ALTIMETER

instrument that measures the altitude of the land surface or any object such as an airplane. The two main types are the pressure altimeter, which is an indicator of atmospheric pressure, and the radio altimeter, which measures the time required for a pulse of radio energy to travel from an object in the atmosphere to the ground and back. Figure 1: Pressure altimeter A typical pressure altimeter is illustrated in Figure 1. The instrument is enclosed in a case that is connected to the outside of the aircraft by an air pressure inlet at the rear of the housing. Two or more aneroid capsulesi.e., thin corrugated metallic bellows from which air has been exhaustedare positioned near the inlet. These capsules expand when the outside air pressure falls (as in climbing), and contract when the outside air pressure rises (as in descending). By a mechanical arrangement of sector gears, pinion, backlash spring, and crankshaft, the expansion or contraction of the aneroid capsules is converted to the movement of pointers on a dial. The graduated scale dial is marked off in feet (or metres), and a series of pointers may be used to indicate the altitude in units of hundreds, thousands, or ten thousands. The barometric scale dial records the air pressure in millibars (mb). Because atmospheric pressure is measured relative to sea level, a pressure altimeter must be adjusted with a barosetting knob in order to compensate for variations in barometric pressure caused by weather changes. Figure 2: Graph plotting altitude vs. pressure As atmospheric pressure decreases with atltitude, a pressure versus altitude relationship exists, and any altitude can be determined by measuring the pressure associated with it. In Figure 2, for example, if the pressure reading on the Boeing 747's altimeter is 194.0 mb, then the aircraft is flying at a height of about 12 km. The radio altimeter measures the distance of an aircraft above the ground rather than above sea level. A cathode-ray tube indicates the time that a pulse of radio energy takes to travel from the aircraft to the ground and back to the aircraft. The altitude is equal to one-half the time multiplied by the speed of the pulse. Radio altimeters are used in automatic navigation and blind-landing systems. Another device for measuring altitude utilizes the effect of atmospheric pressure on the boiling point of a liquid. The instrument, known as a hypsometer, consists of a cylindrical vessel in which the liquid (usually water) is boiled, surrounded by a jacketed column through which the vapour circulates around a shielded thermometer; the higher the altitude, the lower the pressure and the lower the temperature at which the liquid boils. The boiling temperature is thus inversely proportional to the altitude.