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Ligne 91 :
Ligne 91 : + + + + − + Ligne 104 :
Ligne 108 : − + − − Ligne 112 :
Ligne 114 : − + − Ligne 144 :
Ligne 145 : − + − # Use that value as baseline (don't hesitate to multiply it by 100) + − + − − When you have to "correction" value you don't have to care anymore about the baseline.+ + + + + + + + + − Indeed, we can read the current pressure (without caring about the baseline) THEN we add the "correction" value --> Tadaaa! We have the SLP pressure (the same than displayed on Reference Weather Station).+ + + + + + + + + − <nowiki># mean pressure at the sea level (not critical for SLP pressure)+ − p.baseline = 101325 − # SLP pressure − p = bmp280.pressure + compensation</nowiki> − − where {{fname|p}} would contains pressure normalized at sea level (so the SLP value). − − The downside of the "mean pressure value" approach (101325 Pa) is that you cannot estimate the altitude with accuracy. − However, by updating the baseline value every day with the "day's pressure" at sea level (see on Internet broadcast website) then the altitude calculation would be fairly precise. − + − + Ligne 174 :
Ligne 182 : − + − So I have fixed the baseline as follow before reading the altitude:+ − − Serial.print(bme.readAltitude(1002.00)); − Serial.println(" m"); + Ligne 191 :
Ligne 197 : − + − bme.readAltitude(1002.00)+ − − The sensor would return the right altitude (104m) for the sensor which is quite good since we are not in the tower (like the reference weather station). − − − + − + − + − * that we have to set the baseline to the current pressure at day's sea level pressure (baseline=100200) − * once done, we can use the sensor to calculate the current altitude of the sensor (in our case, it is 104 m) − * the pressure decrease of 1hPa every time we increase the altitude of 8.3m . − + − The weather station does normalize the atmospheric pressure calculate local pressure at the Sea Level altitude (called SLP for "Sea Level Pressure" also named "PNM" for ''Pression Niveau Mer'').+ + + + + + + + + + − This means that reference weather station applies the correction (compensating) to the read value. + − + − For the reference station next to house, its height is 120m. so the correction adds the air column of 120m height over the sensor value to get the normalized SLP pressure. So the correction is evaluated to (120 / 8.3) hPa = 14.45 hPa.+ − + − {{underline|Let's apply the same principle to our BMP280 readings:}}+ − + − In our case, we know that the altitude of the BMP280 sensor is 104m. Remind, the pressure is reduced of 1hPa every time our altitude increase of 8.3m.+ − + − For 104m, the air column until the sea level correspond to 104 / 8.3 = 12.53 hPa additional pressure.+ − + − As the sensor give the 989.09 hPavalue, the correction to have the Normalized SLP is 989.09 + 12.53 = 1001.62 hPa. Great! it is almost the same value than reference weather station next to home (to remind, it communicates 1002 hPa).+ − {{ambox|text=Please note that the today's pressure is 1002 hPa and our normalized pressure is also 1002 hPa. This is rare situation and means that the air will not flow in any way between our location and the sea.}}+ + + + + + + +
ENG-CANSAT-PICO-BMP280 (voir la source)
Version du 26 février 2022 à 23:57
, 26 février 2022 à 23:57→Testing the sensor
<syntaxhighlight lang="python">
<syntaxhighlight lang="python">
# Read the sensor values:
# (temperature_celcius, pressure_hpa, humidity_percent)
# Humidity only applies to BME280 only, not BMP280.
#
from machine import I2C
from machine import I2C
# BME280 aslo work for BMP280
# BME280 aslo work for BMP280
from bme280 import BME280, BMP280_I2CADDR
from bme280 import BME280, BMP280_I2CADDR
from time import sleep
from time import sleep
i2c = I2C(1)
i2c = I2C(0)
bmp = BME280( i2c=i2c, address=BMP280_I2CADDR )
bmp = BME280( i2c=i2c, address=BMP280_I2CADDR )
while True:
while True:
Which produce the following results:
Which produce the following results:
<nowiki>(22.28, 1017.68, 0.0)
<syntaxhighlight lang="bash">
(22.28, 1017.68, 0.0)
(22.27, 1017.66, 0.0)
(22.27, 1017.66, 0.0)
(21.87, 1017.67, 0.0)
(21.87, 1017.67, 0.0)
(21.83, 1017.68, 0.0)
(21.83, 1017.68, 0.0)
(21.81, 1017.68, 0.0)
(21.81, 1017.68, 0.0)
(21.81, 1017.68, 0.0)
(21.81, 1017.68, 0.0)</nowiki>
</syntaxhighlight>
By activating the Plotter, the value can even be made visible as a graph. However, to see a proper graph evolution, the best is to replace the {{fname|sleep(1)}} to {{fname|sleep( 60*30 )}} (30 min).
By activating the Plotter, the value can even be made visible as a graph. However, to see a proper graph evolution, the best is to replace the {{fname|sleep(1)}} to {{fname|sleep( 60*30 )}} (30 min).
