While travelling in a plane, have you ever looked down and wondered why everything on the earth looked so small? The reason is that planes fly at great heights. The greater the height, the smaller will be the appearance of the objects on the earth. The height of the plane is measured by an instrument called the altimeter.
There are five main types of altimeters. The first is the pressure altimeter. It basically measures the decrease in air pressure as the altitude increases. The second is the radio altimeter. This is used to measure the time required by a radio pulse to travel from the airplane to the ground and back, much like radar. The third type is the sonic altimeter which makes use of sound waves. The fourth type measures the altitude by using the effect of atmospheric pressure on the boiling point of a liquid. The fifth type is the capacitance altimeter.
The pressure altimeter is basically an aneroid barometer. It contains an aneroid capsule made of metallic bellows from which air has been exhausted. The bellows expand or contract as the atmospheric pressure changes. As the atmospheric pressure decreases with altitude, it measures the change in pressure by means of an arrangement of levers and gears. The expansion or contraction of the bellows changes the rotation of pointers relative to a dial. A pressure altimeter has to be adjusted to the air pressure on the ground before each flight. It is also adjusted for variations in atmospheric pressure caused by weather changes.
In a radio altimeter, the radio pulses are sent to the ground. They get reflected from the ground, and are picked up by a receiver in the aircraft. The time taken by the radio pulse to travel from the aircraft to the ground and back is measured. Half of this time, when multiplied by the speed of the radio pulse, gives the altitude of the plane. These types of altimeters are very accurate and hence are called absolute altimeters. They are used in automatic navigation and in blind landing systems.
The third type of altimeter makes use of sound waves instead of radio pulses. This is called sonic altimeter. It sends sound waves from the aircraft to the grounds which bounce back. The time taken by the sound waves, multiplied with the velocity of sound in air, gives double the height of the airplane. Half of this distance is the real altitude of the plane.
The fourth type of altimeter makes use of 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 water is boiled. This is surrounded by a jacket through which the water-vapour circulates around a thermometer. The higher the altitude, the lower the pressure, and consequently lower the temperature at which the liquid boils. The boiling temperature is thus inversely proportional to the altitude. Hence by measuring the boiling point of the liquid, one can measure the altitude of the airplane.
A capacitance altimeter indicates altitude by measuring the difference between electrical changes of the earth and the plane. Altimeters are thus very useful instruments for guiding and controlling aircrafts.