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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/494,075, filed on Jun. 12, 2012. The patent application identified above is incorporated herein by reference in its entirety to provide continuity of disclosure. 
    
    
     BACKGROUND 
     An oil well typically goes through a “completion” process after it is drilled. Casing is installed in the well bore and cement is poured around the casing. This process stabilizes the well bore and keeps it from collapsing. Part of the completion process involves perforating the casing and cement so that fluids in the formations can flow through the cement and casing and be brought to the surface. The perforation process is often accomplished with shaped explosive charges. These perforation charges are often fired by applying electrical power to an initiator. Applying the power to the initiator in the downhole environment is a challenge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a perforation system. 
         FIG. 2  illustrates a perforation apparatus. 
         FIG. 3  illustrates the perforation system after one of the perforation charges has been fired. 
         FIG. 4  is a block diagram of a perforation apparatus. 
         FIG. 5  is an exploded view of a pressure activated switch. 
         FIG. 6  is a perspective view of elements of a pressure activated switch. 
         FIG. 7  is a perspective view of a pressure activated switch. 
         FIG. 8  is a cross-sectional view of a pressure activated switch before it is actuated. 
         FIG. 9  is a cross-sectional view of a pressure activated switch after it is actuated. 
         FIGS. 10, 11, and 12  are schematics of a perforation apparatus. 
         FIG. 13  is a block diagram of an environment for a perforation system. 
     
    
    
     DETAILED DESCRIPTION 
     In one embodiment of a perforation system  100  at a drilling site, as depicted in  FIG. 1 , a logging truck or skid  102  on the earth&#39;s surface  104  houses a shooting panel  106  and a winch  108  from which a cable  110  extends through a derrick  112  into a well bore  114  drilled into a hydrocarbon-producing formation  116 . In one embodiment, the derrick  112  is replaced by a truck with a crane (not shown). The well bore  114  is lined with casing  118  and cement  120 . The cable  110  suspends a perforation apparatus  122  within the well bore  114 . 
     In one embodiment shown in  FIGS. 1 and 2 , the perforation apparatus  122  includes a cable head/rope socket  124  to which the cable  110  is coupled. In one embodiment, an apparatus to facilitate fishing the perforation apparatus (not shown) is included above the cable head/rope socket  124 . In one embodiment, the perforation apparatus  122  includes a casing collar locator (“CCL”)  126 , which facilitates the use of magnetic fields to locate the thicker metal in the casing collars (not shown). The information collected by the CCL can be used to locate the perforation apparatus  122  in the well bore  114 . A gamma-perforator (not shown), which includes a CCL, may be included as a depth correlation device in the perforation apparatus  122 . 
     In one embodiment, the perforation apparatus  122  includes an adapter (“ADR”)  128  that provides an electrical and control interface between the shooting panel  106  on the surface and the rest of the equipment in the perforation apparatus  122 . 
     In one embodiment, the perforation apparatus  122  includes a plurality of select fire subs (“SFS”)  130 ,  132 ,  134 ,  135  and a plurality of perforation charge elements (or perforating gun or “PG”)  136 ,  138 ,  140 , and  142 . In one embodiment, the number of select fire subs is one less than the number of perforation charge elements. 
     The perforation charge elements  136 ,  138 ,  140 , and  142  are described in more detail in the discussion of  FIG. 4 . It will be understood by persons of ordinary skill in the art that the number of select fire subs and perforation charge elements shown in  FIGS. 1 and 2  is merely illustrative and is not a limitation. Any number of select fire subs and sets of perforation charge elements can be included in the perforation apparatus  122 . 
     In one embodiment, the perforation apparatus  122  includes a bull plug (“BP”)  144  that facilitates the downward motion of the perforation apparatus  122  in the well bore  114  and provides a pressure barrier for protection of internal components of the perforation apparatus  122 . In one embodiment, the perforation apparatus  122  includes magnetic decentralizers (not shown) that are magnetically drawn to the casing causing the perforation apparatus  122  to draw close to the casing as shown in  FIG. 1 . In one embodiment, a setting tool (not shown) is included to deploy and set a bridge or frac plug in the borehole. 
       FIG. 3  shows the result of the explosion of the lowest perforation charge element. Passages  302  (only one is labeled) have been created from the formation  116  through the concrete  120  and the casing  118 . As a result, fluids can flow out of the formation  116  to the surface  104 . Further, stimulation fluids may be pumped out of the casing  118  and into the formation  116  to serve various purposes in producing fluids from the formation  116 . 
