Editing Pressure measurement (section)

====Bellows====
[[File:Barograph 03.jpg|thumb|A pile of pressure capsules with corrugated diaphragms in an aneroid [[barograph]]]]

In gauges intended to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an '''aneroid'''. (Early [[barometer]]s used a column of liquid such as water or the liquid metal [[mercury (element)|mercury]] suspended by a [[vacuum]].) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), [[altimeter]]s, altitude recording [[barograph]]s, and the altitude [[telemetry]] instruments used in [[weather balloon]] [[radiosonde]]s. These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as [[airspeed indicator|air speed indicators]] and rate of climb indicators ([[variometer]]s) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber.