An aneroid barometer senses atmospheric pressure with a sealed metal capsule. Changes in external pressure compress or relax its flexible walls, and a mechanical movement amplifies the small displacement into pointer rotation. This animation shows that principle. The calculator maps an entered pressure onto an assumed linear travel and dial range; it does not design or calibrate a physical capsule.
Aneroid Barometer Interactive Calculator
Vary the pressure scale, capsule travel, and pointer sweep to see capsule deflection and dial motion.
Equation Used
The calculator maps the selected atmospheric pressure across the low-to-high dial range. The capsule deflection is a linear fraction of the entered full travel, and the same fraction drives the pointer through its full angular sweep.
- Capsule response is linear over the selected pressure range.
- Scale low point is the zero-deflection reference.
- Internal capsule pressure is fixed at 50 hPa for pressure difference.
- Temperature compensation, backlash, and hysteresis are ignored.
The pressure difference assumes a fixed 50 hPa residual capsule pressure. The pointer and capsule travel are clamped to the selected scale endpoints. Animation cycles from low-scale pressure to the selected endpoint; tiles show that endpoint.
From capsule movement to pointer movement
A sealed aneroid capsule contains a reduced internal pressure. External air pressure acts on its flexible walls, while the capsule’s elasticity and any associated spring resist deformation. An increase in external pressure compresses the capsule; a decrease allows it to expand.
Levers, a chain or geared movement can amplify that motion into pointer travel. Construction varies: some instruments use a capsule stack, and temperature compensation may be built into the capsule or movement. Neither a particular capsule count nor a compensation method guarantees accuracy by itself.
The drawing greatly enlarges capsule motion so that it is visible. It cycles from the selected scale-low pressure to the entered pressure and back. The result tiles describe the selected endpoint, not the changing pressure in the playback.
Capsule Deflection is measured from the calculator’s scale-low reference. It is not the capsule’s absolute compression from an unloaded shape.
Pressure indication and pressure records
Mechanical aneroid instruments can indicate atmospheric pressure on a dial or move a pen across a barograph chart. Pressure altimeters use pressure-sensitive elements with an altitude calibration and setting mechanism. A barometer reading alone is not an altitude or a complete weather forecast.
Electronic pressure sensors may also use a diaphragm, but their electrical sensing and compensation are different from the lever-and-pointer mechanism shown here. Product accuracy, orientation, temperature range and shock resistance require the particular instrument’s specifications.
For comparisons with a weather report, distinguish the pressure at the instrument from pressure reduced to sea level. A difference may reflect reference level, location, time or calibration; it does not prove a capsule leak.
The selected linear scale
Define span = Phigh − Plow in hPa and f = (P − Plow)/span, limited to the interval 0 to 1. Capsule Deflection is f × entered full travel in mm. Pointer Angle is f × entered full sweep in degrees.
Capsule Sensitivity is 1000 × full travel/span in µm/hPa. It is the assumed slope inside the selected range, not the clipped response outside that range. Pressure minus 50 hPa is simply P − 50, an independent pressure difference using an assumed fixed internal pressure.
The 50 hPa value is a model assumption, not a universal residual pressure for aneroid cells. That output does not drive the travel calculation. No diaphragm area, stiffness, spring preload, hysteresis, temperature or linkage geometry is solved.
Pressure outside the selected dial range leaves the pointer and travel at the corresponding endpoint and produces an out-of-range message. The pressure-difference tile still follows the entered pressure. The scale is an illustrative mapping, not a specification for a real instrument.
Worked example: a 100 hPa dial span
Set Scale Low to 950 hPa, Scale High to 1050 hPa, Capsule Travel to 0.30 mm and Pointer Sweep to 270°. At Atmos Pressure 1000 hPa, the fraction is 0.5. Capsule Deflection is 0.15 mm and Pointer Angle is 135°.
Capsule Sensitivity is 1000 × 0.30/100 = 3.0 µm/hPa. The separate pressure-difference tile is 1000 − 50 = 950 hPa.
At 950 hPa, deflection and pointer angle are zero by the chosen reference. At 1050 hPa they reach 0.30 mm and 270°. At 1100 hPa they remain at those endpoints, while the pressure-difference tile rises to 1050 hPa.
Doubling Capsule Travel to 0.60 mm at 1000 hPa doubles calculated deflection to 0.30 mm and sensitivity to 6.0 µm/hPa, but leaves pointer angle at 135°. The dial mapping is specified independently; this is not a prediction of how changing an actual capsule would affect its movement.
Measurement principles
| Instrument | Principle | Key distinction |
|---|---|---|
| Mechanical aneroid | Elastic capsule and pointer movement. | Mechanical calibration, friction and temperature behavior matter. |
| Liquid-column barometer | Balances pressure against a liquid column. | Fluid properties and the required orientation matter. |
| Electronic barometer | A sensor and electronics convert pressure to an output. | Sensor calibration, compensation and power requirements matter. |
Readability, resolution and accuracy are separate. Fine dial markings or many digital digits do not establish a small measurement error. Calibration and permitted conditions must match the actual instrument.
Aneroid-barometer questions
No. It is a fixed assumption used only for the pressure-difference tile here. The linear travel and pointer calculations use the selected dial limits instead.
The model maps both independently from the same fraction of the pressure span. Full travel sets capsule displacement, while full sweep sets pointer angle.
No. It is only the reference point for incremental travel across the chosen scale.
The calculation clips travel and pointer angle at the chosen scale endpoints. It does not extrapolate beyond that range or model damage to a real capsule.
First check whether both values refer to station pressure, sea-level pressure or another reporting convention, and compare location and time. Follow the specific instrument’s instructions for adjustment.
No. Offset adjustment is not a substitute for a specified pressure range or full calibration. Do not alter range or internal movement adjustments without the manufacturer’s procedure.
No. Friction, calibration, temperature, capsule condition and other causes can affect readings. A traceable comparison and suitable servicing are needed; this calculator cannot diagnose a leak or prescribe lubrication.
No. It is a pressure-to-display illustration without altitude conversion, setting procedures, instrument qualification or flight-system checks.
References and further reading
Fischer: precision aneroid barometers describes capsules, mechanical transmission and model-specific specifications. The Fischer barograph manual distinguishes station pressure from sea-level reports and identifies adjustments that must not be altered. Consult the instructions for the particular instrument.
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