     One embodiment of a perforation charge element  136 ,  138 ,  140 ,  142 , illustrated in  FIG. 4 , includes 7 perforating charges (or “PC”)  402 ,  404 ,  406 ,  408 ,  410 ,  412 , and  414 . It will be understood that by a person of ordinary skill in the art that each perforation charge element  136 ,  138 ,  140 ,  142  can include any number of perforating charges. 
     In one embodiment, the perforating charges are linked together by a detonating cord  416  which is attached to a detonator  418 . In one embodiment, when the detonator  418  is detonated, the detonating cord  416  links the explosive event to all the perforating charges  402 ,  404 ,  406 ,  408 ,  410 ,  412 ,  414 , detonating them simultaneously. In one embodiment, a select fire sub  130 ,  132 ,  134 ,  135  containing a single pressure activated switch (“PAS”)  420  is attached to the lower portion of the perforating charge element  136 ,  138 ,  140 ,  142 . In one embodiment, the select fire sub  130 ,  132 ,  134 ,  135  defines the polarity of the voltage required to detonate the detonator in the perforating charge element above the select fire sub. Thus in one embodiment, referring to  FIG. 2 , select fire sub  130  defines the polarity of perforating charge element  136 , select fire sub  132  defines the polarity of perforating charge element  138 , select fire sub  134  defines the polarity of perforating charge element  140 , and select fire sub  135  defines the polarity of perforating charge element  142 . In one embodiment not shown in  FIG. 2 , the bottom-most perforating charge element  142  is not coupled to a select fire sub (i.e., select fire sub  135  is not present) and thus can be detonated by a voltage of either polarity. 
     One embodiment of a pressure activated switch  420 , shown in  FIGS. 5-9 , includes a housing  502  that fits within a housing, not shown, for a select fire sub  130 ,  132 ,  134 ,  135 . In one embodiment, O-rings  806  and  808 , not shown in  FIG. 5, 6 , or  7  but shown in  FIGS. 8 and 9 , provide a seal between the housing  502  and the housing for the select fire sub  130 ,  132 ,  134 ,  135 . In one embodiment, the housing  502  has a large opening  504  at one end and a small opening  506  at the other end. In one embodiment, a large chamber  508  extends from the large opening  504  to a shoulder  510 . In one embodiment, a small chamber  512  extends from the shoulder  510  to the small opening  506 . 
     In one embodiment, a piston housing  514  houses a piston  516 . In one embodiment, the piston housing  514  is cylindrical. In other embodiments (not shown), the piston housing  514  has other shapes, in which the cross-section of the piston housing  514  is square, rectangular, oval, or some other shape. In one embodiment, the piston housing  514  has an outside diameter that fits within the inside diameter of the large chamber  508 . In one embodiment, the piston  516  is cylindrical. In other embodiments (not shown), the piston  516  has other shapes, in which the cross-section of the piston  516  is square, rectangular, oval, or some other shape. In one embodiment, the piston  516  has an outside diameter that is substantially the same (i.e., with enough of a difference to allow for the insertion of O-rings  802  and  804 , not shown in  FIG. 5, 6 , or  7  but shown in  FIGS. 8 and 9 ) as the small piston-receiving chamber  610  (described below). In one embodiment, the piston housing  514  and the piston  516  are made of polyether ether ketone (or “PEEK”). In one embodiment, the piston includes O-rings  802  and  804 , not shown in  FIG. 5, 6 , or  7  but shown in  FIGS. 8 and 9 , that provide a seal between the piston  516  and the piston housing  514 . 
     The piston housing  514 , shown in more detail in  FIG. 6 , has a large contact-housing-receiving opening  602  and a small piston-receiving opening  604 . A large contact-housing-receiving chamber  606  extends from the large contact-housing-receiving opening  602  to a piston-housing shoulder  608 . A small piston-receiving chamber  610  extends from the piston-housing shoulder  608  to the small piston-receiving opening  604 . 
     In one embodiment, the piston housing  514  and the piston  516  are made of a non-conductive material. In one embodiment, the piston housing  514  and the piston  516  are made of PEEK. 
