Final answer:
Aircraft altimeters determine the pressure altitude by dialing in a reference pressure setting, commonly the standard sea level pressure. These devices work similarly to mercury barometers, where the height of the mercury column is correlated with atmospheric pressure, which changes with altitude.
Step-by-step explanation:
Aircraft altimeters operate based on the pressure-height relationship and can determine the pressure altitude by dialing in a reference pressure setting. This setting is usually the standard sea level pressure, which is 29.92 inches of mercury (inHg) or 1013.25 millibars (hPa). The principle behind this is similar to how a mercury barometer works.
A barometer is a device that measures atmospheric pressure. The classic mercury barometer, invented by Evangelista Torricelli, is constructed from a glass tube closed at one end and filled with mercury. When the tube is inverted and placed in a pool of mercury, the height of the mercury column 'h' correlates with the atmospheric pressure, applying the formula hpg = Patm. In an aircraft altimeter, this pressure reading is used to calculate altitude, since atmospheric pressure decreases with increasing altitude.
The altimeter can be set to reflect the standard pressure or an actual pressure obtained from a weather station. When the pilot adjusts the altimeter setting, the device is calibrated to show the height above sea level, taking into account the current atmospheric pressure conditions.