be well known that the weather bureau reports of barometric pressure are corrected to sea level even though describing conditions in the mountains, etc. Table IV gives the barometric pressure at various altitudes these should be used when applicable in equations 5 and 6. Representative cities and the variations in their respective altitudes are given in Table V. 

Observe and record the barometric pressure at the time of the tests. When the pressure differs from 760mm Hg, correct the fl p... 

The lack of chemical interactions in the noble gases makes them ideal gases, and as such their solubility in water is directly proportional to their partial pressure in air. This, in turn, depends on the barometric pressure, which is linearly correlated to the altitude. The higher the altitude, the less noble gases dissolve in water. This relation is expressed in Fig. 13.2. The correction factor, read on the right vertical axis in Fig. 13.2, is multiplied by... 

Compute the correction factors for the forced-draft fan. Commercial fan-capacity tables are based on fans handling standard air at 70°F at a barometric pressure of29.92 inHg and having a density... 

Apply this correction to all flash point determinations between 200 and 325 F (93.3 and 168C). Round off corrections to the nearest yhole number according to Recommended Practice E 29 Note 8 The calibration procedure provided in this method will cancel out the effect of barometric pressure if calibration and tests are run at same pressure... 

The determination of the boiling point has already been mentioned briefly in the discussion on distillation, and here we shall deal with the corrections which may be necessary. It is well known that the boiling point is dependent on the pressure and therefore the pressure must be taken into account in an accurate boiling point determination. This correction is not very large, not exceeding about 2°C., at barometric pressures above 700 mm. A larger error under some conditions... 

Repeat the above procedure for each of the gases to be studied. Also record the temperature and the corrected barometric pressure p2- Record at least approximate values for all the pertinent apparatus factors—L, r, V. 

AH = moisture-content correction of air when barometric pressure differs from standard barometer, gr/lb dry air... 

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