     In one embodiment, an electrically conductive leaf spring  612  is embedded in the piston housing  514  at one end and has a securing bead  614  at the other end. In one embodiment, the spring  612  is made of an electrically conductive spring material, such as copper or bronze. In one embodiment, the spring  612  is a wire. In one embodiment, the spring  612  has a ribbon shape. 
     In one embodiment, the securing bead  614  is a ball of conductive material, such as copper or bronze, welded or soldered to the end of the spring  612 . In one embodiment, the securing bead  614  is formed from the spring  612  by, for example, flattening the end of a wire. In one embodiment, a hole is drilled or otherwise formed in the securing bead  614  to receive a pin as described below. 
     In one embodiment, a conductive bead contact  616  is coupled, e.g., using an adhesive, to a wall of the large contact-housing-receiving chamber  606 . In one embodiment, a hole is drilled or otherwise formed in the bead contact  616  to receive a pin as described below. 
     In one embodiment, the piston  516  has threads  618  at its threaded end  620 . In one embodiment, the threads  618  receive the stop  532  (not shown in  FIG. 6 ). In one embodiment, a tip contact  622  extends from the threaded end  620  of the piston  516 . In one embodiment, a conductor  624 , such as a wire, extends from the tip contact  622  to a pin contact  626 . In one embodiment, the piston housing  614  has holes  628 ,  630 ,  632 , and  634  drilled through from the outer circumference of the piston housing  614  to the large contact-housing-receiving chamber  606 . In one embodiment, hole  628  is substantially (i.e., within 10 degrees) collinear with hole  630  and hole  632  is substantially (i.e., within 10 degrees) collinear with hole  634 . In one embodiment, piston  516  includes holes  636  and  638  that are substantially (i.e., within 10 degrees) perpendicular to a longitudinal axis of the piston  516  and are spaced apart by substantially (i.e., within 1 millimeter) the same amount as holes  628  and  632  and holes  630  and  634 . In one embodiment, the piston  516  can be rotated so that hole  636  is substantially (i.e., within 10 degrees) collinear with holes  628  and  630  and hole  638  is substantially (i.e., within 10 degrees) collinear with holes  632  and  634 . 
     In one embodiment, the hole in bead contact  616  is alignable with hole  634 . 
     In one embodiment, a trigger pin  640  (represented by a hidden line) passes through hole  628  (which is not distinguished in  FIG. 6  from the hidden line representing the trigger pin  640 ), a portion of the large contact-housing-receiving chamber  606  above (as seen in  FIG. 6 ) the piston  516 , hole  636  (which is not distinguished in  FIG. 6  from the hidden line representing the trigger pin  640 ), a portion of the large contact-housing-receiving chamber  606  below (as seen in  FIG. 6 ) the piston  516 , the securing bead  614  and hole  630  (which is not distinguished in  FIG. 6  from the hidden line representing the trigger pin  640 ). In one embodiment, the spring  612  is deflected from a position in which it is relaxed into the position shown in  FIG. 6 , in which the spring  612  is in tension and is urging the securing bead  614  toward the large contact-housing-receiving opening  602 . In one embodiment, the securing bead  614 , which is held in position by the trigger pin  640 , keeps the spring  612  in tension. 
     In one embodiment, when the spring bead  614  is in the position shown in  FIG. 6  it is in electrical contact with the bead contact  616 . In one embodiment (not shown), the bead contact  616  includes a geometrically-shaped object (i.e., a cube, sphere, cone, ovoid, cylinder, parallelpiped, etc., or variations on those shapes) that is projected from the surface of the bead contact  616  by a captive spring imbedded in the surface of the bead contact  616  and can be pressed into the surface of the bead contact  616  by the spring bead  614  while maintaining contact with the spring bead  614 . In one embodiment, the captive spring is conductive and provides an electrical connection to the spring bead  614  and the spring  612 . 
     In one embodiment, a conductive pin  642  (represented by a hidden line) passes through hole  632  (which is not distinguished in  FIG. 6  from the hidden line representing the conductive pin  642 ), a portion of the large contact-housing-receiving chamber  606  above (as seen in  FIG. 6 ) the piston  516 , hole  638  (which is not distinguished in  FIG. 6  from the hidden line representing the conductive pin  642 ), a portion of the large contact-housing-receiving chamber  606  below (as seen in  FIG. 6 ) the piston  516 , the hole in the bead contact  616  and hole  634  (which is not distinguished in  FIG. 6  from the hidden line representing the conductive pin  642 ). In one embodiment, as conductive pin  642  passes through hole  638  it makes electrical contact with pin contact  626  and with bead contact  616 . Thus, in the configuration shown in  FIG. 6 , tip contact  622  is electrically coupled to spring  612  through a pin conductor  624 , pin  642 , bead contact  616 , and securing bead  614 . 
