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The reference value for P b for b = 0 is the defined sea level value, P 0 = 101 325 Pa or 29.92126 inHg. Values of P b of b = 1 through b = 6 are obtained from the application of the appropriate member of the pair equations 1 and 2 for the case when h = h b +1 .
Old altimeters were typically limited to displaying the altitude when set between 950 mb and 1030 mb. Standard pressure, the baseline used universally, is 1013.25 hectopascals (hPa), which is equivalent to 1013.25 mb or 29.92 inches of mercury (inHg). This setting is equivalent to the atmospheric pressure at mean sea level (MSL) in the ISA
The graph on the right above was developed for a temperature of 15 °C and a relative humidity of 0%. At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h:
Comparison of a graph of International Standard Atmosphere temperature and pressure and approximate altitudes of various objects and successful stratospheric jumps The International Standard Atmosphere ( ISA ) is a static atmospheric model of how the pressure , temperature , density , and viscosity of the Earth's atmosphere change over a wide ...
Thus the standard consists of a tabulation of values at various altitudes, plus some formulas by which those values were derived. To allow modeling conditions below mean sea level , the troposphere is actually extended to −2,000 feet (−610 m), where the temperature is 66.1 °F (18.9 °C), pressure is 15.79 pounds per square inch (108,900 Pa ...
The calibration of an altimeter follows the equation = (/), [1] where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and P o is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air.
For example, IUPAC has, since 1982, defined standard reference conditions as being 0 °C and 100 kPa (1 bar), in contrast to its old standard of 0 °C and 101.325 kPa (1 atm). [2] The new value is the mean atmospheric pressure at an altitude of about 112 metres, which is closer to the worldwide median altitude of human habitation (194 m).
1–10 kPa Typical explosion peak overpressure needed to break glass windows (approximate) [40] 2 kPa Pressure of popping popcorn (very approximate) [41] [42] 2.6 kPa 0.38 psi Pressure at which water boils at room temperature (22 °C) (20 mmHg) [43] 5 kPa 0.8 psi Blood pressure fluctuation (40 mmHg) between heartbeats for a typical healthy ...