Using the BMP (API v2)

This page is based on the new v2 of the BMP085 driver, which uses Adafruit's new Unified Sensor Driver. The driver provides better support for altitude calculations, and makes it easy to switch between the BMP085 and any other supported pressure sensor in your projects.

If you haven't already done so, you'll need to install the Adafrut_Sensor library on your system as well, since Adafruit_BMP085 relies on this library to generate the sensor data in a universal manner.

Using the BMP085 / BMP180

To use this sensor and calculate the altitude and barometric pressure, there's a lot of very hairy and unpleasant math. You can check out the math in the datasheet but really, its not intuitive or educational - its just how the sensor works. So we took care of all the icky math and wrapped it up into a nice Arduino library.

You can find the Arduino library repository on github To install it, click this button to download the compressed ZIP file then install it. This guide will help you with the install process if you have never installed an Arduino library.

The same code is used for both the BMP085 and BMP180 (they are compatible!)

Restart the IDE

Now you can run this first example sketch

#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_BMP085_U.h>
   
Adafruit_BMP085_Unified bmp = Adafruit_BMP085_Unified(10085);

void setup(void) 
{
  Serial.begin(9600);
  Serial.println("Pressure Sensor Test"); Serial.println("");
  
  /* Initialise the sensor */
  if(!bmp.begin())
  {
    /* There was a problem detecting the BMP085 ... check your connections */
    Serial.print("Ooops, no BMP085 detected ... Check your wiring or I2C ADDR!");
    while(1);
  }
}

void loop(void) 
{
  /* Get a new sensor event */ 
  sensors_event_t event;
  bmp.getEvent(&event);
 
  /* Display the results (barometric pressure is measure in hPa) */
  if (event.pressure)
  {
    /* Display atmospheric pressure in hPa */
    Serial.print("Pressure: "); Serial.print(event.pressure); Serial.println(" hPa");
  }
  else
  {
    Serial.println("Sensor error");
  }
  delay(250);
}
Then open up the serial monitor at 9600 baud. The sketch will continuously print out the pressure in hPa (hectoPascals). You can test that the sensor is measuring variations in pressure by placing your fingertip over the open port hole in the top of the sensor. The pressure will increase as you can see here:

Altitude Measurements

Since we know that pressure drops as we gain altitude (that's why air is so thin on mountain-tops) we can compute the current altitude knowing the pressure and temperature. Again, there's a bit of hairy math involved, you can read about the calculations on wikipedia (where this graph is from).
With the Arduino library, we take care of that for you! Simply update the 'void loop()' function above with the code below to get the altitude based on the pressure and temperature:
void loop(void) 
{
  /* Get a new sensor event */ 
  sensors_event_t event;
  bmp.getEvent(&event);
 
  /* Display the results (barometric pressure is measure in hPa) */
  if (event.pressure)
  {
    /* Display atmospheric pressue in hPa */
    Serial.print("Pressure:    ");
    Serial.print(event.pressure);
    Serial.println(" hPa");
    
    /* Calculating altitude with reasonable accuracy requires pressure    *
     * sea level pressure for your position at the moment the data is     *
     * converted, as well as the ambient temperature in degress           *
     * celcius.  If you don't have these values, a 'generic' value of     *
     * 1013.25 hPa can be used (defined as SENSORS_PRESSURE_SEALEVELHPA   *
     * in sensors.h), but this isn't ideal and will give variable         *
     * results from one day to the next.                                  *
     *                                                                    *
     * You can usually find the current SLP value by looking at weather   *
     * websites or from environmental information centers near any major  *
     * airport.                                                           *
     *                                                                    *
     * For example, for Paris, France you can check the current mean      *
     * pressure and sea level at: http://bit.ly/16Au8ol                   */
     
    /* First we get the current temperature from the BMP085 */
    float temperature;
    bmp.getTemperature(&temperature);
    Serial.print("Temperature: ");
    Serial.print(temperature);
    Serial.println(" C");

    /* Then convert the atmospheric pressure, SLP and temp 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)); 
    Serial.println(" m");
    Serial.println("");
  }
  else
  {
    Serial.println("Sensor error");
  }
  delay(1000);
}
Run the sketch to see the calculated altitude.
The data above is reasonably close to what I'd expect at my location, but we can improve the accuracy by changing the reference sea level pressure, which will change depending on the weather conditions. Every 1 hPa that we are off on the sea level pressure equals about 8.5 m of error in the altitude calculations!

Many weather sites, (particularly near major airports) will provide pressure readings. If you happened to be near Paris, France, for example, you might look up the current air pressure at Charles de Gaulle airport, which we can see is 1009 hPa (a meaningful difference from the generoc 1013.25 hPa value we are plugging in via the SENSORS_PRESSURE_SEALEVELHPA macro):
Updating the following line to 1009 will give us a more accurate altitude:
float seaLevelPressure = 1009;
If you don't know the current mean pressure at sea level for your current weather conditions and location, SENSORS_PRESSURE_SEALEVELHPA is still a safe starting point:
This now gives us the following results, which shows that calibrating for your local conditions is often worthwhile when working with low altitudes!
Just be careful looking for local mean pressure at sea level values, since the functions in the driver are expecting hPa units, not one of the dozens of other values you may encounter, but you should be able to convert anything you find to hPa which is a standard SI unit.
Last updated on 2016-09-19 at 01.06.04 PM Published on 2012-07-29 at 11.58.38 AM