Application Note

Bosch Sensortec

BlueBox Demo-Board for the digital sensors AN18002

SMB365 (acceleration) and SMD500 (pressure)

3.3Custom 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 (p0) can be related as followed:

⎛

 

 

 

1

⎞

 

⎛

p

⎞ 5.255

 

⎜

⎟

 

 

⎜

 

⎟

⎟

h in meter

 

 

h = 44262.7 ⋅ ⎜1- ⎜

 

⎟

⎜

⎝ p0

⎠

⎟

 

⎝

 

 

 

 

⎠

 

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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