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Calibration of the QNH or of the altitude (known datum) makes it possible to re-adjust the Xplorer
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in relation to the measured ambient pressure . This varies continually according to the altitude and
the current meteorological conditions .
The only reference for the Xplorer is the air pressure which it is measuring, from which it deduces
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the altitude . Any variations in this pressure makes the altitude change: 9 metres per hPa a low
altitudes, 14 metres at 5,000 metres, and this is why it is necessary to calibrate as often as
possible to maintain a credible display (even airline pilots have this to comply with!) .
Historical Recording Modes:
The Xplorer 4 records the air pressure, even when it is OFF, every hour for the past 48 hours . By default
this setting is turned OFF to save battery economy . There are three recording modes, for air pressure
trend, relative air pressure (QNH) and altitude . However all three records are stored based on the air
pressure at the time of measurement and the current calibration of altitude/QNH .
Trend:
This is the difference in pressure over the last 24 hours (or 48 hours, or 1 hour etc .) .
This is valid if the Xplorer is in the same place (same altitude). A negative figure indicates
a drop in pressure, a positive figure, an increase. Pressure naturally varies during the day,
and the weather trend is more accurate at 12 to 24 hours . Increasing air pressure generally
indicates clearing weather, decreasing air pressure worsening weather, and steady pressure
no weather change . A large drop in air pressure (0 .06 inHg within 12 hours) can indicate an
approaching storm . A large increase in air pressure can bring strong winds .
QNH:
This is the relative pressure at 24 hours ago (or 48 hours, etc .) . Valid if the Xplorer
is at the same place and is recalibrated .
Altitude:
If you set off for a mountain walk or go paragliding, you can check the altitudes
achieved hour by hour . The altitude will be accurate if you force the QNH calibration of the
Xplorer on the weather pressure that was present when you recorded .