Capsule gauges get their name from an internal pocket created by joining two circular membranes along the outer edges. The membranes act as diaphragms. One has an opening in the center allowing the gas being measured into the capsule.
As pressure changes, the diaphragms expand or contract. The change in shape is relayed via deflection lever, which translates the change into a rotary movement. That rotary movement is displayed on the gauge’s face, providing a reading.
Gauges with similar designs have been used since the 1920s.