Atmospheric-pressure sensors are called barometers. Almost all barometers measure the pressure difference between atmospheric pressure on one side of the sensor, and a reference pressure on the other side. This pressure difference causes a net force that pushes against a spring or a weight. For most barometers, the reference pressure is a vacuum (zero pressure).
Aneroid barometers use a corrugated metallic can (the aneroid element) with a vacuum inside the can. A spring forces the can sides outward against the inward-pushing atmospheric-pressure force. The relative inflation of the can is measured with levers and gears that amplify the minuscule deflection of the can, and display the result as a moving needle on a barometer or a moving pen on a barograph (a recording barometer). The scale on an aneroid barometer can be calibrated to read in any pressure units (see Table 1-3)
Mercury (Hg) barometers (developed by Evangelista Torricelli in the 1600s) are made from a U-shaped tube of glass that is closed on one end. The closed end has a vacuum, and the other end is open to atmospheric pressure. Between the vacuum and the air is a column of mercury inside the tube, the weight of which balances atmospheric pressure.
Atmospheric pressure is proportional to the height difference ∆z between the top of the mercury column on the vacuum side, and the height on the side of the U-tube open to the atmosphere. Typical ∆z scales are millimeters of mercury (mm Hg), centimeters of mercury (cm Hg), or inches of mercury (in Hg). To amplify the height signal, contra-barometers (developed by Christiaan Huygens in the 1600s) use mercury on one side of the U-tube and another fluid (e.g., alcohol) on the other.
Because mercury is a poison, modern Torricelli (U-tube) barometers use a heavy silicon-based fluid instead. Also, instead of using a vacuum as a reference pressure, they use a fixed amount of gas in the closed end of the tube. All Torricelli barometers require temperature corrections, because of thermal expansion of the fluid.
Electronic barometers have a small can with a vacuum or fixed amount of gas inside. Deflection of the can is measured by strain gauges, or by changes in capacitance between the top and bottom metal ends of an otherwise non-conductive can. Digital barometers are electronic barometers that include analog-to-digital circuitry to send pressure data to digital computers. More info about all weather instruments is in WMO-No. 8 Guide to Meteorological Instruments and Methods of Observation.