Modifications

=== Pressure, SLP and altitude ===

=== Pressure, SLP and altitude ===

SLP means Sea Level Pressure (also knwon as PNM pressure). Most of the advanced the meteo station does transform the current pressure to SLP pressure before displaying it. If all pressure in the country are expressed as SLP pressure then it is more easy to determine the wind (and cloud) movements across the country, from higher pressure to lower pressure.  

SLP means Sea Level Pressure (also knwon as PNM pressure). Most of the advanced the weather station does transform the current pressure to normalized SLP pressure before displaying it. If all pressure in the country are expressed as SLP pressure then it is more easy to determine the wind (and cloud) movements across the country, from higher pressure to lower pressure.  

Here is a small picture that shows the relation between SLP pressure, altitude and pressure given by a sensor (like the BMP280).  

Here is a small picture that shows the relation between normalized SLP pressure, altitude and pressure given by a sensor (like the BMP280).  

[[Fichier:ENG-CANSAT-BMP280-50.jpg|800px]]

[[Fichier:ENG-CANSAT-BMP280-50.jpg|800px]]

Let's say that we readed the pressure at house with the BMP280. The house sit at 523m of altitude.

Let's say that we readed the pressure of 950 hPa at our house with the BMP280. The house sit at 523m of altitude.

Nous venons donc de dire qu'il faut "creuser un trou sous la maison pour revenir jusqu'au niveau de la mer". Pas besoin de faire un vrai trou, il suffit de connaître la hauteur de ce trou (donc notre altitude) puis d'ajouter la pression correspondant à cette colonne d'air.  

If we want to know the SLP pressure, it is like sink a well under the house (down to the sea level). At the bottom of the well, we have more air over our head so the pressure will be greater than 950 hPa. In real world, we do not have to sink a well, we do know the altitude of our house (at 523m) so we can estimate the correction to apply (corresponding to a column of 523m of air). So, from the 950hPa read at house, we can calculate the corresponding pressure at sea level (SLP pressure). Together with the other SLP values, we can estimate the movement of clouds :-) .  

Voici la marche à suivre:

# Relevez la pression hPa (ou mmbar) au niveau de la mer (sur une station météo de référence)

 

# saisissez cette valeur comme baseline (n'oubliez pas de multiplier par 100)  

Here the steps to follow with the BMP280 to estimate the altitude of the house:

# Utilisez le senseur pour déterminer votre l'altitude.  

# Find the hPa (or mmbar) pressure at Sea Level on a WebSite

# Calculer la valeur de compensation (colonne d'air) en hPa = hauteur-en-m / 8.3

# Use that value as baseline (don't hesitate to multiply it by 100)  

# Use the sensor and the BMP280 library to read the altitude.  

# Calculate the correction value (for the air's colonne) in hPa = height-in-meter / 8.3

 

 

  <nowiki>

  <nowiki>

# pression moyenne au niveau de la mer (non essentielle pour Pression PNM)

# mean pressure at the sea level (not critical for SLP pressure)

p.baseline = 101325

p.baseline = 101325

# pression PNM

# SLP pressure

p = bmp180.pressure + compensation

p = bmp280.pressure + compensation

  </nowiki>

  </nowiki>

p contiendra alors la valeur de pression normalisée au niveau de la mer (PNM).

where {{fname|p}} would contains pressure normalized at sea level (so the SLP value).

Par contre, l'altitude absolue ne sera plus mesurée fiablement (à moins de pouvoir faire une mise-à-jour de p.baseline régulièrement). Il sera toujours possible d'utiliser la valeur de l'altitude pour détecter une différence de niveau si votre senseur est embarqué sur un "ballon sonde". Dans le cas d'une station météo fixe, l'altitude ne sera plus d'une très grande utilité :-) ... c'est la pression PNM qui nous intéresse

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.

{{ambox|text=Prenez le temps de lire les points suivants, ils contiennent toutes les informations nécessaires pour comprendre et réaliser les opérations décrites ci-dessus.}}

* '''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.

=== La pression au niveau de la mer varie! ===

=== La pression au niveau de la mer varie! ===