vertical speed indicator

A flight instrument that indicates the rate of climb or descent or the zero, or level, position at 9 o’clock. The VSI is a sensitive pressure gauge that senses the rate of change of static pressure. Atmospheric pressure is led directly into an aneroid capsule or a diaphragm contained within the case of the instrument. Air is permitted to leak at a relatively slow rate through a capillary tube into the casing. The difference between the quick change in pressure within the aneroid capsule and the relatively slow rate at which this pressure is equalized within the case causes the capsule to expand or contract. This movement is magnified and indicated on the dial through a system of gearing. See also instant rate of climb indicator.