Discussion about wrong barometric Pressure by BM280 sensor.
https://forums.adafruit.com/viewtopic.php?f=22&t=118039

"The BME280 is simply a pressure sensor and will report the actual pressure as designed. It can't possibly know what altitude it is being operated at - or even what you intend to use the pressure reading for. If you want to use it to control a hyperbaric chamber or maintain a positive pressure differential in a clean-room, it will give you exactly the readings you need.

If you want to compare readings reported by the local weather station, you will need to apply the same correction function that the weather stations apply to their barometric readings. By convention, the pressures reported by airports and weather stations are not the raw readings from the instrument. They are 'normalized' to sea level so that air pilots can use the reported pressure to gauge altitude above sea-level."

Here is the correction page:
https://keisan.casio.com/exec/system/1224579725

When barometers in the home are set to match the local weather reports, they measure pressure adjusted to sea level, not the actual local atmospheric pressure. 

1,013.25 hPa is the sea level average pressure

The average value of surface pressure on Earth is 985 hPa. This is in contrast to mean sea-level pressure, which involves the extrapolation of pressure to sea-level for locations above or below sea-level. The average pressure at mean sea-level (MSL) in the International Standard Atmosphere (ISA) is 1013.25 hPa, or 1 atmosphere (Atm), or 29.92 inches of mercury.

Correction when using Arduino [ https://github.com/adafruit/Adafruit_BME280_Library/ ]:

// Keisan Calculation for Sea Level Pressure from current BME280 readings
// http://keisan.casio.com/exec/system/1224575267
void getP() {
int a = 43; // My altitude in meters = 142 (ft)* 0.30488
float k = .0065; // altitude multiplier
volatile float z = (((k * a) / (bme.readTemperature() + (k * a) + 273.15))) ;
volatile float y = (bme.readPressure());
P = y * (pow ((1 - z) , -5.257)) ;
P = P * 0.0002953; // convert to Inches
return P;
}




Re: BME280 Pressure Accuracy

by WhidbeyBill on Sun Dec 17, 2017 12:09 am
Sea Level Barometer Function: (check the sketch and comments for double to string conversion. Arduino cannot print doubles. #include <Math.h>)

// Keisan Calculation for Sea Level Pressure from current BME280 readings
// http://keisan.casio.com/exec/system/1224575267
void getP() {
int a = 43; // My altitude in meters = 142 (ft)* 0.30488
float k = .0065; // altitude multiplier
volatile float z = (((k * a) / (bme.readTemperature() + (k * a) + 273.15))) ;
volatile float y = (bme.readPressure());
P = y * (pow ((1 - z) , -5.257)) ;
P = P * 0.0002953; // convert to Inches
return P;
}

Attachments

    BYun_20171216_SeaLevelBarometer.txt
        Sea Level Barometer with fixed altitude.
        (3.81 KiB) Downloaded 290 times








Pressure (P), mass (m), and the acceleration due to gravity (g), are related by P = F/A = (m*g)/A, where A is surface area. Atmospheric pressure is thus proportional to the weight per unit area of the atmospheric mass above that location. 

https://en.wikipedia.org/wiki/Atmospheric_pressure

Pressure on Earth varies with the altitude of the surface; so air pressure on mountains is usually lower than air pressure at sea level. Pressure varies smoothly from the Earth's surface to the top of the mesosphere. Although the pressure changes with the weather, NASA has averaged the conditions for all parts of the earth year-round. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude.[8] Temperature and humidity also affect the atmospheric pressure, and it is necessary to know these to compute an accurate figure. The graph at right was developed for a temperature of 15 °C and a relative humidity of 0%.

At low altitudes above sea level, the pressure decreases by about 1.2 kPa (12 hPa) for every 100 metres. For higher altitudes within the troposphere, the following equation (the barometric formula) relates atmospheric pressure p to altitude h: 

http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/barfor.html

https://www.e-education.psu.edu/meteo3/l6_p3.html