Altitude calculators

  •   ALTITUDE & PRESSURE...    'a' This option computes an estimate of mean atmospheric pressure as a function of altitude, or vice versa.  It is based on a polynomial approximation of the International Standard Atmosphere equation, obtained from the Smithsonian Meteorological Tables (as a rough approximation, pressure decreases by 50% for every 5500 meter increase in altitude).  The results should be accurate within 1-2% of actual pressure or altitude, unless weather conditions are really unusual.  Nevertheless, if you use this calculator, you need to keep a few caveats in mind:

  •       ALTITUDE SIMULATION...      This computes the concentration of an artificial gas mixture used to simulate gas concentration at a different altitude or pressure.   For example, it might be desirable to simulate the hypoxia at high altitude in a near-sea-level laboratory; this is done by using an artificial gas mixture with an oxygen concentration lower than the standard 20.95% atmospheric concentration: e.g., to simulate the oxygen partial pressure at 3,800 meters in a laboratory at sea level, one would use a gas mixture containing about 13% oxygen.
         The calculator allows estimations from different altitudes (in which case it estimates ambient pressures from a polynomial approximation of the International Standard Atmosphere equation, obtained from the Smithsonian Meteorological Tables).   It also allows estimation directly from user-entered ambient pressures -- this permits adjustments for weather or other local phenomena that shift ambient barometric pressures.

  •   pO2 and pH2O ESTIMATION...    'o'   You can use this calculator to determine the partial pressure of oxygen (pO2) -- or any other gas species in a mixture -- from ambient temperature, ambient pressure (in the gas phase), fractional concentration of the gas species of interest in a dry gas mix, and the percent saturation of water vapor (i.e., relative humidity) in the gas phase. Oxygen (or other gases) are diluted by water vapor, and the degree of that dilution depends on RH and temperature.

    In the example at right, pressure is sea level standard atmospheric pressure (760 torr), temperature is the typical mammalian body temperature (37 °C), etc.  Note that at this temperature the saturation vapor pressure of water is about 47.6 torr (this is not affected by the total pressure in the system).

    Other considerations for this calculator:

  •   DISSOLVED OXYGEN...    'w'   You can use this calculator to determine the amount of oxygen dissolved in a given volume water, as a function of partial pressure, temperature, salinity (dissolved solutes), and other factors.   Results include both the oxygen content per liter, and the total oxygen content in the specified volume of water.  Output units are user-selectable as ml O2 (STP) or mg O2 .
          The default salinity value (2 parts/thousand) is reasonable for freshwater.   For seawater, use a value of 35 parts/thousand, and for typical physiological saline, use 9 parts/thousand.

    In the example at right, pressure is sea level standard atmospheric pressure (760 torr), temperature 10 °C, the water is fully saturated with oxygen, and there are 2 parts/thousand of dissolved solutes (reasonable for fairly fresh water).  The calculator provides the dissolved oxygen per unit volume, and for the total volume.


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