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Update pressure calculation to use equations from BMP datasheet instead of wikipedia.

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This commit is contained in:
Tony DiCola
2014-09-19 13:57:19 -07:00
parent cf6e244865
commit 67ae2e69a6
3 changed files with 74 additions and 27 deletions
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91
Adafruit_BMP085_U.cpp
View File

@ -353,7 +353,34 @@ void Adafruit_BMP085_Unified::getTemperature(float *temp)
/**************************************************************************/
/*!
Calculates the altitude (in meters) from the specified atmospheric
pressure (in hPa), sea-level pressure (in hPa), and temperature (in <20>C)
pressure (in hPa), and sea-level pressure (in hPa).
@param seaLevel Sea-level pressure in hPa
@param atmospheric Atmospheric pressure in hPa
*/
/**************************************************************************/
float Adafruit_BMP085_Unified::pressureToAltitude(float seaLevel, float atmospheric)
{
// Equation taken from BMP180 datasheet (page 16):
// http://www.adafruit.com/datasheets/BST-BMP180-DS000-09.pdf
// Note that using the equation from wikipedia can give bad results
// at high altitude. See this thread for more information:
// http://forums.adafruit.com/viewtopic.php?f=22&t=58064
return 44330.0 * (1.0 - pow(atmospheric / seaLevel, 0.1903));
}
/**************************************************************************/
/*!
Calculates the altitude (in meters) from the specified atmospheric
pressure (in hPa), and sea-level pressure (in hPa). Note that this
function just calls the overload of pressureToAltitude which takes
seaLevel and atmospheric pressure--temperature is ignored. The original
implementation of this function was based on calculations from Wikipedia
which are not accurate at higher altitudes. To keep compatibility with
old code this function remains with the same interface, but it calls the
more accurate calculation.
@param seaLevel Sea-level pressure in hPa
@param atmospheric Atmospheric pressure in hPa
@ -362,33 +389,49 @@ void Adafruit_BMP085_Unified::getTemperature(float *temp)
/**************************************************************************/
float Adafruit_BMP085_Unified::pressureToAltitude(float seaLevel, float atmospheric, float temp)
{
/* Hyposometric formula: */
/* */
/* ((P0/P)^(1/5.257) - 1) * (T + 273.15) */
/* h = ------------------------------------- */
/* 0.0065 */
/* */
/* where: h = height (in meters) */
/* P0 = sea-level pressure (in hPa) */
/* P = atmospheric pressure (in hPa) */
/* T = temperature (in <20>C) */
return (((float)pow((seaLevel/atmospheric), 0.190223F) - 1.0F)
* (temp + 273.15F)) / 0.0065F;
return pressureToAltitude(seaLevel, atmospheric);
}
/**************************************************************************/
/*!
Calculates the pressure at sea level (in hPa) from the specified altitude
(in meters), and atmospheric pressure (in hPa).
@param altitude Altitude in meters
@param atmospheric Atmospheric pressure in hPa
*/
/**************************************************************************/
float Adafruit_BMP085_Unified::seaLevelForAltitude(float altitude, float atmospheric)
{
// Equation taken from BMP180 datasheet (page 17):
// http://www.adafruit.com/datasheets/BST-BMP180-DS000-09.pdf
// Note that using the equation from wikipedia can give bad results
// at high altitude. See this thread for more information:
// http://forums.adafruit.com/viewtopic.php?f=22&t=58064
return atmospheric / pow(1.0 - (altitude/44330.0), 5.255);
}
/**************************************************************************/
/*!
Calculates the pressure at sea level (in hPa) from the specified altitude
(in meters), and atmospheric pressure (in hPa). Note that this
function just calls the overload of seaLevelForAltitude which takes
altitude and atmospheric pressure--temperature is ignored. The original
implementation of this function was based on calculations from Wikipedia
which are not accurate at higher altitudes. To keep compatibility with
old code this function remains with the same interface, but it calls the
more accurate calculation.
@param altitude Altitude in meters
@param atmospheric Atmospheric pressure in hPa
@param temp Temperature in degrees Celsius
*/
/**************************************************************************/
float Adafruit_BMP085_Unified::seaLevelForAltitude(float altitude, float atmospheric, float temp)
{
/* Hyposometric formula: */
/* */
/* P0=((((h*0.0065)/(temp + 273.15F))+1)^(^/0.190223F))*P */
/* */
/* where: h = height (in meters) */
/* P0 = sea-level pressure (in hPa) */
/* P = atmospheric pressure (in hPa) */
/* T = temperature (in <20>C) */
return (float)pow((((altitude*0.0065)/(temp + 273.15F))+1), (1.0/0.190223F))*atmospheric;
return seaLevelForAltitude(altitude, atmospheric);
}

5
Adafruit_BMP085_U.h
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@ -104,6 +104,11 @@ class Adafruit_BMP085_Unified : public Adafruit_Sensor
bool begin(bmp085_mode_t mode = BMP085_MODE_ULTRAHIGHRES);
void getTemperature(float *temp);
void getPressure(float *pressure);
float pressureToAltitude(float seaLvel, float atmospheric);
float seaLevelForAltitude(float altitude, float atmospheric);
// Note that the next two functions are just for compatibility with old
// code that passed the temperature as a third parameter. A newer
// calculation is used which does not need temperature.
float pressureToAltitude(float seaLevel, float atmospheric, float temp);
float seaLevelForAltitude(float altitude, float atmospheric, float temp);
void getEvent(sensors_event_t*);

5
examples/sensorapi/sensorapi.pde
View File

@ -115,13 +115,12 @@ void loop(void)
Serial.print(temperature);
Serial.println(" C");
/* Then convert the atmospheric pressure, SLP and temp to altitude */
/* Then convert the atmospheric pressure, and SLP to altitude */
/* Update this next line with the current SLP for better results */
float seaLevelPressure = SENSORS_PRESSURE_SEALEVELHPA;
Serial.print("Altitude: ");
Serial.print(bmp.pressureToAltitude(seaLevelPressure,
event.pressure,
temperature));
event.pressure));
Serial.println(" m");
Serial.println("");
}