     In one embodiment, the piston  516  has a pinning portion  644  that is the portion of the piston that extends into the large contact-housing-receiving chamber  606  and is pierced by the trigger pin  640  and the conductive pin  642  and a contact portion  646  that includes the portion of the piston that extends outside the piston housing  514 , including the threaded end  622  of the piston  516 . In one embodiment, the pinning portion  644  and the contact portion  646  are adjacent to each other. In one embodiment, there is a portion of the piston  516  between the pinning portion  644  and the contact portion  646 . 
     Returning to  FIG. 5 , in one embodiment, a contact housing  518  includes a first contact  520  and a second contact  522 . In one embodiment, the first contact  520  and second contact  522  are half-circles or half-ovals of spring material as shown in  FIG. 5 . In one embodiment (not shown), the first contact  520  and the second contact  522  are geometrically-shaped objects (i.e., cubes, spheres, cones, ovoids, cylinders, parallelpipeds, etc., or variations on those shapes) that are projected from the surface of the contact housing  518  by captive springs imbedded in the surface of the contact housing  518  and can be pressed into the surface of the contact housing  518  while maintaining contact with the item exerting the pressure. In one embodiment, the captive springs are conductive and provide an electrical connection to the first contact  520  and the second contact  522 . 
     In one embodiment, a first contact conductor  524 , such as a wire, provides an electrical path from the first contact  520  to the rear of the pressure activated switch  420 . In one embodiment, a second contact conductor  526 , such as a wire, provides an electrical path from the second contact  522  to the rear of the pressure activated switch  420 . In one embodiment, the contact housing  518  is cylindrical and has an outside diameter that fits within the piston housing  514 . In one embodiment, a contact housing shoulder  528  and contact housing shelf  530  are sized so that the contact housing shelf  530  fits within the large contract-housing-receiving chamber  606  and the contact housing  518  can be inserted into the piston housing  514  far enough so that the first contact  520  makes contact with the spring  612  but the second contact  522  does not make contact with the spring  612 . This can be seen in  FIG. 7 , which shows an embodiment of an assembled version of the pressure activated switch  420 . In one embodiment, the first contact  520  is in contact with spring  612  but there is a gap  702  between second contact  522  and spring  612 . In the configuration shown in  FIG. 7 , there is an electrical connection between conductor  524  and spring  612  through first contact  520  but no electrical connection between spring  612  and second contact  522 . 
     In one embodiment, the contact housing  518  is made of a non-conductive material. In one embodiment, the contact housing  518  is made of PEEK. 
     Returning to  FIG. 5 , a threaded stop  532  attaches to the threaded end  620  of the piston  516  via threads  618  (see also  FIG. 6 ). In one embodiment, a cap  534 , which in some embodiments is threaded, and a wave washer  536  hold the contact housing  518  in place inside the housing  502 . 
     In one embodiment, the assembly of the pressure activated switch begins by assembling the piston  515 , pins  640  and  642 , and spring  612  as shown in  FIG. 6 . In one embodiment, this assembly is inserted into the housing  502 , with the tip contact  622  and the threaded end  620  of the piston  516  passing through the small opening  506  in the housing  502 . The stop  532  is then screwed on to the threaded end  620  of the piston  516  where it acts to prevent the piston  516  from moving into the piston housing  514  beyond the point where the stop  532  engages the piston housing  514 . In one embodiment, the cap  534  and wave washer  536  secure the contact housing  518  within the housing  502 . 
