What causes the pressure differential in a vertical speed indicator?

The pressure differential in a vertical speed indicator (VSI) is caused by the use of two aneroid wafers that are differentially sensitive to changes in atmospheric pressure. When the aircraft climbs or descends, the external atmospheric pressure changes, causing a pressure difference across the aneroid wafers. This difference in pressure results in the movement of the wafers, which is translated into a visual indication of the aircraft's vertical speed on the VSI dial.

One aneroid is exposed to the static pressure, while the other has a calibrated leak that allows it to respond to changes in pressure at a slower rate. As the aircraft ascends or descends, the VSI measures the rate of change of this pressure difference over time, indicating whether the aircraft is climbing, descending, or in level flight. Thus, the mechanism behind the VSI relies fundamentally on the pressure differential created by the two aneroids with their specific design and calibration.