A barometer measures atmospheric pressure — the weight of the column of air above the instrument. A precision aneroid barometer typically resolves pressure to ±0.3 mbar (0.009 inHg) over a 950 to 1050 mbar range, while a mercury barometer can hit ±0.1 mbar in lab conditions. The reading tells you what the atmosphere is doing locally: rising pressure forecasts clearing weather, falling pressure warns of storms. Aviation altimeters, marine GPS units, and weather stations like the Davis Vantage Pro2 all rely on barometric sensors as their primary input.
Aneroid Barometer Interactive Calculator
Vary the pressure swing, capsule sensitivity, lever ratio, and animation cycle to see capsule deflection amplified into pointer travel.
Equation Used
The calculator uses the aneroid-barometer relationship from the article: a pressure change flexes the sealed capsule by about 25 to 50 um per 100 mbar, then the lever train multiplies that tiny motion into pointer travel.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Capsule deflection is linear over the selected pressure swing.
- Sensitivity is entered in um per 100 mbar, matching the article range of 25 to 50 um.
- Lever train is treated as ideal, with no hysteresis, pivot friction, or temperature drift.
How the Barometer Works
Every barometer answers the same question — how heavy is the column of air sitting on top of the sensor right now. Air at sea level pushes down at roughly 101,325 Pa (1013.25 mbar, 29.92 inHg), and that number changes as weather systems move through, as you climb a mountain, or as a closed building's HVAC swings the indoor pressure. The instrument converts that push into a readable number using one of two methods: balancing the air column against a liquid column (mercury or water), or letting the air flex a sealed metal capsule and reading the deflection mechanically or electrically.
The mercury barometer, invented by Torricelli in 1643, inverts a glass tube full of mercury into an open dish. Atmospheric pressure on the dish pushes mercury up the tube until the mercury column's weight equals the air column's weight — about 760 mm at standard sea-level pressure. The vacuum at the top of the tube is the Torricelli vacuum. Tube bore matters: capillary depression in a 6 mm bore knocks the reading down by ~0.5 mbar, so precision instruments use 12 mm or larger bores and apply a temperature correction because mercury expands at 0.018%/°C.
An aneroid barometer replaces the liquid with a sealed, partially evacuated metal capsule — the Vidie capsule — held open by a spring. As atmospheric pressure rises the capsule compresses by tens of microns, and a lever train multiplies that motion to drive a needle. If the capsule develops even a pinhole leak the reading drifts low and never recovers; if the lever pivots wear out you get hysteresis where the pointer reads differently going up vs coming down. Field aneroids should be re-zeroed against a known reference every 6 to 12 months because the capsule spring slowly creeps.
Key Components
- Aneroid capsule (Vidie cell): A thin-walled corrugated metal disc, partially evacuated to around 10 mbar internal pressure and held open by an internal spring. It compresses roughly 25 to 50 µm per 100 mbar of external pressure change. Wall thickness sits at 0.1 to 0.2 mm — too thin and it buckles under storm-low pressure swings, too thick and you lose sensitivity.
- Mercury column and cistern: A glass tube of 12 mm or larger bore, sealed at the top and dipped into a mercury reservoir. The column height directly equals the atmospheric pressure in millimetres of mercury at 0°C and standard gravity. Tube verticality must be within 0.1° or the reading reads high by the cosine error.
- Lever and rack mechanism: Multiplies the tiny capsule deflection (microns) into needle rotation across a 270° dial. Typical mechanical advantage is 200:1 to 500:1. Pivot bushings must run with under 5 µm of clearance — any more and you get measurable hysteresis between rising and falling pressure.
- Bimetallic temperature compensator: A small bimetal strip mechanically subtracts the thermal expansion of the lever train so the reading stays accurate from -10°C to +40°C. Without it a 20°C shift in room temperature shifts the indicated pressure by 2 to 4 mbar.
- Piezoresistive or capacitive sensor (digital): Modern electronic barometers like the Bosch BMP390 use a silicon diaphragm whose resistance or capacitance changes with applied pressure. Resolution down to 0.02 mbar (about 17 cm altitude resolution), but they need factory calibration and temperature compensation in firmware.
- Cistern adjustment screw: On Fortin-type mercury barometers, raises or lowers the cistern mercury level to a fixed ivory-pointer datum before each reading. Skip this and the column-height reading is offset by however much mercury volume sits in the cistern that day.
Industries That Rely on the Barometer
Pressure measurement underlies a surprisingly wide spread of industries. Weather forecasting is the obvious one, but the same sensor sits inside aircraft altimeters, dive computers, smartphone fitness apps, and HVAC building-management systems. The reason a builder cares which type to specify comes down to accuracy versus environment: a mercury barometer is a calibration reference but cannot move, an aneroid survives a ship's bridge or a backpack but drifts over years, and a piezoresistive digital sensor offers altitude resolution down to 17 cm but fails silently if its temperature compensation table is wrong.
