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.