Image Number 2 for United States Patent #4648470.
A firing head for a tubing conveyed perforating gun is shown which is operated by pressuring the interior of the tubing string. A sliding sleeve having a pressure responsive seal area changes position at a predetermined increase in tubing pressure. Movement of the sliding sleeve drains an initially filled, hydraulic cylinder within the interior of the tool. As the cylinder is drained, an associated fluid piston moves to occupy the evacuated chamber. The fluid piston is connected to a firing pin holder which releases the firing pin. Well annulus pressure acts through a port in the tool upon a pressure responsive seal area on the firing pin, thereby propelling the firing pin into a percussion detonator to fire the perforating gun.