Bosch SMD500 Datasheet Page 21

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Application Note
BlueBox Demo-Board for the digital sensors
SMB365 (acceleration) and SMD500 (pressure)
Bosch Sensortec
AN18002
Rev. 1.0
Page 21
© Bosch Sensortec GmbH reserves all ri
g
hts even in the event of industrial propert
y
ri
g
hts. We reserve all ri
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hts of disposal such
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and passin
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onto third parties.
7211RB11.
3.3 Custom calibration of the SMD500 altimeter function
With the SMD500, there is the possibility to precisely measure the ambient air- or atmospheric
pressure. According to the international barometric formula it is possible to determine the
current altitude out of the measured atmospheric pressure. Before using the SMD500 on the
BlueBox multi sensor demo board for exact altitude measurements, some facts are good to
know:
The ambient air- or atmospheric pressure is a typical weather and temperature related
phenomena. As a law of nature, the atmospheric pressure varies in a certain range over the day
and can be referred in two ways. Either to the ambient (as is) pressure, including the actual
position altitude or to the pressure at sea level (virtual), excluding the altitude of the actual
position. This “virtual” or “re-calculated” pressure, the so called pressure at sea level, always
refers to the height of “zero” above sea level. According to the international barometric formula
mentioned above, altitude (h) in meter, atmospheric pressure (p) and pressure at sea level (p
0
)
can be related as followed:
=
255.5
1
0
p
p
-1 44262.7 h h in meter
According to the formula above, the SMD500 always measures precisely the absolute
atmospheric pressure, including height, temperature and weather effects. In combination with
the pressure at sea level, it is possible e.g. to determine the altitude. For accurate altitude
measurements, using the SMD500 on the BlueBox demo board, it is necessary to recalibrate
the board at least once a day. For an exact altitude measurement we recommend to calibrate
the board straight before the measurement.
Generally, there are three possibilities for a calibration. On the one hand, if the exact sea level
pressure is known or possible to get (e.g. from internet or a nearby airport), there is the
possibility to “calibrate at sea level” either “via internet” or “manual”. On the other hand, if the
exact altitude is known, there is the possibility for a calibration using the function “calibrate
altitude”.
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