Pressure altimeter

Last updated November 02, 2024

Digital barometric pressure sensor for altitude measurement in consumer electronic applications Digital-barometric-pressure-sensor.jpg — Digital barometric pressure sensor for altitude measurement in consumer electronic applications

Altitude can be determined based on the measurement of atmospheric pressure. The greater the altitude, the lower the pressure. When a barometer is supplied with a nonlinear calibration so as to indicate altitude, the instrument is a type of altimeter called a pressure altimeter or barometric altimeter. A pressure altimeter is the altimeter found in most aircraft, and skydivers use wrist-mounted versions for similar purposes. Hikers and mountain climbers use wrist-mounted or hand-held altimeters, in addition to other navigational tools such as a map, magnetic compass, or GPS receiver.

Calibration

The calibration of an altimeter follows the equation

z=c\;T\;\log(P_{o}/P),

^[1]

where c is a constant, T is the absolute temperature, P is the pressure at altitude z, and P_o is the pressure at sea level. The constant c depends on the acceleration of gravity and the molar mass of the air. However, one must be aware that this type of altimeter relies on "density altitude" and its readings can vary by hundreds of feet owing to a sudden change in air pressure, such as from a cold front, without any actual change in altitude.^[2]

The most common unit of measurement used for altimeter calibration worldwide is hectopascals (hPa), except for North America (other than Canada^[3] ) and Japan where inches of mercury (inHg) are used.^[4] To obtain an accurate altitude reading in either feet or meters, the local barometric pressure must be calibrated correctly using the barometric formula.

History

The scientific principles behind the pressure altimeter were first written by Rev. Alexander Bryce, a Scottish minister and astronomer in 1772 who realised that the principles of a barometer could be adjusted to measure height.^[5]

Applications

Use in hiking, climbing and skiing

A barometric altimeter, used along with a topographic map, can help to verify one's location. It is more reliable, and often more accurate, than a GPS receiver for measuring altitude; the GPS signal may be unavailable, for example, when one is deep in a canyon, or it may give wildly inaccurate altitudes when all available satellites are near the horizon. Because barometric pressure changes with the weather, hikers must periodically re-calibrate their altimeters when they reach a known altitude, such as a trail junction or peak marked on a topographical map.

Skydiving

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

An altimeter is the most important piece of skydiving equipment, after the parachute itself. Altitude awareness is crucial at all times during the jump, and determines the appropriate response to maintain safety.

Since altitude awareness is so important in skydiving, there is a wide variety of altimeter designs made specifically for use in the sport, and a non-student skydiver will typically use two or more altimeters in a single jump:^[6]

Hand, wrist or chest-mounted mechanical analogue visual altimeters are the most basic and common devices, and are used by (and commonly mandated for) virtually all student skydivers. The common design has a face marked from 0 to 4000 m (or 0 to 12000 ft, mimicking the clock face), with an arrow pointing to the current altitude. The face plate has sections prominently marked with yellow and red, signifying the recommended deployment altitude and the emergency procedure decision altitude (commonly known as "hard deck") respectively. A mechanical altimeter has a knob that needs to be manually adjusted to make it point to 0 on the ground before jump, and if the landing spot is not at the same altitude as the takeoff spot, the user needs to adjust it appropriately. Some advanced electronic altimeters are also available which make use of the familiar analogue display, despite internally operating digitally.
Digital visual altimeters are mounted on the wrist or hand. This type always operates electronically, and conveys the altitude as a number, rather than a pointer on a dial. Since these altimeters already contain all the electronic circuitry necessary for altitude calculation, they are commonly equipped with auxiliary functions such as electronic logbook, real-time jump profile replay, speed indication, simulator mode for use in ground training, etc. An electronic altimeter is activated on the ground before the jump, and calibrates automatically to point to 0. It is thus essential that the user not turn it on earlier than necessary to avoid, for example, the drive to a dropzone located at a different altitude than one's home which could cause a potentially fatal false reading. If the intended landing zone is at a different elevation than the takeoff point, the user needs to input the appropriate offset by using a designated function.
Audible altimeters (also known as "dytters", a genericised trademark of the first such product on the market) are inserted into one's helmet, and emit a warning tone at a predefined altitude. Contemporary audibles have evolved significantly from their crude beginnings, and sport a vast array of functions, such as multiple tones at different altitudes, multiple saved profiles that can be switched quickly, electronic logbook with data transfer to a PC for later analysis, distinct free fall and canopy modes with different warning altitudes, swoop approach guiding tones, etc. Audibles are strictly auxiliary devices, and do not replace, but complement a visual altimeter which remains the primary tool for maintaining altitude awareness. The advent of modern skydiving disciplines such as freeflying, in which the ground might not be in one's field of view for long periods of time, has made the use of audibles nearly universal, and virtually all skydiving helmets come with one or more built-in ports in which an audible might be placed. Audibles are not recommended and often banned from use by student skydivers, who need to build up a proper altitude awareness regime for themselves.
Auxiliary visual altimeters do not show the precise altitude, but rather help maintain a general indicator in one's peripheral vision. They might either operate in tandem with an audible equipped with an appropriate port, in which case they emit warning flashes complementing the audible tones, or be standalone and use another display mode, such as showing either green or red light depending on the altitude.^{[ citation needed ]}