     As can be seen in the cross-sectional view of one embodiment of the pressure activated switch  420  in  FIG. 8 , while the piston  516  is not restricted in movement by the piston housing  514  (except for the action of the O-rings  802  and  804  which provide a seal between the piston  516  and the housing  502 ), the trigger pin  640  and conductive pin  642  restrict the movement of the piston  516  within the piston housing  514  and the housing  502 . If, in one embodiment, enough force (“F” in  FIG. 8 ) is exerted on the piston  516 , the trigger pin  640  and the conductive pin  641  will break. This is shown in  FIG. 9 , which shows that the piston  516  has moved into the piston housing  514  and has broken the trigger pin  640  and the conductive pin  641  (represented by broken pieces  902  and  904 ). In one embodiment, this will free the securing bead  614  and allow the spring  612  to relax into the state shown in  FIG. 9  in which the spring  612  completes an electrical circuit between conductor  524  and conductor  526 . In one embodiment, increases in the force F caused by the elevated temperatures at depth in an oil well are offset by increased pressure in the large contact-housing-receiving chamber  606  caused by the elevated temperatures. 
     In one embodiment, the pressure activated switch  420  shown in  FIGS. 5-9  is “actuated,” as that word is used in this application, when the transition from the state of the pressure activated switch  420  shown in  FIG. 8  (the “first state”) to the state of the pressure activated switch shown in  FIG. 9  (the “second state”). In the first state, there is no electrical connection between first contact conductor  524  and second contact conductor  526 . In the second state, there is an electrical connection between first contact conductor  524  and second contact conductor  526 . In the first state, there is an electrical connection between the first contact conductor  524  and the tip contact  622 . In the second state, there is no electrical connection between the first contact conductor  524  and the tip contact  622 . 
     In one embodiment, O-rings  806  and  808  provide a seal between the housing  502  and a select fire sub housing (not shown). In one embodiment, a diode  810  determines the polarity of current that can flow through the circuit formed by conductor  524 , first contact  520 , spring  612 , second contact  522 , and conductor  526 . In one embodiment, with the diode  810  arranged as shown in  FIGS. 8 and 9 , current can flow in conductor  524  and out conductor  526 . In an embodiment that is not shown in which the polarity of the diode  810  is reversed, current can flow in conductor  526  and out conductor  524 . 
     In one embodiment, the diode  810  is inside or attached to the contact housing  518 . In one embodiment, the diode  810  is outside the contact housing  518  and is attached to the select fire sub  420  in another way. 
     In one embodiment, the amount of force F required to break the trigger pin  640  and the conductive pin  642  is determined by the following equation:
 
 F=A×P=T  
 
where:
 
     A is the cross-sectional area of the piston  516 , 
     P is the pressure exerted on the piston in the direction of Force F in  FIG. 8  (P out ) minus the pressure inside the piston housing  514  (P in ), i.e., P=P out −P in , and 
     T is the combined tensile breaking strength of the trigger pin  640  and the conductive pin  642 , where tensile breaking strength is the stress required to cause a break. 
     In one embodiment, the conductive pin  642  is not secured to the piston housing  514  so that a trigger-pin-breaking pressure differential, P trigger , generating a force F trigger , needs to be only sufficient to break the trigger pin  640 . In that case, T is the tensile breaking strength of the trigger pin  640 . In an embodiment in which both the conductive pin  642  and the trigger pin  640  are present, a two-pin-breaking pressure differential, P two-pin , generating a force F two-pin , needs to be sufficient to break both pins. 
     In one embodiment, the combined tensile breaking strength of the trigger pin  640  and the conductive pin  642  is between 400 and 600 pounds per square inch. In one embodiment, the combined tensile breaking strength of the trigger pin  640  and the conductive pin  642  is between 300 and 800 pounds per square inch. In one embodiment, the combined tensile breaking strength of the trigger pin  640  and the conductive pin  642  is between 200 and 1000 pounds per square inch. 
     In one embodiment, the trigger pin is non-conductive. In one embodiment, the trigger pin  640  is made of plastic, such as PEEK. In one embodiment, the trigger pin  640  is made of glass. In one embodiment, the trigger pin  640  is made of a ceramic material. In one embodiment, the trigger pin  640  is conductive. In one embodiment, the trigger pin  640  is a thin gauge wire (e.g., AWG 28 or higher) made of metal such as copper or a copper alloy. If the trigger pin  640  is conductive, in one embodiment the trigger pin  640  is installed so that it does not touch or make electrical contact with housing  502 . 
     In one embodiment, the conductive pin  642  is a thin gauge wire (i.e., AWG 28 or higher) made of metal such as copper or a copper alloy. 
