A Bourdon tube pressure gauge has an elastic tube that is soldered or welded toward a single side into a socket. An adjustment of pressure causes a deflection within the tube; This is often proportional for the used pressure and is shipped to the rotary gear which has a pointer. https://lt1464acn8-pbf76307.tinyblogging.com/capsule-pressure-gauge-can-be-fun-for-anyone-74917872