Speaking altimeter with helmet for skydiving Speaking-Altimeter.jpg — Speaking altimeter with helmet for skydiving

Speaking altimeters (also known as voice altimeters) combine both audible and visual altimeter functions. Units of this type operate at all altitudes experienced in skydiving, and announce their altitude as a number in the diver's native language. These are inserted into helmet like audible altimeters, and have automatic volume adjustment dependent on the skydiver's speed. The main goal of this type of altimeter is to ensure that skydivers always know their position; this feature is useful for experienced skydivers as well as formation skydiving load organizers or accelerated freefall instructors.^{[ citation needed ]}

The exact choice of altimeters depends heavily on the individual skydiver's preferences, experience level, primary disciplines, as well as the type of the jump.^[7] On one end of the spectrum, a low-altitude demonstration jump with water landing and no free fall might waive the mandated use of altimeters and use none at all. In contrast, a jumper doing freeflying jumps and flying a high performance canopy might use a mechanical analogue altimeter for easy reference in free fall, an in-helmet audible for breakaway altitude warning, additionally programmed with swoop guide tones for canopy flying, as well as a digital altimeter on an armband for quickly glancing the precise altitude on approach. Another skydiver doing similar types of jumps might wear a digital altimeter for their primary visual one, preferring the direct altitude readout of a numeric display.

Use in aircraft

A drum-type aircraft altimeter, showing the small Kollsman windows at the bottom left (hectopascals) and bottom right (inches of mercury) of the face. Drum-Altimeter.png — A drum-type aircraft altimeter, showing the small *Kollsman windows* at the bottom left (hectopascals) and bottom right (inches of mercury) of the face.

In aircraft, an aneroid altimeter or aneroid barometer measures the atmospheric pressure from a static port outside the aircraft. Air pressure decreases with an increase of altitude—approximately 100 hectopascals per 800 meters or one inch of mercury per 1000 feet or 1 hectopascals per 30 feet near sea level.

The aneroid altimeter is calibrated to show the pressure directly as an altitude above mean sea level, in accordance with a mathematical model atmosphere defined by the International Standard Atmosphere (ISA). Older aircraft used a simple aneroid barometer where the needle made less than one revolution around the face from zero to full scale. This design evolved to three-pointer altimeters with a primary needle and one or more secondary needles that show the number of revolutions, similar to a clock face. In other words, each needle points to a different digit of the current altitude measurement. However, this design has fallen out of favor due to the risk of misreading in stressful situations. The design evolved further to drum-type altimeters, the final step in analogue instrumentation, where each revolution of a single needle accounted for 1,000 feet (300 metres), with thousand foot increments recorded on a numerical odometer-type drum. To determine altitude, a pilot had first to read the drum to determine the thousands of feet, then look at the needle for the hundreds of feet. Modern analogue altimeters in transport aircraft are typically drum-type. The latest development in clarity is an Electronic flight instrument system with integrated digital altimeter displays. This technology has trickled down from airliners and military planes until it is now standard in many general aviation aircraft.

