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Pressure Measurement: Difference between revisions
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Atmospheric pressure (also known as barometric pressure) is the pressure exerted by the weight of the air above the Earth's surface. It is measured using a [http://en.wikipedia.org/wiki/Barometer barometer]. Atmospheric pressure and its rate of change are important parameters for weather and the ocean.
==Locating the Barometer==▼
For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. Unlike the barometer in an automatic weather station, they have no temperature compensation. Therefore, for best pressure readings, you should locate the console in a place where the temperature is reasonably constant, not in direct sunlight or near a heater.▼
==Units==
The units most often used by meterorologists in much of the world for atmospheric pressure are hPa (hecto Pascals, although not a System International unit because of its prefix
Alternatives units are
*mb (millibars, this is the equivalent used for public communications, but its scientific definition has varied from 101,325 Pa to (from 1982) 100,000 Pa),
*mmHg (millimetres of mercury, strictly a function of temperature and gravitational force as well as atmospheric pressure, abandoned in 1929),
*torr (now defined as exactly 1/760 of a standard atmosphere, almost equivalent to mmHg, but mmHg varies by temperature and gravitational strength),
*kPa (kiloPascals is a SI unit, used in Canada),
*inHg (inches of mercury, used in USA and by some UK instruments, but strictly a function of temperature and gravitational force as well as atmospheric pressure).
Cumulus [[cumulus.ini|supports]]:
The best time to set your datum is when the pressure is not changing either with time or between nearby locations. Note most sites only give pressure to the nearest hPa, so you can be 0.5 hPa out if you use that, whereas most personal weather stations give pressure with 0.1 hPa precision.
If you find that your sensor is inaccurate, it may be possible to apply a calibration within cumulus (configuration menu).▼
▲==Locating the Barometer==
▲For most personal weather stations, the barometer is in the console, usually attached to the printed circuit board. These barometers are very cheap devices that simply record the load on a diaphragm. Unlike the barometer in an automatic weather station, they have no temperature compensation. Therefore, for best pressure readings, you should locate the console in a place where the temperature is reasonably constant, not in direct sunlight or near a heater.
==How Accurate are PWS Barometers?==
The figure below shows a comparison between the pressure measured by a Fine Offset PWS and a nearby AWS. You can see that the comparison is excellent.
▲If you find that your sensor is inaccurate, it may be possible to apply a calibration
[[file:KyleVsBrockworthPress.png|500 px]]
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