What Are Barometers?

Barometers are instruments that measure the absolute pressure of the atmosphere, conventionally reported in hectopascals, millibars, inches of mercury, or millimeters of mercury. Standard sea level pressure is 1013.25 hPa, and the quantity varies by a few percent with weather systems and falls roughly exponentially with altitude, halving near 5.5 km. Evangelista Torricelli built the first working instrument in 1643 by inverting a mercury-filled tube into a reservoir and observing that the column settled at a height set by the atmosphere pressing on the open surface. That experiment established both the measurement principle and, in the void above the column, the first sustained laboratory vacuum.

A barometer differs from a general pressure gauge in that it reads absolute rather than differential or gauge pressure, so its reference is an evacuated volume rather than the surrounding air. That requirement shapes the design of every implementation, from a sealed metal capsule to a micromachined silicon diaphragm with a cavity beneath it.

Liquid Column and Aneroid Instruments

The mercury barometer remained the reference instrument for three centuries. Its reading requires correction for the thermal expansion of mercury and of the scale, for local gravitational acceleration, and for the capillary depression of the meniscus, after which a well-maintained laboratory instrument can reach a few parts in ten thousand. Environmental and occupational restrictions on mercury have retired most of these instruments from field service. The aneroid barometer, patented by Lucien Vidie in 1844, replaced the liquid column with a partially evacuated corrugated metal capsule whose deflection under changing pressure is amplified through a lever train to a pointer. Aneroid capsules are compact, rugged, and orientation-tolerant, which made them the basis of the aircraft altimeter and of the recording barograph, though they exhibit hysteresis and require periodic recalibration against a reference.

Electronic and Micromachined Sensors

Most barometers manufactured today are silicon micromachined devices. A thin diaphragm is etched over a sealed reference cavity, and its deflection is transduced either piezoresistively, using implanted resistors in a Wheatstone bridge whose resistance changes with strain, or capacitively, by sensing the changing gap between the diaphragm and a fixed electrode. Capacitive designs generally offer lower power and better long-term stability, while piezoresistive designs give a simpler linear output. On-die temperature sensors and factory-stored polynomial coefficients correct the substantial temperature dependence of both mechanisms, and integrated sigma-delta converters deliver a digital reading over a serial bus. Resonant silicon and quartz devices, in which pressure shifts the resonant frequency of a vibrating element, achieve the highest stability among solid-state options and are used in aviation air data computers.

Calibration and Pressure Metrology

Barometric readings are only as good as their traceability chain. National measurement institutes maintain primary realizations of the pascal, historically through mercury manometers, and have moved toward optical methods that infer pressure from the refractive index of a gas. The fixed length optical cavity pressure standards developed at NIST compare the optical path length through a gas-filled Fabry-Perot cavity with its evacuated length, giving an SI-traceable pressure without a mercury column, and the working equation relating cavity resonance to gas pressure sets out the underlying model. Field instruments are then calibrated against transfer standards, and operational networks add redundancy, with the Automated Surface Observing System stations operated by the National Weather Service, FAA, and Department of Defense reporting sea level pressure and altimeter setting from roughly 950 sites.

Applications

Barometers are used across measurement and engineering domains, including:

• Surface weather observation and numerical forecast data assimilation
• Aircraft altimetry and air data computation
• Indoor floor-level positioning in smartphones and wearables
• Altitude compensation in engine control and fuel injection systems
• Hydrological and oceanographic gauges that need atmospheric correction
• Building automation and pressurization monitoring
• Laboratory reference measurements in gas metrology and vacuum science