a seismic gas exploder is coupled to the movable piston of a hydraulic catching system so that the piston is constrained to move with the exploder as it rebounds upward from a shot and then falls back to earth . the weight of the gas exploder is applied through the piston to increase the pressure of the hydraulic fluid in the system when the exploder is moving in a downward direction . a two - position diaphragm actuated pressure switch monitors the fluid pressure , the position of the switch contacts being effective to control an exploder firing circuit or provide other indication of the exploder &# 39 ; s position .