A "classic" VSI is supplied with static pressure which is fed both directly into a capsule and into the instrument case through a calibrated orifice. The pointer deflection is a function of the differential pressure between the capsule and the interior of the instrument case. Thus, the instrument does not measure pressure it measures the Rate of Change of Pressure. The calibrated leak is self-compensated by the change in the viscosity of the air that occurs with the change in source pressure. The cross-sectional area is matched to the length of the orifice so as to obtain a virtually constant characteristic based on the rate of change of pressure regardless of the source pressure. Obviously there are limits to which this tactic can work for, as you approach a vacuum, there is insufficient energy in the gas to support the capsule. There are slight errors but the instrument may be considered accurate enough at the comparitively low altitudes achieved by aircraft using air-breathing engines.
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Through difficulties to the cinema
[This message has been edited by Blacksheep (edited 21 March 2001).]