Vertical Speed Indicator (VSI) shows a RATE of change in pressure. In order to achieve this the capsule is not a sealed one, like altimeters, but inside the capsule we receive the value of the atmospheric pressure. At the same line, there is a restrictor that feeds the case of the instrument with the value of atmospheric pressure but with a delay due to the restrictor. That leads the capsule to expand or retract not immediately with the change of the value, but with a small delay due to the delay of the restrictor.

This difference between the present Static pressure and the Static pressure of the previous moment, is translated as a Rate of Climb or Descent. When the pressure of the capsule is the same as the case, it means that the aeroplane is not climbing or descending, showing a value of zero differential pressure so 0 feet per minute.

The working principle of a Vertical Speed Indicator is measuring the present Static air pressure and comparing it to the Static air pressure inside a reservoir.