Editing Variometer (section)

==Description==
[[Image:Faa vertical air speed.JPG|thumb|left|Schematic drawing of the internals of a classic aircraft vertical speed indicator]]
According to [[Paul MacCready]], "A variometer is essentially a pressure altimeter with a leak which tends to make it read the altitude of a moment earlier.  It consists of a container vented to the outside air in such a way that the pressure inside the flask lags slightly behind the outside static pressure.  The rate of climb measurement comes from the rate-of-air inflow or outflow from the container."<ref name="pm">{{cite journal |last1=MacCready |first1=Paul |title=Measurements of Vertical Currents |journal=Soaring |date=1954 |volume=18 |issue=3 |pages=11–19 |publisher=Soaring Society of America}}</ref>

Variometers measure the rate of change of altitude by detecting the change in air pressure (static pressure) as altitude changes. Common types of variometers include those based on a diaphragm, a vane (horn), a taut band, or are electric based.  The vane variometer consists of a rotating vane, centered by a coil spring, dividing a chamber into two parts, one connected to a static port, and the other to an expansion chamber. Electric variometers use [[thermistor]]s sensitive to airflow, or [[circuit board]]s consisting of variable resistors connected to the membrane of a tiny vacuum cavity.<ref name="hr">{{cite book |last1=Reichmann |first1=Helmut |title=Cross-Country Soaring, A Handbook for Performance and Competition Soaring |date=1993 |publisher=Soaring Society of America, Inc. |location=Iceland |isbn=1883813018 |pages=142–152}}</ref><ref>{{cite web |title=Variometer, Vanetype Variometers |url=https://shop.segelflugbedarf24.de/Glider-equipment/Instruments/Variometer:::107_124_125.html?language=en&filter_id=30 |website=Segelflugbedarf |access-date=13 December 2020}}</ref><ref>{{cite web |title=Sage Variometers |url=https://www.sagevariometers.com/sagequst.htm |access-date=13 December 2020}}</ref><ref name=faa>{{cite web |title=Glider Flying Handbook, FAA-H-8083-13B |url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/glider_handbook/gfh_chapter_4.pdf |publisher=U.S. Department of Transportation |access-date=10 January 2025 |pages=4-11 to 4-17 |date=2024}}</ref>

A simple variometer can be constructed by adding a large reservoir (a [[vacuum flask]]) to augment the storage capacity of a common aircraft rate-of-climb instrument. In its simplest electronic form, the instrument consists of an air bottle connected to the external atmosphere through a sensitive air flow meter.  As the aircraft changes altitude, the atmospheric pressure outside the aircraft changes and air flows into or out of the air bottle to equalise the pressure inside the bottle and outside the aircraft.  The rate and direction of flowing air is measured by the cooling of one of two self-heating [[thermistor]]s and the difference between the thermistor resistances will cause a voltage difference; this is amplified and displayed to the pilot.  The faster the aircraft is ascending (or descending), the faster the air flows.  Air flowing out of the bottle indicates that the altitude of the aircraft is increasing.  Air flowing into the bottle indicates that the aircraft is descending.

Newer variometer designs directly measure the static pressure of the atmosphere using a pressure sensor and detect changes in altitude directly from the change in air pressure instead of by measuring air flow.  These designs tend to be smaller as they do not need the air bottle. They are more reliable as there is no bottle to be affected by changes in temperature and fewer chances for leaks to occur in the connecting tubes.

The designs described above, which measure the rate of change of altitude by automatically detecting the change in static pressure as the aircraft changes altitude are referred to as "uncompensated" variometers.  The term "vertical speed indicator" or "VSI" is most often used for the instrument when it is installed in a powered aircraft.  The term "variometer" is most often used when the instrument is installed in a glider or sailplane.

An "Inertial-lead" or "Instantaneous" VSI (IVSI) uses accelerometers to provide a quicker response to changes in vertical speed.<ref>{{Cite book | url=https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-15B.pdf | title = Instrument Flying Handbook | last = Federal Aviation Administration | author-link = Federal Aviation Administration | place = Washington, DC | pages = 5–8 | year = 2012 | access-date = 2016-07-12}}</ref>

[[File:Cair-Xk10-vario.jpeg|thumb|Panel mounted variometer for [[Glider (sailplane)|gliders]], showing vertical speed in knots (kn).]]
[[Image:Gleitschirmvario.jpg|thumb|upright|A variometer for [[paraglider]]s, [[hang glider]]s, and [[balloon]]eers, showing vertical speed with both a ribbon indicator and a numeric readout, showing vertical speed in meters per second (m/s).]]