Fine offset HP2550 Operation Manual page 64

over time due to contamination. In addition, location has an adverse
affect on humidity readings (installation over dirt vs. lawn for
example).
Official stations recalibrate or replace humidity sensors on a yearly
basis. Due to manufacturing tolerances, the humidity is accurate to
± 5%. To improve this accuracy, the indoor and outdoor humidity can
be calibrated using an accurate source, such as a sling psychrometer.
(3) The display console displays two different pressures: absolute
(measured) and relative (corrected to sea-level).
To compare pressure conditions from one location to another,
meteorologists correct pressure to sea-level conditions. Because the
air pressure decreases as you rise in altitude, the sea-level corrected
pressure (the pressure your location would be at if located at sea-level)
is generally higher than your measured pressure.
Thus, your absolute pressure may read 28.62 inHg (969 mb) at an
altitude of 1000 feet (305 m), but the relative pressure is 30.00 inHg
(1016 mb).
The standard sea-level pressure is 29.92 in Hg (1013 mb). This is the
average sea-level pressure around the world. Relative pressure
measurements greater than 29.92 inHg (1013 mb) are considered high
pressure and relative pressure measurements less than 29.92 inHg are
considered low pressure.
To determine the relative pressure for your location, locate an official
reporting station near you (the internet is the best source for real time
barometer conditions, such as Weather.com or Wunderground.com),
and set your weather station to match the official reporting station.
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