An Aneroid Barometer measures atmospheric pressure using a sealed, partially evacuated metal capsule that flexes as air pressure changes. French engineer Lucien Vidi patented the design in 1844, replacing the column of mercury with a compact mechanical sensor. As pressure rises the capsule compresses and as it falls the capsule expands, and a lever train amplifies that tiny motion into a needle reading on a dial in hPa or inHg. The result is a portable, mercury-free instrument that drives weather forecasting, aircraft altimeters, and pressure recording for surface stations worldwide.
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 full 0.3 mm 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.
How the Aneroid Barometer Works
The heart of every Aneroid Barometer is the Vidi capsule — a thin corrugated metal disc, usually beryllium copper or phosphor bronze, sealed and partially evacuated to roughly 5% of atmospheric pressure. Outside air pushes on the capsule walls and a calibrated internal spring (or the elastic stiffness of the corrugations themselves) resists. When pressure rises the capsule squeezes by tens of microns. When pressure falls it expands. That displacement is the entire raw signal — everything else is amplification.
A chain-and-lever train multiplies the capsule deflection by roughly 200:1 to 400:1 to drive a pointer across a dial. Why so much amplification? Because the full atmospheric range from a deep low (950 hPa) to a strong high (1050 hPa) only moves the capsule face by around 0.3 mm in a typical 50 mm diameter cell. The hairspring takes up backlash in the lever pivots so the needle settles cleanly instead of drifting. If the hairspring loses tension or a pivot jewel wears, you will see hysteresis — the needle reads differently going up than coming down, often by 2-3 hPa, which is enough to ruin altimeter accuracy.
Tolerances matter. The capsule diaphragm thickness must hold to about ±5 µm across the corrugation, otherwise the pressure-deflection curve goes non-linear at the ends of the scale. Temperature is the other enemy — the capsule metal expands with heat and would read pressure changes that aren't there, so a bimetallic compensating link in the lever train cancels roughly 90% of the thermal drift between 0°C and 40°C. Common failure modes are a leaking capsule (slow drift toward atmospheric, the needle climbs a few hPa per month), a bent lever from drop shock, and corroded pivots in marine units that cause the needle to stick until you tap the glass.
Key Components
- Aneroid Cell (Vidi Capsule): A sealed corrugated metal disc, typically 30-60 mm diameter, evacuated to about 50 hPa internal pressure. The corrugations let it flex axially while staying stiff radially. Full-range deflection is around 0.3 mm — small but repeatable to within 5 µm if the capsule is undamaged.
- Capsule Stack or Spring: Higher-precision instruments stack 2 to 6 capsules in series to multiply deflection. Lower-cost units use a single capsule with an external leaf spring to set the pressure-deflection slope. The spring rate must match the capsule stiffness within ±2% or the dial reads non-linearly at the extremes.
- Lever and Chain Amplifier: A two- or three-stage lever train converts the sub-millimetre capsule motion into 270° of pointer rotation. Total mechanical advantage runs 200:1 to 400:1. Pivot clearance must stay under 10 µm — anything more and you get visible hysteresis between rising and falling readings.
- Hairspring: A fine spiral spring preloads the lever train against backlash so the pointer follows pressure changes smoothly rather than jumping in steps. If it weakens you'll see the pointer lag behind real pressure changes, particularly during fast frontal passages.
- Bimetallic Compensator: A short bimetallic strip in the lever train counteracts thermal expansion of the capsule metal. Without it the instrument would shift roughly 0.3 hPa per °C, drowning out real pressure trends. Properly compensated units hold within ±0.5 hPa from 0°C to 40°C.
- Dial and Pointer: Calibrated in hPa (typically 950-1050), inches of mercury (28.0-31.0 inHg), or millibars. A second movable pointer set by hand lets the user mark the previous reading so trend direction (rising or falling) is obvious at a glance.
Industries That Rely on the Aneroid Barometer
Aneroid Barometers show up anywhere a compact, mercury-free pressure reading matters — and that's a lot of places. Pilots stake their lives on the same mechanism inside altimeters. Weather services use bank after bank of recording aneroids called barographs to log pressure trends. Hikers, sailors, and meteorology hobbyists pick them because they survive vibration, work in any orientation, and need no power. The mechanism scales from a 50 mm wristwatch barometer to industrial barographs the size of a shoebox.
- Aviation: Sensitive altimeters in general aviation aircraft like the Cessna 172 use a stack of three to four aneroid capsules driving a 1000-foot-per-revolution pointer, calibrated against standard atmosphere ICAO tables.
- Marine Navigation: Ship's barometers from makers like Plastimo and Weems & Plath use brass-cased aneroids with gimbal mounts to track pressure trends ahead of weather fronts at sea.