Modern aircraft use a "sensitive altimeter". On a sensitive altimeter, the sea-level reference pressure can be adjusted with a setting knob. The reference pressure, in inches of mercury in Canada and the United States, and hectopascals (previously millibars) elsewhere, is displayed in the small Kollsman window,^[9] on the face of the aircraft altimeter. This is necessary, since sea level reference atmospheric pressure at a given location varies over time with temperature and the movement of pressure systems in the atmosphere.

In aviation terminology, the regional or local air pressure at mean sea level (MSL) is called the QNH or "altimeter setting", and the pressure that will calibrate the altimeter to show the height above ground at a given airfield is called the QFE of the field. An altimeter cannot, however, be adjusted for variations in air temperature. Differences in temperature from the ISA model will accordingly cause errors in indicated altitude.

In aerospace, the mechanical stand-alone altimeters which are based on diaphragm bellows were replaced by integrated measurement systems which are called air data computers (ADC). This module measures altitude, speed of flight and outside temperature to provide more precise output data allowing automatic flight control and flight level division. Multiple altimeters can be used to design a pressure reference system to provide information about the airplane's position angles to further support inertial navigation system calculations.

Pilots can perform preflight altimeter checks by setting the barometric scale to the current reported altimeter setting. The altimeter pointers should indicate the surveyed field elevation of the airport.^[10] Federal Aviation Administration requires that if the indication is off by more than 75 ft (23 m) from the surveyed field elevation, the instrument should be recalibrated.^[11]

Other modes of transport

The altimeter is an instrument optional in off-road vehicles to aid in navigation. Some high-performance luxury cars that were never intended to leave paved roads, such as the Duesenberg in the 1930s, have also been equipped with altimeters.

Related Research Articles

<span class="mw-page-title-main">Pressure measurement</span>

Pressure measurement is the measurement of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure mechanically are called pressure gauges,vacuum gauges or compound gauges. The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

An altimeter or an altitude meter is an instrument used to measure the altitude of an object above a fixed level. The measurement of altitude is called altimetry, which is related to the term bathymetry, the measurement of depth under water.

Atmospheric pressure, also known as air pressure or barometric pressure, is the pressure within the atmosphere of Earth. The standard atmosphere is a unit of pressure defined as 101,325 Pa (1,013.25 hPa), which is equivalent to 1,013.25 millibars, 760 mm Hg, 29.9212 inches Hg, or 14.696 psi. The atm unit is roughly equivalent to the mean sea-level atmospheric pressure on Earth; that is, the Earth's atmospheric pressure at sea level is approximately 1 atm.

A barometer is a scientific instrument that is used to measure air pressure in a certain environment. Pressure tendency can forecast short term changes in the weather. Many measurements of air pressure are used within surface weather analysis to help find surface troughs, pressure systems and frontal boundaries.

<span class="mw-page-title-main">Altitude</span> Height in relation to a specified reference point

Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context. Although the term altitude is commonly used to mean the height above sea level of a location, in geography the term elevation is often preferred for this usage.

<span class="mw-page-title-main">Flight instruments</span> Aircraft instrument that gives information during flight

Flight instruments are the instruments in the cockpit of an aircraft that provide the pilot with data about the flight situation of that aircraft, such as altitude, airspeed, vertical speed, heading and much more other crucial information in flight. They improve safety by allowing the pilot to fly the aircraft in level flight, and make turns, without a reference outside the aircraft such as the horizon. Visual flight rules (VFR) require an airspeed indicator, an altimeter, and a compass or other suitable magnetic direction indicator. Instrument flight rules (IFR) additionally require a gyroscopic pitch-bank, direction and rate of turn indicator, plus a slip-skid indicator, adjustable altimeter, and a clock. Flight into instrument meteorological conditions (IMC) require radio navigation instruments for precise takeoffs and landings.

