AN1921 Motorola / Freescale Semiconductor, AN1921 Datasheet - Page 28

• Manufacturer Part Number: AN1921
• Description: AN1921 General DSP568xx Interface Examples using the Embedded SDK
• Manufacturer: Motorola / Freescale Semiconductor

Integrating Devices With the EVM Board
Appendix D.
28
This section discusses the apparent error seen when using the MPX4115A Integrated Pressure Sensor.
The units of barometric pressure used here are in metric units (SI) millibars. If the demo is performed
at or near sea level, the displayed result should be identical to a reading provided by the local weather
service via either the internet or radio weather services. As a standard, all barometric pressure values
recorded and provided by the weather service are always referenced to sea level, regardless of the
altitude of the weather measuring station.
When the demo is performed at a location above sea level, a divergence takes place between the
demo-measured value and the current weather service value. The demo value will always be less and
will decrease as the height above sea level increases. Under these circumstances, the MPX4115A is
performing nothing more than an altimeter function. A mathematical relationship exists that relates the
height above sea level to the pressure measurement.
For any given location, there is a means of obtaining the correction for the altitude. One way is to
contact the local office of the weather service via its internet web site and observe the barometric
pressure readings provided. A second means requires listening to the local radio outlet of the weather
service (usually found around 162.0 MHz VHF using a NBFM radio) for the current barometric value.
With the current values, a correction term ( ) can be determined by the following expression:
The value (D) is a constant for a given location and it is added to the measured barometric value as
follows:
The station pressure varies with altitude above sea level in accordance with the following expression:
For example, a local radio station announces the barometric or sea level pressure as 1013 millibars (or
hectoPascals) and you live in the mountains at 1760 meters. Using the above equation, your station
pressure is 788 millibars. The
P
P
sea-level
station
Altitude Pressure Adjustment
= P
weather_service
= P
where
= P
H = 7000
sea-level
z = height above sea-level (meters)
measure
* exp(-z/H)
- P
Interface Examples Using the Embedded SDK
+
measured
Determining the Pressure Transducer’s
correction in this case is 225 millibars.