     In one embodiment, the cross-section of the piston  526  is a disk measuring 0.5 inches in diameter, in which case its cross-sectional area is 0.196 inches. If the differential pressure across the piston is 1000 psi, the force F exerted on pins  640  and  642  would be 196 pounds. If the pins are made to break at a tensile force of 100 pounds, a differential pressure of approximately 510 psi (producing a force F of approximately 100 pounds) would be sufficient to break them. Such pressures are common in oil wells deeper than approximately 1500 feet. In one embodiment, for shallower wells in which the pressure is less, the pins are designed to break at lower forces. Similarly, in one embodiment, for deeper wells in which the pressure is greater, the pins may be designed to break at higher forces. 
       FIGS. 10, 11, and 12  are schematic diagrams of a portion of perforation apparatus  122 . Only perforating guns  142 ,  138 , and  140  and select fire subs  134  and  132  are illustrated. It will be understood that the perforation apparatus  122  can include any number of perforating guns and any number of select fire subs by repeating the arrangement shown in  FIG. 10 . Select fire sub  134  provides the switching for perforating gun  140  and select fire sub  132  provides the switching for perforating gun  138 . In one embodiment, select fire subs  134  and  132  have the elements illustrated above in  FIGS. 5-9 . In the discussion of  FIGS. 10 and 11  to follow those elements will be referred to by the select fire sub reference number (i.e.,  132  or  134 ) followed by the element number. For example, the first contact (element  520  in  FIGS. 5, 7, 8, and 9 ) in select fire sub  132  will be referred to as first contact  132 / 520 . In one embodiment, there is no select fire sub associated with perforating gun  142 , which means that the detonator  1010  of perforating gun  142  is electrically coupled to pin  134 / 622  by way of a conducting wire and a diode  1008 . A diode  1008  assures that perforating gun  142  is fired with a selected polarity. 
     As can be seen in  FIG. 10 , in one embodiment, a power line  1002  enters at the top of the apparatus. In one embodiment, the power line  1002  is coupled to a power line that flows through other perforating guns, other select fire subs, a CCL, a gamma ray correlator, and other equipment higher (i.e. closer to the earth&#39;s surface  104 ) than the equipment shown in  FIGS. 10, 11, and 12 . In one embodiment, the power line  1002  is coupled to a pass-through line  1004  in perforating gun  138  which passes any voltage present on the pass-through line  1004  to the first contact conductor  132 / 524  of select fire sub  132 . In one embodiment, the first contact conductor  132 / 524  is coupled to the first contact  132 / 520  which is connected to the spring  132 / 612 . In one embodiment, the spring  132 / 612  is in its deflected state in which it is under tension. In one embodiment, the securing bead  132 / 614  at the end of the spring  132 / 612  is in contact with the bead contact  132 / 616 . In one embodiment, the bead contact  132 / 616  provides an electrical connection to the tip contact  132 / 622  through conductive pin  132 / 642  and pin conductor  132 / 624 . 
     In one embodiment, the tip contact  132 / 622  is electrically coupled to a pass-through line  1006  in perforating gun  140  which passes any voltage present on the pass-through line  1006  to the first contact conductor  134 / 254  of select fire sub  134 . In one embodiment, the first contact conductor  134 / 524  is coupled to the first contact  134 / 520  which is connected to the spring  134 / 612 . In one embodiment, the spring  134 / 612  is in its deflected state in which it is under tension. In one embodiment, the securing bead  134 / 614  at the end of the spring  134 / 612  is in contact with the bead contact  134 / 616 . In one embodiment, the bead contact  134 / 616  provides an electrical connection to the tip contact  134 / 622  through conductive pin  134 / 642  and pin conductor  134 / 624 . 
     In one embodiment, the tip contact  134 / 622  is coupled to the cathode of diode  1008 . The anode of diode  1008  is coupled to a detonator  1010 , which is coupled to one or more perforating charges  1012  (i.e., such as perforating charges  402 ,  404 ,  406 ,  408 ,  410 ,  412 , and  414  shown in  FIG. 4 ) through a detonating cord  1014 . The other electrical contact of the detonator  1010  is coupled to the housing of perforating gun  142 , which serves as a ground. 
     In one embodiment, with the perforation apparatus  122  configured as shown in  FIG. 10 , any voltage or power applied to the power line  1002  will be applied to the cathode of diode  1008 . In one embodiment, the detonators on the other two perforating guns  138  and  140 , i.e. detonators  1016  and  1018 , are protected from detonation because the springs  132 / 612  and  134 / 612  are in their deflected positions which means there is no connection between the detonators  1016  and  1018  and the power line  1002 . 
