The VSI is in a sealed case connected to the static line through a calibrated leak (restricted diffuser). Inside the case, a diaphragm attached to the pointer by a system of linkages is vented to the static line without restrictions.

As the aircraft climbs, the diaphragm contracts and the pressure drops faster than the case pressure can escape through the restrictor, resulting in climb indications; the reverse is true during descent. If level flight is resumed, pressure equalizes in the case and diaphragm within six to nine seconds and the pointer returns to zero rate of climb. The vertical speed indicator has 100-ft calibrations with numbers every 500 ft (see Face of Vertical Speed Indicator figure below).

Although the vertical-speed indicator operates from the static pressure source, it is a differential pressure instrument. The differential pressure is established between the instantaneous static pressure in the diaphragm and the trapped static pressure within the case.

When the pressures are equalized in level flight, the needle reads zero. As static pressure in the diaphragm changes during entry to a climb or descent, the needle immediately shows a change of vertical direction. However, until the differential pressure stabilizes at a definite ratio, reliable rate indications cannot be read. Because of the restriction in air flow through the calibrated leak, a 6- to 9-second lag is required to equalize or stabilize the pressures.

Limitations in the use of the vertical-speed indicator are due to the calibrated leak. Sudden or abrupt changes in aircraft attitude cause erroneous instrument readings as the air flow fluctuates over the static ports. Both rough control technique and turbulent air result in unreliable needle indications. When used properly, the instrument provides reliable information to establish and maintain level flight and rate climbs or descents.

The instantaneous vertical-speed indicator incorporates acceleration pumps to eliminate the limitations associated with the calibrated leak. For example, during climb entry, vertical acceleration causes the pumps to supply extra air into the diaphragm to stabilize the pressure differential without the usual lag time. During the level flight and steady rate climbs and descents, the instrument operates on the same principles as the earlier conventional type.