- Meteorology: Recording barographs such as the Fischer Precision 110 use a 6-capsule stack and a clock-driven drum to ink a 7-day pressure trace for surface weather stations.
- Outdoor Recreation: Wrist barometers in Suunto Core and Garmin Fenix watches use miniaturised silicon-MEMS aneroid sensors based on the same evacuated-diaphragm principle for storm-warning alarms.
- Industrial Test Equipment: Calibration-grade aneroid altimeter test sets like the Druck DPI 740 use precision capsules to check aircraft pitot-static systems against a traceable reference.
- Home and Office Instruments: Decorative wall barometers from companies like Fischer Barometer GmbH put the Vidi capsule mechanism behind a brass dial as both a forecasting tool and a desk piece.
The Formula Behind the Aneroid Barometer
The working relationship inside an aneroid is simple: the capsule deflects in proportion to the pressure difference between outside and the near-vacuum inside, divided by the capsule's effective stiffness. The needle position is then that deflection multiplied by the lever ratio. At the low end of the typical operating range — say 950 hPa during a deep storm - the capsule sits near its maximum expansion and the lever train is at one end of its travel, where small mechanical errors get magnified. At the high end, 1050 hPa under a strong continental high, the capsule is nearly fully compressed and you risk bottoming out the corrugations if the design margin is thin. The sweet spot for linearity is right around standard atmosphere, 1013.25 hPa, where the capsule operates in the centre of its stress-strain curve and lever pivots sit mid-travel.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| θpointer | Angular deflection of the pointer | radians | degrees |
| G | Mechanical amplification of the lever train | rad/m | deg/in |
| Patm | Atmospheric pressure (the measured quantity) | Pa or hPa | inHg or psi |
| Pinternal | Residual pressure inside the evacuated capsule | Pa | psi |
| Aeff | Effective area of the capsule diaphragm | m² | in² |
| kcapsule | Combined stiffness of capsule and any external spring | N/m | lbf/in |
Worked Example: Aneroid Barometer in a precision marine wall barometer build
You are designing a precision marine wall barometer for a 40-foot sailing yacht using a single Vidi capsule of 50 mm diameter and a lever train with G = 300 deg per mm of capsule travel. The capsule effective area is 1.96 × 10-3 m², its combined stiffness is 6500 N/m, and internal residual pressure is 50 hPa. You need to know how far the pointer swings between 950 hPa (storm) and 1050 hPa (fair weather high) and whether the lever range is sized correctly.
Given
- Dcapsule = 50 mm
- Aeff = 1.96 × 10⁻³ m²
- kcapsule = 6500 N/m
- Pinternal = 50 hPa (5000 Pa)
- G = 300 deg/mm
Solution
Step 1 — at nominal standard atmosphere (1013.25 hPa = 101325 Pa), compute the net force on the capsule diaphragm:
Step 2 — convert that force to capsule deflection through the stiffness:
What matters for the dial is the change in deflection across the working range, not the absolute number. Step 3 — at the low end of the typical operating range, 950 hPa (95000 Pa):
Step 4 — at the high end, 1050 hPa (105000 Pa):
So the working stroke between storm and fair-weather is xhigh − xlow = 3.02 mm. Step 5 — apply the lever amplification G to get pointer swing:
That is far more than the 270° a typical dial provides, which means the lever ratio is too aggressive for a single capsule of this stiffness. Drop G to roughly 90 deg/mm and you land at 270° of pointer travel between 950 and 1050 hPa — a clean fit. At the low end of pressure the pointer sits at the storm mark with the lever train near the soft-stop. At the high end it rests at the fair-weather mark. The sweet spot in the middle of the dial corresponds to standard atmosphere, where pivot wear and corrugation non-linearity have the least effect on accuracy.
Result
With the corrected lever ratio of 90 deg/mm, the pointer swings the full 270° dial between 950 and 1050 hPa, with each hPa corresponding to 2. 7° of pointer rotation. That is fine enough resolution to read pressure to within 1 hPa by eye — the smallest meaningful trend change for short-term forecasting. At the 950 hPa low end the needle pegs near the storm zone with capsule at 27.13 mm deflection, at 1013 hPa nominal it sits mid-dial near 28.6 mm, and at 1050 hPa the needle reaches the fair-weather mark at 30.15 mm — the sweet spot for linearity is the middle third of that range. If your built barometer reads 5+ hPa off the predicted value, suspect three things: a slow capsule leak that has equalised internal pressure above the design 50 hPa (test by watching the zero shift over a week), a bent or unseated hairspring that lets the lever train rest against a hard stop instead of preloading the chain, or a temperature-compensation bimetal that has been soldered in the wrong orientation, doubling thermal drift instead of cancelling it.
