barometer

barometer An instrument for determining atmospheric pressure. There are four general types: mercurial, aneroid, hypsometric, and solid-state (piezoresistive or capacitance). The original mercurial design in which the weight of the column of air is balanced by a column of mercury is still used for synoptic or climatological purposes, the Kew-pattern and Fortin types being widely encountered. Aneroid barometers, including the precision aneroid type, make use of one or more evacuated capsules that deform with changes in pressure.

The hypsometric barometer (or hypsometer) determines pressure by measuring the water vapour temperature (boiling point). It was particularly useful for high-altitude work, where other forms were less accurate. Increasing use is now made of robust solid-state sensors, either the piezoresistive type, in which electrical resistance changes in response to pressure, or the capacitance form, in which flexure of a silicon diaphragm changes the device's capacitance. Versions of the latter exist for the complete pressure range, from surface measurements to use in radiosondes, and this form has many other advantages, not least its accuracy and repeatability
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