Altimeter

An altimeter or altimeters (latin altus, high ' and ancient Greek. Μέτρον métron, measure, scale ') is a device by which the height of an object above a defined level or reference surface is measured. The level measurement can barometrically, done with sound, with microwaves ( radar) or with laser beams. The term is used in various applications and technical areas:

Mountain climbing, hiking, orienteering

Such altimeters are barometers that show instead of air pressure height above sea level. Most of the hand makes one revolution per 1000 m; the km value appears in a small window (usually measuring range 5 or 8 km). The accuracy is 2-20 meters, when a correct output level or the pressure at sea level ( geoid ) has been set.

Digital Altimeter

Digital altimeter used in the above applications frequently. They show, in addition to differences in height, maximum values ​​or the time course of the passage of different heights. For a precise digital altimeter, a calibrated and temperature compensated pressure sensor. Typically, these sensors provide a suitable data interface ( SPI microcontroller; Serial or USB interface for PC). The accuracy good barometric sensors is less than 1 m ( at sea level ). GPS can also be used as an altimeter.

Skydiving

An analog skydiving altimeter is an airtight metal box, similar to a barometer, depending on the ambient air pressure more or less the contracts or diverges. This deformation is transmitted via a mechanical lever mechanism to a pointer, which indicates the appropriate height on a scale. It is thus not the absolute amount displayed above sea level, but only the ambient air pressure. When air pressure fluctuations can be observed a change of the altitude of a barometric altimeter.

The altimeter will be used by the parachutist at the back of the hand or on a chest pad. The altimeter usually has a scale of 0 to 4,000 m or 6,000 m or a corresponding scale reading in feet. Before the start of the altimeter is set to 0 m square height. If is assumed to land on another place than on the starts, the differential height of the intended landing site is set. The parachutist shall use the altimeter when to open the parachute (limited height).

Very often parachutists have an opening machines in addition to the harness. This continuously measures electronically the amount and calculates the rate of fall. Join this that the jumpers in a defined height has too high a speed, the computer ahead of an emergency situation and initiates the opening of reserve parachute.

An additional audible altimeter is a supplement to the optical altimeter, which are defined in different heights warning sounds.

Geodesy, navigation

Instruments as above, but more accurate. By calibrating means of temperature or pressure gradient accuracies are possible up to a few decimeters.

• See cover protractor.

Aviation

→ Main article: Barometric altitude measurement in aviation

All aircraft measure their flight altitude based on the external air pressure, ie barometric. The indicator scale is usually calibrated in feet. Only in Russia, in France, in some gliders and ultralight aircraft, the height is given in meters.

To accurately display the altitude of each pilot 's altimeter has to adjust to the current meteorological air pressure before starting. Either he shall the altimeter so that it displays the ( known to him ) height of the airfield, or if he asks for the current reference air pressure at the flight line. The PMR - abbreviation for this is QNH. A set to QNH altimeter shows the altitude above sea level ( MSL, English mean sea level ) to. Divergences arising from deviating from the ISA temperature. This should be considered at low temperatures of pilots mathematically, for example, to ensure the required obstacle clearance.

Above the transition altitude are all barometric altimeter to atmospheric pressure, ie 1013.25 hPa or 29.92 inHg set. The thus measured flight altitudes do not represent absolute heights more, but relative heights above the calculated normal pressure surface. They are known as flight levels. Example: An airplane is flying at flight level 120 when his altimeter is set to the reference pressure 1013.25 hPa, indicating an altitude of 12,000 feet with this setting.

In addition to the barometric altimeters are available in most aircraft variometer to indicate the ascent or descent rate.

Aircraft that can land in very bad weather conditions have, such as large commercial aircraft are in addition to barometric also equipped with a radar altimeter, which sends a radar signal towards the ground and determined from the reflection of the height above ground. The radio altimeter is an aid in the landing, in cruising flight only applies to the barometric height measured.

By comparing print and radio altimeter can be high and low pressure areas and locate the drift caused by crosswinds calculate ( Meteorological navigation ).