The device known as an altimeter is most commonly used in aircraft to ascertain the height, or altitude of the aircraft above sea level. Variations in atmospheric pressure are measured, and interpreted by an instrument to give the pilot an accurate idea of altitude.
How a traditional altimeter works
An altimeter works on the hydrostatic principle of atmospheric pressure, whereby the pressure of a gas at a constant temperature, in this case air, decreases at an exponential rate as altitude increases. Air decreases to half of its sea-level pressure at 18,000 feet, and down to a third at 27,000 feet. In order to measure these variations in pressure, aircraft have a barometer fixed to the outside fuselage.
The instrument is calibrated with the barometric formula, so that as the outside air pressure drops with the increase in altitude, the instrument is able to give the pilot a reading of the altitude. The instrument is calibrated with the altitude above sea level, and the atmospheric pressure before take off, to ensure the instrument is accurate.
How the instrument is interpreted
In traditional altimeters, the instrument takes the form of a clock face, with three needles, and a scale of 1-10. Depending on the units (modern aviation is standardised to feet), the largest hand will show hundreds of feet, the medium hand thousands of feet, and the smallest tens of thousands of feet.
By taking the reading of the smallest needle, and then adding the two larger ones, the pilot may quickly ascertain the altitude above sea level. For instance, if the smallest needle shows 2, the medium needle 3, and the largest 4, the altitude will be 23,400 feet.
With the advent of ‘glass cockpits’ or digital instrument displays, altimeters have become computerised. This means that the information from the external barometer is interpreted by a computer, with variations in temperature and atmospheric pressure at sea level automatically taken into consideration for a more accurate reading. This reading is shown as a numerical readout, making interpretation easier, and also making finer readings of altitude possible.
Furthermore, radar altimeters are often used in conjunction with a standard barometric altimeter. These use the Doppler principle to ascertain altitude above the terrain as local topography changes. This is especially useful in military aircraft, and in low-visibility conditions.