How does the Vertical Speed Indicator (VSI) provide readings?

The Vertical Speed Indicator (VSI) specifically measures the rate of climb or descent of the aircraft and operates based on the principle of a calibrated leak. Inside the VSI, there is a diaphragm that responds to changes in atmospheric pressure. As the aircraft climbs or descends, the rate of change in static pressure surrounding the diaphragm affects its movement. The calibrated leak allows the internal pressure of the VSI to equalize with the static pressure at a controlled rate, creating a differential pressure that results in a reading of how quickly the aircraft is climbing or descending.

This method is essential because it enables the VSI to provide a smooth and accurate indication of vertical speed, without being immediately reactive to transitory changes in altitude, thereby giving the pilot a reliable measure of their vertical lift or descent rate over time.