A barometric altimeter is a pressure gauge with a height scale: near sea level the air pressure falls by about 12 Pa for each metre of climb, so a 100 m climb lowers it by about 1,200 Pa, roughly 9 mmHg. Because weather moves the pressure by as much, the instrument must be set to the pressure at take-off and read only as a change in height, and it never shows the clearance over the ground below. Its scale must be calibrated against known heights.
Part of Aerodynamics and Flight
Measuring height by the falling pressure of the air, and why the reading drifts with weather and temperature.
How Pressure Falls With Height
The pressure at a given level is the weight of the air column above it. Climbing a small height h removes a slice of that column of weight ρ × g × h per unit area. Worked example: ρ = 1.225 kg/m³ (the sea-level value used in the corpus), g = 9.81 m/s², h = 1 m: 1.225 × 9.81 × 1 = 12.0 Pa. For 100 m: 12.0 × 100 = 1,202 Pa; one millimetre of mercury is 133.3 Pa, so 1,202 ÷ 133.3 = 9.0 mmHg. Rounded, that is about 12 Pa per metre, 1,200 Pa for 100 m and 9 mmHg. The figure holds only for small climbs near sea level, because the air thins as you go up; the real relation across a larger range is a published table, not a constant.
The Instrument
An aneroid capsule, a sealed thin metal box with a partial vacuum inside, flexes with the outside pressure and drives a needle through a linkage. The carrier suggests adapting a good aneroid barometer (Aerodynamics and Flight). A Bourdon gauge such as the one in Pressure Gauge reads pressures far too coarsely for this. The pressure change for the heights a light aircraft flies is a few thousand pascals out of about 101,000 Pa, a few per cent, so the capsule needs good sensitivity.
Calibration: Take It To The Hill
Step 1: Choose a hill or building with a known height difference you can measure by another means (for instance the levelling methods of Elevation Measurement).
Step 2: Read the instrument at the bottom and at several heights up the slope, noting the time, because the weather moves the pressure.
Step 3: Repeat on the way down. If the up and down readings differ at the same height, the instrument or the weather has drifted; the difference estimates the error.
Step 4: Mark the scale at the known heights. This calibrates height changes, not absolute height.
Weather And Temperature Move The Reading
A falling barometer on a day of approaching bad weather will read as a climb: the corpus’s Pressure Signs describe how fast pressure drops before a storm. Temperature changes the instrument as well (Bottle Barometer shows a sealed-bottle instrument that a 5 degree temperature swing can mimic a pressure shift). Set the altimeter to zero at the take-off field before each flight, and read only the change.
What It Cannot Tell You
It does not show height above the ground beneath you, only above where you set it. A hill, a tree line or a descent into a valley is invisible to it. For the first low hops, you can see the ground and the altimeter is irrelevant; it matters when you climb out of sight of a landmark, and then it is a navigation aid, not a safeguard.
Common Mistakes
- Not re-zeroing before each flight.
- Trusting it in changing weather.
- Using it to judge clearance over terrain.
- A leaky or wind-exposed case — a leak, or wind pressure on the case, changes the reading in flight.
Safety
The altimeter says nothing about the ground
A pilot descending to land by altimeter alone, in poor visibility or over unmapped land, can fly into rising ground. Fly low-level tests only in good visibility over ground you know.
What’s Next
- Learn next: Compass Navigation, knowing where you are going
- Combine with: Airspeed Indicator, the other pressure instrument