When to Use a Aneroid Barometer and When Not To
Aneroid is one of three pressure-measurement traditions you'll see in field instruments. Each has a clear niche based on accuracy, ruggedness, power needs, and cost. The comparison below is the practical decision matrix.
| Property | Aneroid Barometer | Mercury Barometer | Digital MEMS Barometer |
|---|---|---|---|
| Typical accuracy | ±0.5 to ±2 hPa | ±0.1 hPa (reference grade) | ±0.1 to ±1 hPa |
| Long-term drift | 1-3 hPa per year (capsule creep) | Negligible if clean | 0.5-1 hPa per year |
| Power requirement | None — fully mechanical | None | Battery, microamp range |
| Operating orientation | Any orientation | Vertical only | Any orientation |
| Shock and vibration | Tolerates 5g routinely | Fragile — column breaks | Tolerates 1000g+ |
| Service life | 20-50 years typical | Indefinite if sealed | 10-15 years (electronics) |
| Cost (consumer grade) | $30-300 | $200-2000 | $5-50 (sensor IC) |
| Best application fit | Aviation altimeters, marine, home weather | Reference calibration labs | Smartphones, wearables, drones |
Frequently Asked Questions About Aneroid Barometer
Almost always elevation, not the instrument. Atmospheric pressure drops about 1 hPa for every 8.3 m of altitude near sea level, so if your house sits 40 m above the airport you'd expect about a 5 hPa lower reading — that is correct, not an error.
Most home barometers have a small adjustment screw on the back that lets you set the needle to match the local airport's mean sea level pressure. Once set, your barometer will then read the same MSL pressure as the airport even though the actual air pressure at your location is lower. If after adjusting it still drifts more than 2 hPa over a week, the capsule is leaking.
Watch the dial during a known calm-pressure spell — typically the centre of a strong high — and compare against three or four official METAR readings over 48 hours. A healthy aneroid will track within ±1 hPa across the day. If the needle creeps in one direction (almost always upward, toward higher reading) by more than 1 hPa per day with steady real pressure, the capsule has a slow leak and internal pressure is climbing toward atmospheric. Once internal pressure passes about 200 hPa the dial range collapses and the needle barely moves at all between weather systems. The fix is capsule replacement — they cannot be re-evacuated in the field.
Stack them when you need either higher resolution or wider angular swing on the dial. Each capsule contributes its own deflection to the same lever input, so a 4-capsule stack delivers roughly 4× the raw motion of a single capsule of the same diameter and stiffness. That lets you use a smaller lever ratio, which means less pivot-wear amplification and better long-term accuracy.
For a basic wall barometer reading to 1 hPa, a single 50 mm capsule is plenty. For an altimeter or barograph reading to 0.2 hPa, go with 3-6 capsules. The downside is cost and assembly complexity — every capsule in the stack adds another sealed joint that can leak.
That tells you the lever pivots have static friction higher than the hairspring preload can overcome. The needle sits wherever pressure last pushed it, and the lever doesn't catch up until you give it a mechanical kick. On a healthy instrument the tap should move the needle less than 0.5 hPa.
Causes are corroded pivot jewels (common on marine units exposed to salt air), a hairspring that has lost tension from age or shock, or oil migration from the lever pivots into the chain drive gumming up the linkage. A drop of clock oil on each pivot and a careful re-tension of the hairspring fixes most cases, but if the jewels are pitted you need a new movement.
Only if the dial and capsule were designed for it. A standard sea-level barometer with a 950-1050 hPa scale will peg at the low end above roughly 600 m elevation because the needle runs out of dial. Aviation altimeters use the same mechanism but with much wider scales — 540 to 1050 hPa is typical — and tighter capsules tuned for the lower end of the range.
If you take a sea-level aneroid up a mountain, you can recalibrate it by adjusting the zero screw, but the linearity will be poor because the capsule is operating away from its design centre. For altitude work above 3000 m use a purpose-built sensitive altimeter or a digital MEMS sensor with a 300-1100 hPa range.
Sawtooth or stepped traces almost always come from stick-slip in the inking arm pivot, not the capsule. The capsule is moving smoothly with pressure, but the arm holding the pen sticks against its bearing until the lever force builds enough to break it free, then jumps. Each jump prints as a vertical step on the drum.
Check that the pen arm bearing has clean clock oil and no dust. The pen itself can also drag if the ink is too thick — fresh, low-viscosity barograph ink gives the smoothest trace. If the steps repeat at exactly the same time each day, suspect temperature compensation drift instead, since daily heating and cooling cycles will print as a sawtooth even on a steady-pressure day.
References & Further Reading
- Wikipedia contributors. Barometer. Wikipedia
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