Safety device to prevent premature firing of explosive well tools

Apparatus for use in preventing the firing of an explosive well tool until said well tool has been lowered to or below a predetermined depth in a fluid-filled well bore, comprising:

• a generally tubular body having threaded connection means at its upper and lower ends, said body defining upper and lower chambers, passage means for communicating adjacent ends of said chambers, and a transverse bore intersecting said passage means;
• piston means slidably arranged in said bore for movement between an outer position and an inner position, said piston means having an outer face that is subject to the hydrostatic pressure in the well bore, said piston means having an inner section that extends across said passage means in said outer position and an opening that is substantially aligned with said passage means in said inner position;
• spring means for yieldably resisting movement of said piston means toward said inner position;
• first explosive means mounted in said upper chamber;
• and second explosive means mounted in said lower chamber and arranged to be exploded in response to pressure forces generated by detonation of said first explosive means, said explosive forces being blocked from reaching said second explosive means until said opening in said piston means is substantially aligned with said passage means.