Smartphone barometer: measuring atmospheric pressure

A barometer is an instrument that measures atmospheric pressure, that is, the force per unit of surface exerted by the weight of the column of air above the measurement point. This pressure is expressed in hectopascals (hPa) and averages 1013 hPa at sea level.

Discover FizziQ

How to measure it in class

With the FizziQ app, the smartphone’s pressure sensor tracks live variations of atmospheric pressure, whether of meteorological origin or due to a change of altitude.

Steps:

Open FizziQ and select the “Barometer” instrument. First check that the instrument appears: not all smartphones carry a pressure sensor.
Place the device on a table and observe the displayed value, in hectopascals. Compare it with the pressure reported by a local weather station.
Start a recording over several hours or days to visualise the slow drift of the pressure, and correlate a marked drop with the arrival of a disturbance.
Move the smartphone vertically by one or two floors: note that climbing lowers the pressure by about 0.12 hPa per metre.
Distinguish two qualities of measurement: resolution (the sensor detects very small variations, of the order of 0.01 to 0.1 hPa) and absolute accuracy (the displayed value can be offset by several hPa from the true pressure, without calibration).

Scientific activities on this topic

Possible extensions with FizziQ: keep a weather log over a week and anticipate weather changes, or draw the altitude profile of a hike from pressure variations. See the smartphone sensors guide.

Learn more

How it works:

The pressure sensor of a smartphone is a miniaturised MEMS (Micro Electro Mechanical Systems) component. A silicon membrane, exposed to ambient air, deforms slightly under pressure. This deformation changes an electrical quantity (capacitance or resistance of a strain gauge) that the electronics convert into a pressure value. The same principle drives aneroid barometers, while Torricelli’s historical barometer measured the height of a mercury column supported by atmospheric pressure.

Availability: not all smartphones have a barometer. The sensor equips most mid-range and high-end models, but is absent from many entry-level phones and most tablets. Without the sensor, the “Barometer” instrument does not appear in FizziQ and the measurement is impossible.

Relative resolution vs absolute accuracy: a smartphone barometer detects very fine variations (resolution of the order of 0.01 to 0.1 hPa, a few tens of centimetres of altitude), but its absolute accuracy is far more modest (typical offset of ± 1 hPa or more, without calibration). It is therefore excellent for measuring pressure differences, and mediocre for providing an absolute reference pressure.

Orders of magnitude:

Average sea-level pressure: 1013 hPa (that is, 101,325 Pa).
Pressure gradient near the ground: about 0.12 hPa per metre (12 Pa/m).
Range of weather variations: roughly 970 to 1040 hPa at temperate latitudes.
Resolution of a smartphone sensor: 0.01 to 0.1 hPa (that is, 0.1 to 1 m of altitude).
Typical absolute accuracy without calibration: ± 1 hPa (about ± 8 m).

Formula

Converting a pressure difference into an altitude difference follows from the pressure gradient near the ground:

Δh ≈ (P₁ - P₂) × 8.3

Where:

Δh: altitude difference (m)
P₁, P₂: pressures measured at the two points (hPa)
the factor 8.3 m/hPa is the inverse of the 0.12 hPa/m gradient

This relation only holds for small height differences near the ground and assumes the weather pressure does not change during the measurement. For a wider altitude range, the barometric formula for an isothermal atmosphere is used:

h = H × ln(P₀/P), with H ≈ 8,400 m at 15 °C

where H = RT/(Mg) is the scale height, which depends on temperature (about 8,000 m at 0 °C); P₀ is the reference pressure at level h = 0.

Application examples

Weather tracking: a fast pressure drop often announces the arrival of a depression and disturbed weather.
Altimetry: estimate the height of a building or a tower from the pressure difference between base and top.
Aviation: an aircraft’s altimeter is a barometer set to a reference pressure (QNH or QNE setting).
Hiking: sports watches and GPS devices use a barometer to display elevation gain.
Indoor navigation: smartphones use the barometer to detect floor changes, where GPS is inoperative.
Home forecasting: a living-room barometer shows the trend (rise or fall) over a few hours.

FAQ

Q: Does my smartphone have a barometer? A: Not necessarily. The sensor mostly equips mid-range and high-end models. The simplest check is whether the “Barometer” instrument appears in FizziQ; if it is absent, the device has none.

Q: Why does the displayed value differ from the weather report’s pressure? A: Weather stations reduce their measurement to sea level, while the barometer displays the actual pressure at your altitude. On top of this comes the sensor’s limited absolute accuracy (about ± 1 hPa). The barometer nevertheless remains reliable for measuring variations.

Q: Can altitude be measured with the barometer? A: Yes, through barometric altimetry. Measuring altitude differences is very precise (down to about 1 m), because it relies on the sensor’s resolution. Absolute altitude is less reliable, as it depends on the reference pressure and the weather.

Q: Does the barometer need calibrating? A: For relative measurements (pressure variations, height differences), no: the resolution is enough. For an absolute pressure or altitude, calibration against a known reference value (local QNH) is necessary.

Atmospheric pressure - Barometric altimetry - Altitude - GPS altimeter - GPS geolocation