What is an Altimeter?
An altimeter is a altitude measurement device used by mountaineers, skydivers, pilots, satellites, and many others. The altimeter’s general purpose is to tell its human or machine user how elevated from sea level it is. It performs this function via one of two ways – either by measuring the local air pressure using a barometer, or by bouncing radar off a target. It has been known for several centuries that a thin column of mercury attached to an open mercury reservoir can be used to measure atmospheric pressure. Tiny changes in atmospheric pressure are indicated by the level of the mercury column. This common instrument is called a barometer. At altitudes relatively close to sea level, air pressure varies uniformly with altitude. Every 27 feet (8.23 m) one ascends above sea level, air pressure increases by about a millibar, a unit of pressure that can be measured in inches of mercury. Hence, a barometer can be used as an altimeter. Because it is dependent on air pressure varying a unif
The most basic definition would include reference to an instrument that is active in that it measures altitude and the measurement changes as the person moves closer to or farther from a fixed point (the earth, in this case). That just about covers the importance of altimeters for skydivers. One common and much-used type of altimeter is the pressure or barometric altimeter. The device works because air pressure in the atmosphere decreases with altitude. The altimeter is designed to show this air pressure as a distance above sea level. With this information in mind, why do skydivers use altimeters, exactly? Let the experts answer this question. To paraphrase www.patentstorm.us, in free fall, the altitude above ground is key in deciding when to open a parachute at sufficient altitude to allow for jettisoning of the parachute. In case the main parachute does not open properly, the reserve chute must have enough altitude. In formation flying, when many people may be falling close to one an
Let’s start with a barometer. A barometer gives the atmospheric pressure at its location. An altimeter is just a complicated barometer; if you make an altimeter read zero feet (sorry, air traffic controllers use feet for height – although visibility is in metres and km!), it will give you the atmospheric pressure (pressure) at that position (see alt_zero.jpg). At military airfields, the Met Office take the pressure and, using some complicated formula, work out the pressure on the runway where the aircraft will land (as opposed to one of the other runways). In Q code, this is known as the QFE. (During the Second World War, a three-letter code, known as the Q code because each element began with the letter Q, was used to transmit radio signals between aircraft and air traffic control in Morse. Some of these codes are still in use today, even though the reason why particular letters were selected is no longer known – if there even was a reason in the first place.) Going back to our altime