<span class="mw-page-title-main">Variometer</span> Flight instrument which determines the aircrafts vertical velocity (rate of descent/climb)

In aviation, a variometer – also known as a rate of climb and descent indicator (RCDI), rate-of-climb indicator, vertical speed indicator (VSI), or vertical velocity indicator (VVI) – is one of the flight instruments in an aircraft used to inform the pilot of the rate of descent or climb. It can be calibrated in metres per second, feet per minute or knots, depending on country and type of aircraft. It is typically connected to the aircraft's external static pressure source.

In aviation, a flight level (FL) is an aircraft's altitude as determined by a pressure altimeter using the International Standard Atmosphere. It is expressed in hundreds of feet or metres. The altimeter setting used is the ISA surface pressure of 1013 hPa or (29.92 inHg). The actual surface pressure may vary from this at different locations and times. Therefore, by using a standard pressure setting, every aircraft has the same altimeter setting, and vertical clearance can be maintained.

Given an atmospheric pressure measurement, the pressure altitude is the imputed altitude that the International Standard Atmosphere (ISA) model predicts to have the same pressure as the observed value.

A barograph is a barometer that records the barometric pressure over time in graphical form. This instrument is also used to make a continuous recording of atmospheric pressure. The pressure-sensitive element, a partially evacuated metal cylinder, is linked to a pen arm in such a way that the vertical displacement of the pen is proportional to the changes in the atmospheric pressure.

The true airspeed of an aircraft is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogue TAS indicator, but as GPS has become available for civilian use, the importance of such air-measuring instruments has decreased. Since indicated, as opposed to true, airspeed is a better indicator of margin above the stall, true airspeed is not used for controlling the aircraft; for these purposes the indicated airspeed – IAS or KIAS – is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370 km/h). Neither provides for accurate speed over the ground, since surface winds or winds aloft are not taken into account.

<span class="mw-page-title-main">Depth gauge</span> Instrument that indicates depth below a reference surface

A depth gauge is an instrument for measuring depth below a vertical reference surface. They include depth gauges for underwater diving and similar applications. A diving depth gauge is a pressure gauge that displays the equivalent depth below the free surface in water. The relationship between depth and pressure is linear and accurate enough for most practical purposes, and for many purposes, such as diving, it is actually the pressure that is important. It is a piece of diving equipment used by underwater divers, submarines and submersibles.

The density altitude is the altitude relative to standard atmospheric conditions at which the air density would be equal to the indicated air density at the place of observation. In other words, the density altitude is the air density given as a height above mean sea level. The density altitude can also be considered to be the pressure altitude adjusted for a non-standard temperature.

In aviation, an electronic flight instrument system (EFIS) is a flight instrument display system in an aircraft cockpit that displays flight data electronically rather than electromechanically. An EFIS normally consists of a primary flight display (PFD), multi-function display (MFD), and an engine indicating and crew alerting system (EICAS) display. Early EFIS models used cathode ray tube (CRT) displays, but liquid crystal displays (LCD) are now more common. The complex electromechanical attitude director indicator (ADI) and horizontal situation indicator (HSI) were the first candidates for replacement by EFIS. Now, however, few flight deck instruments cannot be replaced by an electronic display.

In aviation, atmospheric sciences and broadcasting, a height above ground level is a height measured with respect to the underlying ground surface. This is as opposed to height above mean sea level, height above ellipsoid, or height above average terrain. In other words, these expressions indicate where the "zero level" or "reference altitude" – the vertical datum – is located.

A pitot–static system is a system of pressure-sensitive instruments that is most often used in aviation to determine an aircraft's airspeed, Mach number, altitude, and altitude trend. A pitot–static system generally consists of a pitot tube, a static port, and the pitot–static instruments. Other instruments that might be connected are air data computers, flight data recorders, altitude encoders, cabin pressurization controllers, and various airspeed switches. Errors in pitot–static system readings can be extremely dangerous as the information obtained from the pitot static system, such as altitude, is potentially safety-critical. Several commercial airline disasters have been traced to a failure of the pitot–static system.

A primary flight display or PFD is a modern aircraft instrument dedicated to flight information. Much like multi-function displays, primary flight displays are built around a Liquid-crystal display or CRT display device. Representations of older six pack or "steam gauge" instruments are combined on one compact display, simplifying pilot workflow and streamlining cockpit layouts.