- Aviation: Aircraft altimeters in every Cessna 172 or Boeing 737 use an aneroid stack referenced to the local QNH altimeter setting broadcast by ATIS. Above the transition altitude pilots set 1013.25 mbar (29.92 inHg) as the standard reference.
- Meteorology: Automated surface weather stations such as the NOAA ASOS network log barometric pressure every minute using digital sensors traceable to a mercury reference at NIST.
- Marine navigation: Ship's bridge aneroid barometers — typically the Fischer Precision or Weems & Plath models — track storm approach by 3-hour pressure tendency, with a 3 mbar drop in 3 hours triggering heavy-weather preparation.
- Consumer electronics: Smartphones like the iPhone 15 carry a Bosch BMP-series sensor for floor-level indoor positioning and step-counting altitude detection.
- HVAC and cleanrooms: Differential barometric sensors maintain positive or negative pressure offsets in semiconductor fabs and hospital isolation rooms — typically holding ±5 Pa relative to corridor pressure.
- Diving: Dive computers like the Suunto D5 use a piezoresistive pressure sensor calibrated for both atmospheric and hydrostatic water pressure to compute decompression schedules.
The Formula Behind the Barometer
The fundamental equation behind every barometer is the hydrostatic relation — pressure equals fluid density times gravitational acceleration times column height. For a mercury barometer this gives you the pressure directly from the height you measure. The practical question is what range you'll encounter: at sea level on a calm day you'll read around 760 mm Hg (1013 mbar), in the eye of a strong hurricane like Wilma 2005 you might see 882 mbar (662 mm Hg), and at 3000 m elevation the column drops to about 525 mm Hg (700 mbar). The sweet spot for a domestic barometer is the 950 to 1050 mbar window where most weather systems live — that's where you want the dial to read clearly without compressed gradations.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| P | Atmospheric pressure | Pa (or mbar = 100 Pa) | inHg or psi |
| ρ | Density of the barometric fluid (mercury = 13,595 kg/m³ at 0°C) | kg/m³ | lb/ft³ |
| g | Local gravitational acceleration (9.80665 m/s² standard) | m/s² | ft/s² |
| h | Height of the fluid column above the cistern reference | m (or mm Hg) | in (or inHg) |
Worked Example: Barometer in calibrating a reference mercury barometer in a metrology lab
A metrology lab at a university physics department is calibrating a Fortin-pattern mercury barometer against a digital reference standard. The technician measures a mercury column height of 760.0 mm at 20°C ambient and needs to compute the actual atmospheric pressure in pascals, then evaluate how the reading changes if today's weather pushes the column to 740 mm (a strong low-pressure system) or to 775 mm (a high-pressure ridge).
Given
- hnom = 760.0 mm
- ρHg at 20°C = 13,546 kg/m³
- g = 9.80665 m/s²
- hlow = 740.0 mm
- hhigh = 775.0 mm
Solution
Step 1 — convert the nominal column height from mm to m so the SI units line up:
Step 2 — apply the hydrostatic equation at nominal conditions:
Note this is slightly below the textbook 1013.25 mbar at 760 mm because mercury at 20°C is less dense than at 0°C — the temperature correction matters at the 0.4% level. At the low end of the typical operating range, hlow = 740 mm:
That reading would arrive with a frontal storm system passing overhead — falling barometer, gusty winds, the kind of pressure where small craft warnings fly. At the high end hhigh = 775 mm:
That's a strong high-pressure ridge — clear skies, light winds, the kind of stable air mass that sits over the prairies in winter. The full 35 mm column swing across this range maps to a 47 mbar atmospheric swing, which is exactly the resolution band a domestic weather barometer is built to display.
Result
Nominal atmospheric pressure works out to 100,946 Pa, or 1009. 5 mbar. In practical terms that's a slightly-below-average sea-level day — nothing dramatic, but a forecaster would note the trend rather than the absolute value. The low-end reading of 982.9 mbar at 740 mm corresponds to active stormy weather, and the high-end 1029.4 mbar at 775 mm matches a stable fair-weather ridge — that 47 mbar window covers the vast majority of mid-latitude weather you'll encounter. If your measured pressure differs from the predicted hydrostatic value by more than 1 mbar, suspect (1) skipped temperature correction — mercury density drops 0.018%/°C so a lab at 25°C reads 1.4 mbar low if you used 0°C density tables, (2) cistern level not zeroed against the ivory pointer before reading, which offsets the column height by the cistern displacement, or (3) tube tilt — even 1° off vertical introduces a cosine error that drops the reading by 0.15%.