Here the steps to follow with the BMP280 to estimate the altitude of the house:
Here the steps to follow with the BMP280 to estimate the altitude of the house:
# Find the hPa (or mmbar) pressure at Sea Level on a WebSite
# Find the hPa pressure at Sea Level on a WebSite
# Use the BMP280 sensor to read the pressure.
# Use the sensor and the BMP280 library to read the altitude.
# Calculate the height with the following code
# Calculate the correction value (for the air's colonne) in hPa = height-in-meter / 8.3
<syntaxhighlight lang="python">
# Read Local pressure then
# Calculate corresponding Altitude
#
from machine import I2C
# BME280 aslo work for BMP280
from bme280 import BME280, BMP280_I2CADDR
from time import sleep
i2c = I2C(0)
baseline = 1032.0 # day's pressure at sea level
bmp = BME280( i2c=i2c, address=BMP280_I2CADDR )
while True:
# returns a tuple with (temperature, pressure_hPa, humidity)
p = bmp.raw_values[1]
altitude = (baseline - p)*8.3
print( "Altitude: %f m" % altitude )
sleep(1)
</syntaxhighlight>
[[Fichier:ENG-CANSAT-PICO-BMP280-35.png]]
Just remind:
Just remind:
* '''If you plan to do a Weather station''' then you have to care about the correction in order calculate the normalized value at sea level. Altitude is not useful since the weather station doesn't move.
* '''If you plan to do a Weather station''' then you will have to calculate the normalized pressure value at sea level. You have to know the height of the sensor (the weather station doesn't move).
* '''if you need a flying sensor''' in a rocket then you have to care about the baseline value (sea level pressure {{underline|of the day}}) for have a accurate measure the the altitude.
* '''if you need a flying sensor''' in a rocket then you have to care about the baseline value (sea level pressure {{underline|of the day}}) to have an accurate measurement the the altitude.
=== The sea level pressure change every day! ===
=== The sea level pressure change every day! ===
By example, today the pressure is 1002.00 hPa at the Belgian's sea level.
By example, today the pressure is 1002.00 hPa at the Belgian's sea level.
<font color="red">this value is critical if you want to evaluate the altitude of the sensor.</font>. It is also important to know the current altitude if you plan to calculate the "correction" for the normalized SLP pressure.
<font color="red">this value is critical if you want to evaluate the altitude of the sensor</font>.
It is also important to know the current altitude if you plan to calculate the "correction" for the normalized SLP pressure.
{{underline|Remark:}}<br />
It is quite easy to know the current sea level pressure by using an Internet Weather Broadcast like [http://www.meteobelgique.be/observations/temps-reel/stations-meteo.html this link to meteobelgique.be]
It is quite easy to know the current sea level pressure by using an Internet Weather Broadcast like [http://www.meteobelgique.be/observations/temps-reel/stations-meteo.html this link to meteobelgique.be]
If you want to obtain an accurate altitude value with the BMP280 then you need to know the pressure at the sea level (''the baseline'') with precision.
If you want to obtain an accurate altitude value with the BMP280 then you need to know the pressure at the sea level (''the baseline'') with precision.
Once the baseline value corrected, you will have the correct altitude:
Once the baseline value corrected, you will have the correct altitude.
=== The athmospheric pressure is not correct! ===
=== The pressure is not correct atmosphérique semble incorrecte! ===
My sensor returned a pressure of 98909 pascal (so 989.09 hPa) whereas the reference weather station does mentino 1002 hPa!
My sensor returned a pressure of 98909 pascal (so 989.09 hPa) whereas the reference weather station does mentino 1002 hPa!
The sensor value is right, it just not apply the correction to indicates the Normalized SLP pressure (equivalent pressure at the sea level). The reference {{underline|reference}} weather station does applies this correction for yo (so they displays normalized SLP).
The sensor value is right, it just not apply the correction to return the Normalized SLP pressure (equivalent pressure at the sea level). The reference {{underline|reference}} weather station does applies SLP correction for you (so they displays normalized SLP).
Let's do the correction on the value...
Let's do the SLP correction on the sensor's value...
{{underline|First:}} read the preceding point, from it we know :
{{underline|First:}} You must know your altitude (121m in my case, see here before How I did calculate it).
{{underline|Next, on the reference Weather station:}}
{{underline|Then:}} Read the pressure and apply the SLP correction factor.
<syntaxhighlight lang="python">
# Read Local pressure
# Calculate corresponding SLP pressure
# (SLP: sea level pressure)
#
from machine import I2C
# BME280 aslo work for BMP280
from bme280 import BME280, BMP280_I2CADDR
from time import sleep
i2c = I2C(0)
# Sensor altitude (in meter) required to
# calculate SLP (See Level Pressure)
altitude = 120.1
bmp = BME280( i2c=i2c, address=BMP280_I2CADDR )
while True:
# returns a tuple with (temperature, pressure_hPa, humidity)
p = bmp.raw_values[1]
p_sea = p + (altitude/8.3)
print( "Plocal: %6.1f hPa, Psea: %6.1f hPa" % (p,p_sea) )
sleep(1)
</syntaxhighlight>
which returns:
<nowiki>Plocal: 1017.4 hPa, Psea: 1031.8 hPa
Plocal: 1017.4 hPa, Psea: 1031.9 hPa
Plocal: 1017.3 hPa, Psea: 1031.8 hPa
Plocal: 1017.4 hPa, Psea: 1031.8 hPa
Plocal: 1017.4 hPa, Psea: 1031.9 hPa
Plocal: 1017.4 hPa, Psea: 1031.9 hPa
Plocal: 1017.4 hPa, Psea: 1031.8 hPa</nowiki>
{{ENG-CANSAT-PICO-TRAILER}}
{{ENG-CANSAT-PICO-TRAILER}}