Variometers measure vertical (climbing and descending) speed of aircraft.

Vane type variometers.
The change of air pressure caused by the altitude change causes the pressure difference between the instrument's chamber conncted to a comparition vessel ("thermo flask") and the chamber to which the atmospheric static pressure is fed. The chambers are separated by a vane rotating in jevel bearings and centerde by a spring. The narrow gap between the housing and the vane conects the both chambers. The pressure difference between the chambers is a function of the static pressure rate of change and the vane deflects accordingly. The vane is linked to instument's pointer.

Membrane type variometers.
The change of air pressure caused by the altitude change causes the pressure difference between the instrument's membrane capsule and the static atmospheric pressure fed to the instrument. The inside of the membrane capsule is connected with the atmospheric pressure trough the capillary tube, so the pressure difference and capsule response are functions of the static pressure rate of change. Capsule movements are transmitted to the instrument's pointer.

Total energy variometers.
The vane type variometer of a sailplane can be equipped with so-called total energy compensator. In such a case it does not indicate just the vertical speed of the sailplane but rather the speed of vertical air streams - the information essential for a sailplane pilot.