When to Use a Barometer and When Not To
Choosing between barometer types comes down to whether you need a reference standard, a portable field instrument, or a low-cost embedded sensor. Each style trades accuracy against robustness, cost, and how easily it integrates with a digital data logger.
| Property | Aneroid barometer | Mercury barometer | Piezoresistive digital sensor |
|---|---|---|---|
| Accuracy (typical) | ±0.3 to ±1 mbar | ±0.1 mbar (lab) | ±0.06 to ±0.3 mbar |
| Resolution | ~0.5 mbar dial | 0.1 mm Hg (~0.13 mbar) | 0.02 mbar (Bosch BMP390) |
| Long-term drift | 1-2 mbar/year (capsule creep) | Negligible if mercury is clean | 0.1 mbar/year typical |
| Cost | $50 to $500 | $800 to $5,000+ (and hazmat) | $2 to $20 per chip |
| Portability | Excellent — handheld and aviation-rated | Poor — requires vertical mount, fragile | Excellent — fits in a phone |
| Operating temperature range | -20°C to +50°C with bimetal compensation | +10°C to +30°C lab use | -40°C to +85°C typical IC range |
| Integration with data logging | Requires optical or mechanical encoder add-on | Manual reading or float sensor | Native I²C or SPI digital output |
| Best application fit | Marine, aviation, field use | Calibration reference, teaching | Embedded, IoT, consumer devices |
Frequently Asked Questions About Barometer
METAR pressure is reported as QNH — that's the pressure reduced to mean sea level using the station's elevation and a standard atmosphere lapse rate. Your aneroid reads station pressure, the actual local atmospheric pressure where the instrument sits. If you live 50 m above sea level, station pressure runs about 6 mbar below QNH, which matches your 5 mbar offset almost exactly.
Set the reference screw against QNH only if you want the dial to display sea-level-corrected pressure for weather forecasting. If you're using the barometer as an altimeter or for true local pressure, leave it set to the actual measured value and accept the offset from METAR.
Electronic, every time. A modern Bosch BMP390 or Vaisala PTB210 hits ±0.1 mbar accuracy after calibration, matches mercury in real-world precision, and feeds straight into a data logger. Mercury barometers are calibration references — they don't tolerate vibration, can't be transported once filled, and most jurisdictions now restrict mercury handling under the Minamata Convention.
For a sailing club the practical pick is a digital sensor with a quality aneroid as a visual backup. The aneroid gives crew a glance-readable trend on the wall; the digital sensor logs the actual data and triggers storm alerts at a 3 mbar/3 hour fall threshold.
The instrument is doing its job — a tightly sealed building actually changes pressure when the HVAC supply and return imbalance, when an exhaust fan starts, or when an exterior door opens against wind. Differential pressures of 5 to 50 Pa (0.05 to 0.5 mbar) are normal in commercial buildings and your sensor sees them.
If the jumps are larger than 1 mbar and the building isn't a cleanroom or stairwell, suspect either a positive-pressure makeup-air unit cycling against an undersized return path, or an exterior door slamming at exactly the moment of the reading. Mount the sensor away from supply diffusers and high-traffic doorways to get representative readings.
The capsule itself probably survived but the lever train took the hit. A drop frequently bends one of the multiplying links or shifts a pivot bushing, which changes the mechanical advantage non-linearly. The instrument can still be zeroed at one pressure but the slope of pressure-vs-needle-angle is now wrong.
This isn't a field-fixable failure. The instrument needs to go back to the manufacturer or a calibration lab to have the linkage geometry restored. Trying to compensate by re-zeroing only at average pressure will leave you wrong by the full 4 mbar during the storms when accuracy matters most.
Absolute pressure varies by station elevation, latitude, and local conditions, so 1005 mbar in one location can be normal while it's a deep low somewhere else. Tendency — the change over the last 3 hours — strips away those local factors and tells you what the atmosphere is actively doing.
The classic mariner's rule: a fall of 2 mbar in 3 hours signals approaching weather, 3 mbar in 3 hours means significant deterioration, and 5+ mbar in 3 hours warns of severe weather like a strong front or tropical system. The absolute reading sets context; the tendency drives decisions.
For relative altitude over short time windows, yes — a BMP390-class sensor resolves 0.02 mbar, which is about 17 cm of altitude. For absolute altitude or surveys spanning more than 15 minutes, no, because the atmospheric pressure itself drifts faster than the survey accuracy you'd want.
The standard workaround is differential barometry: leave one sensor at a known elevation as a base station, carry a second sensor as the rover, and subtract the base reading from the rover reading in real time. This cancels weather drift and gets you survey-grade vertical accuracy from $20 chips, but you must log both sensors with synchronised timestamps.
References & Further Reading
- Wikipedia contributors. Barometer. Wikipedia
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