﻿ Altitude calculators
 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:

• Due to approximation (and rounding errors), the algorithm is not completely reversible (for example, the estimated pressure for an altitude of 2200 meters is 576.0 torr, but the estimated altitude for a pressure of 576.0 torr is 2199.7 meters).

• The results are estimated mean values and do not account for local pressure fluctuations arising from weather changes.

• The calculator cannot account for any pressure differential inside your system.  For example, if you use a positive-pressure ("push") respirometry system, the pressure inside the plumbing -- and hence, inside your flow meter -- will be slightly higher than atmospheric.  The opposite is true for a negative-pressure ("pull") flow arrangement.  If you want to be compulsive, you might consider using a water manometer or some other pressure gauge to measure the pressure differential and add that to atmospheric pressure during STP calculations.  In most cases, the pressure differential will be small relative to total atmospheric pressure.

• Accuracy will decrease for altitudes above about 9000 meters (or pressures less than about 230 torr).

• The newest versions let you use kilopascals (kPa) for pressure units, in addition to torr.

• The 'store pressure' button will save the current value of barometric pressure.

•       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:

• The calculated pO2 value is applicable for the gas phase, and also for dissolved oxygen, as long as the solution is fully saturated with O2
• The default pressure (torr or kilopascals, kPa) is obtained from the current data file; the default temperature is 37 °C, and the default fractional gas concentration is .2095 (20.95%, the normal oxygen content of dry atmospheric air).

•   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.