What it shows
Air pressure falls with height, by about 12 pascals for each metre near the ground. A BMP280 sensor can measure this so precisely that it can tell one floor of a building from the next.
The science
Pressure at any height is the weight of the air column above it. The pressure falls with height roughly as P = P0 x (1 minus h / 44330) to the power 5.255 with h in metres, so the library can turn pressure into altitude relative to a reference. Near the ground, pressure drops about 1.2 hPa for every 10 m. Weather changes the pressure too, so over several hours a falling reading warns of bad weather, which is the idea of a barometer.
Materials
Cost: under Rs 1,500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An Arduino Uno or Nano
- A BMP280 pressure sensor module (check whether it is 3.3 V or has a 5 V regulator)
- A 0.96 inch OLED display (SSD1306, I2C)
- A power bank
- A breadboard and jumper wires
- A notebook, a tape measure
Safety
For this project: USB power only. Check the voltage rating of the BMP280 module: a bare 3.3 V chip is damaged by 5 V.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Connect the BMP280 to 3.3 V (or 5 V if your module has a regulator), GND, SDA to A4 and SCL to A5.
Install the Adafruit BMP280 and SSD1306 libraries and run the example to find the I2C address (0x76 or 0x77).
Print pressure in hectopascals and temperature on the OLED every second.
Press a button to record the present pressure as the reference and show the height above it using the library function readAltitude.
Zero the sensor on the ground floor and walk up the stairs, noting the height at each floor against a tape-measure estimate.
Compare the altitude change for each floor: one floor is about 3 m, which is a pressure change of about 0.35 hPa.
Leave it still on a shelf for six hours and log pressure every ten minutes to see the weather drift.
Plot the logged pressure and compare it with the daily weather forecast.
What you should see
The altimeter shows heights that increase by about 3 metres for each floor, and the barometer log shows slow changes as the weather changes.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does pressure fall with height?
There is less air above you, so there is less weight pressing down.
2How much does pressure change per 10 m near the ground?
About 1.2 hectopascals.
3Why does the altitude drift over hours?
Real weather changes the pressure, which the sensor cannot tell from a height change.
4What unit does the sensor give?
Pascals, usually shown as hectopascals.
Take it further
Carry it in a bag to a hill station and compare the altitude reading with a map height.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
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