The VSI consists of several components that are all housed in a sealed case which is connected to the static pressure source. These components comprise of a metering unite, a diaphragm, and an indicating element. A diaphragm inside the instrument expands or contracts according to the pressure change caused while the aircraft is gaining or losing altitude. Movement of the diaphragm is then translated through a mechanical linkage into a needle movement. The needle then indicates the rate of change on a dial presentation which is such that zero is at the 9 o'clock position (see VSI image above).
The diaphragm itself is directly connected to static pressure, and indirectly connected to the area around it through a calibrated leak. Its purpose is to create a lag in static pressure across the system and in this way establish the required pressure differences. If the aircraft climbs, the static pressure inside the diaphragm instantly decreases, while the metering unit restricts the airflow in and out of the sealed case. As a result a differential pressure is created, causing the diaphragm to collapse and so make the pointer indicate a climb.