Pressure altimeter Altimeter

1 pressure altimeter

1.1 use in hiking, climbing , skiing
1.2 skydiving
1.3 use in aircraft

pressure altimeter

digital barometric pressure sensor altitude measurement in consumer electronic applications

altitude can determined based on measurement of atmospheric pressure. greater altitude, lower pressure. when barometer supplied nonlinear calibration indicate altitude, instrument called pressure altimeter or barometric altimeter. pressure altimeter altimeter found in aircraft, , skydivers use wrist-mounted versions similar purposes. hikers , mountain climbers use wrist-mounted or hand-held altimeters, in addition other navigational tools such map, magnetic compass, or gps receiver.

the calibration of altimeter follows equation

z
=
c

t

log
⁡
(

p

o



/

p
)
,


{\displaystyle z=c\;t\;\log(p_{o}/p),}

where c constant, t absolute temperature, p pressure @ altitude z, , po pressure @ sea level. constant c depends on acceleration of gravity , molar mass of air. however, 1 must aware type of altimeter relies on density altitude , readings can vary hundreds of feet owing sudden change in air pressure, such cold front, without actual change in altitude.

use in hiking, climbing , skiing

a barometric altimeter, used along topographic map, can verify 1 s location. more reliable, , more accurate, gps receiver measuring altitude; gps signal may unavailable, example, when 1 deep in canyon, or may give wildly inaccurate altitudes when available satellites near horizon. because barometric pressure changes weather, hikers must periodically re-calibrate altimeters when reach known altitude, such trail junction or peak marked on topographical map.

skydiving

digital wrist-mounted skydiving altimeter in logbook mode, displaying last recorded jump profile.



skydiver in free fall, making use of hand-mounted altimeter. analogue face visible, showing colour-coded decision altitudes. depicted altimeter electronic, despite using analogue display.

an altimeter important piece of skydiving equipment, after parachute itself. altitude awareness crucial @ times during jump, , determines appropriate response maintain safety.

since altitude awareness important in skydiving, there wide variety of altimeter designs made use in sport, , non-student skydiver typically use 2 or more altimeters in single jump:

hand, wrist or chest-mounted mechanical analogue visual altimeters. basic , common type, , used (and commonly mandated for) virtually student skydivers. common design has face marked 0 4000m (or 0 12000 ft, mimicking clock face), on arrow points current altitude. face plate sports sections prominently marked yellow , red respectively, signifying recommended deployment altitude, emergency procedure decision altitude (commonly known hard deck ). mechanical altimeter has knob needs manually adjusted make point 0 on ground before jump, , if landing spot not @ same altitude takeoff spot, user needs adjust appropriately. advanced electronic altimeters available make use of familiar analogue display, despite internally operating digitally.
digital visual altimeters, mounted on wrist or hand. type operates electronically, , conveys altitude number, rather pointer on dial. since these altimeters contain electronic circuitry necessary altitude calculation, commonly equipped auxiliary functions such electronic logbook, real-time jump profile replay, speed indication, simulator mode use in ground training, etc. electronic altimeter activated on ground before jump, , calibrates automatically point 0. essential user not turn on earlier necessary avoid, example, drive dropzone located @ different altitude 1 s home cause potentially fatal false reading. if intended landing zone @ different elevation takeoff point, user needs input appropriate offset using designated function.
audible altimeters (also known dytters , genericised trademark of first such product on market). these inserted 1 s helmet, , emit warning tone @ predefined altitude. contemporary audibles have evolved crude beginnings, , sport vast array of functions, such multiple tones @ different altitudes, multiple saved profiles can switched quickly, electronic logbook data transfer pc later analysis, distinct free fall , canopy modes different warning altitudes, swoop approach guiding tones, etc. audibles strictly auxiliary devices, , not replace, complement visual altimeter remains primary tool maintaining altitude awareness. advent of modern skydiving disciplines such freeflying, in ground might not in 1 s field of view long periods of time, has made use of audibles universal, , virtually skydiving helmets come 1 or more built-in ports in audible might placed. audibles not recommended , banned use student skydivers, need build proper altitude awareness regime themselves.
auxiliary visual altimeters. these not show precise altitude, rather maintain general indicator in 1 s peripheral vision. might either operate in tandem audible equipped appropriate port, in case emit warning flashes complementing audible tones, or standalone , use display mode, such showing either green or red light depending on altitude.

the exact choice of altimeters depends heavily on individual skydiver s preferences, experience level, primary disciplines, type of jump. on 1 end of spectrum, low-altitude demonstration jump water landing , no free fall might waive mandated use of altimeters , use none @ all. in contrast, jumper doing freeflying jumps , flying high performance canopy might use mechanical analogue altimeter easy reference in free fall, in-helmet audible breakaway altitude warning, additionally programmed swoop guide tones canopy flying, digital altimeter on armband glancing precise altitude on approach. skydiver doing similar types of jumps might wear digital altimeter primary visual one, preferring direct altitude readout of numeric display.

use in aircraft

schematic of drum-type aircraft altimeter, showing small kollsman windows @ bottom left , bottom right of face.

in aircraft, aneroid barometer measures atmospheric pressure static port outside aircraft. air pressure decreases increase of altitude—approximately 100 hectopascals per 800 meters or 1 inch of mercury per 1000 feet near sea level.

the aneroid altimeter calibrated show pressure directly altitude above mean sea level, in accordance mathematical model atmosphere defined international standard atmosphere (isa). older aircraft used simple aneroid barometer needle made less 1 revolution around face 0 full scale. design evolved altimeters primary needle , 1 or more secondary needles show number of revolutions, similar clock face. in other words, each needle points different digit of current altitude measurement. design has fallen out of favor due risk of misreading in stressful situations. design evolved further drum-type altimeters, final step in analogue instrumentation, each revolution of single needle accounted 1,000 feet, thousand foot increments recorded on numerical odometer-type drum. determine altitude, pilot had first read drum determine thousands of feet, @ needle hundreds of feet. modern analogue altimeters in transport aircraft typically drum-type. latest development in clarity electronic flight instrument system integrated digital altimeter displays. technology has trickled down airliners , military planes until standard in many general aviation aircraft.

modern aircraft use sensitive altimeter . on sensitive altimeter, sea-level reference pressure can adjusted setting knob. reference pressure, in inches of mercury in canada , united states, , hectopascals (previously millibars) elsewhere, displayed in small kollsman window, on face of aircraft altimeter. necessary, since sea level reference atmospheric pressure @ given location varies on time temperature , movement of pressure systems in atmosphere.

diagram showing internal components of sensitive aircraft altimeter.

in aviation terminology, regional or local air pressure @ mean sea level (msl) called qnh or altimeter setting , , pressure calibrate altimeter show height above ground @ given airfield called qfe of field. altimeter cannot, however, adjusted variations in air temperature. differences in temperature isa model accordingly cause errors in indicated altitude.

in aerospace, mechanical stand-alone altimeters based on diaphragm bellows replaced integrated measurement systems called air data computers (adc). module measures altitude, speed of flight , outside temperature provide more precise output data allowing automatic flight control , flight level division. multiple altimeters can used design pressure reference system provide information airplane s position angles further support inertial navigation system calculations.