Inch of mercury is a non-SI unit of measurement for pressure. It is used for barometric pressure in weather reports, refrigeration and aviation in the United States.

Parachuting and skydiving are methods of descending from a high point in an atmosphere to the ground or ocean surface with the aid of gravity, involving the control of speed during the descent using a parachute or multiple parachutes.

Altimeter setting is the value of the atmospheric pressure used to adjust the scale of a pressure altimeter so that it indicates the height of an aircraft above a known reference surface. This reference can be the mean sea level pressure (QNH), the pressure at a nearby surface airport (QFE), or the "standard pressure level" of 1,013.25 hectopascals which gives pressure altitude and is used to maintain one of the standard flight levels.

References

↑ Crocker, Graham Jackson, Chris. "The use of altimeters in height measurement". www.hills-database.co.uk. Archived from the original on 25 October 2017. Retrieved 29 April 2018.{{cite web}}: CS1 maint: multiple names: authors list (link)
↑ "How Aircraft Instruments Work." Popular Science, March 1944, p. 118.
↑ "Manual of Surface Weather Observation Standards (MANOBS) 8th Edition, Amendment". canada.ca. Government of Canada. December 2021. Retrieved 2 August 2022. 9.1.3 Units of measurement: The unit of measurement of atmospheric pressure is hectopascals; the corresponding symbol is hPa.
↑ "Aviation's Crazy, Mixed Up Units of Measure - AeroSavvy". 5 September 2014.
↑ "Bryce, (The Rev) Alexander".
↑ "What's a Skydiving Altimeter (and How Does It Work?)". Skydive The Wasatch. Archived from the original on 23 April 2015. Retrieved 2 February 2015.
↑ Hawke, John. "Digital or Analog Altimeter". Dropzone.com. Archived from the original on 6 February 2015. Retrieved 2 February 2015.
↑ "Chapter 8: Flight Instruments". Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C ed.). Federal Aviation Administration. 2023-07-17. pp. 4–5.
↑ "Archived copy". Archived from the original on 2006-06-25. Retrieved 2006-06-15.{{cite web}}: CS1 maint: archived copy as title (link)
↑ "Chapter 8: Flight Instruments". Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C ed.). Federal Aviation Administration. 2023-07-17. p. 7.
↑ "Section 2. Barometric Altimeter Errors and Setting Procedures". Aeronautical Information Manual . Federal Aviation Administration . Retrieved 2023-05-20.

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[1] Crocker, Graham Jackson, Chris. "The use of altimeters in height measurement". www.hills-database.co.uk. Archived from the original on 25 October 2017. Retrieved 29 April 2018.{{cite web}}: CS1 maint: multiple names: authors list (link)

[2] "How Aircraft Instruments Work." Popular Science, March 1944, p. 118.

[MANOBS-3] "Manual of Surface Weather Observation Standards (MANOBS) 8th Edition, Amendment". canada.ca. Government of Canada. December 2021. Retrieved 2 August 2022. 9.1.3 Units of measurement: The unit of measurement of atmospheric pressure is hectopascals; the corresponding symbol is hPa.

[4] "Aviation's Crazy, Mixed Up Units of Measure - AeroSavvy". 5 September 2014.

[5] "Bryce, (The Rev) Alexander".

[6] "What's a Skydiving Altimeter (and How Does It Work?)". Skydive The Wasatch. Archived from the original on 23 April 2015. Retrieved 2 February 2015.

[7] Hawke, John. "Digital or Analog Altimeter". Dropzone.com. Archived from the original on 6 February 2015. Retrieved 2 February 2015.

[8] "Chapter 8: Flight Instruments". Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C ed.). Federal Aviation Administration. 2023-07-17. pp. 4–5.

[9] "Archived copy". Archived from the original on 2006-06-25. Retrieved 2006-06-15.{{cite web}}: CS1 maint: archived copy as title (link)

[10] "Chapter 8: Flight Instruments". Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C ed.). Federal Aviation Administration. 2023-07-17. p. 7.

[11] "Section 2. Barometric Altimeter Errors and Setting Procedures". Aeronautical Information Manual . Federal Aviation Administration . Retrieved 2023-05-20.

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