     In one embodiment, a negative voltage is applied to power line  1002  and, through the connections described above, to the cathode of diode  1008 . The same negative voltage, minus a diode drop across diode  1008 , appears at the detonator  1010  causing it to detonate. That detonation causes perforating charge  1012  to explode. 
     The result of the explosion is shown in  FIG. 11 . All or most of the components of the perforating gun  142  have been destroyed and a hole  1102  has been blasted in the housing of perforating gun  142  exposing piston  134 / 516  to fluids from the borehole. Fluids from the borehole (such as formation fluids or drilling mud) enter perforating gun  142  through hole  1102 . These fluids exert pressure on piston  134 / 516  causing it to move into the piston housing  134 / 514 . This movement breaks the conductive pin  134 / 642  and the trigger pin  134 / 640 . The latter action releases the securing bead  134 / 614  and allows the spring  134 / 612  to move to its relaxed position against the second contact  134 / 522 . 
     In this configuration, the perforating gun  140  is armed to fire. In one embodiment, the string of connections from the power line  1002  is the same as described above until it reaches the spring  134 / 612 . In one embodiment, the spring  134 / 612  is in its relaxed position and is in electrical contact with the second contact  134 / 522 . In one embodiment, the second contact  134 / 522  is coupled to the anode of a diode  134 / 810 . In one embodiment, the cathode of the diode is coupled to detonator  1018  in perforating gun  140 , which is coupled one or more perforating charges  1106  (i.e., such as perforating charges  402 ,  404 ,  406 ,  408 ,  410 ,  412 , and  414  shown in  FIG. 4 ) through a detonating cord  1108 . 
     In one embodiment, with the perforation apparatus configured as shown in  FIG. 11  any voltage or power applied to the power line  1002  will be applied to the cathode of diode  134 / 810 . In one embodiment, the detonator on perforating gun  138 , i.e. detonator  1016 , is protected from detonation because the spring  132 / 612  is in its deflected position which means there is no connection between the detonator  1016  and the power line  1002 . 
     In one embodiment, a positive voltage is applied to power line  1002  and, through the connections described above, to the anode of diode  134 / 810 . In one embodiment, the same positive voltage, minus a diode drop across diode  134 / 810 , appears at the detonator  1018  causing it to detonate. In one embodiment, that detonation causes perforating charge  1106  to explode. 
     The result of the explosion is shown in  FIG. 12 . All or most of the components of the perforating gun  140  have been destroyed and a hole  1202  has been blasted in the housing of perforating gun  140  exposing piston  134 / 516  to fluids from the borehole. Fluids from the borehole (such as formation fluids or drilling mud) enter perforating gun  140  through hole  1202 . These fluids exert pressure on piston  132 / 516  causing it to move into the piston housing  132 / 514 . This movement breaks the conductive pin  132 / 642  and the trigger pin  132 / 640 . The latter action releases the securing bead  132 / 614  and allows the spring  132 / 612  to move to its relaxed position against the second contact  132 / 522 . 
     In this configuration, the perforating gun  138  is armed to fire. In one embodiment, the string of connections from the power line  1002  is the same as described above until it reaches the spring  132 / 612 . In one embodiment, the spring  132 / 612  is in its relaxed position and is in electrical contact with the second contact  132 / 522 . In one embodiment, the second contact  132 / 522  is coupled to the cathode of a diode  132 / 810 . In one embodiment, the anode of the diode  132 / 810  is coupled to detonator  1016  in perforating gun  138 , which is coupled one or more perforating charges  1204  (i.e., such as perforating charges  402 ,  404 ,  406 ,  408 ,  410 ,  412 , and  414  shown in  FIG. 4 ) through a detonating cord  1206 . 
     In one embodiment, with the perforation apparatus configured as shown in  FIG. 12  any voltage or power applied to the power line  1002  will be applied to the cathode of diode  132 / 810 . In one embodiment, a negative voltage is applied to power line  1002  and, through the connections described above, to the cathode of diode  132 / 810 . In one embodiment, the same negative voltage, minus a diode drop across diode  132 / 810 , appears at the detonator  1016  causing it to detonate. In one embodiment, that detonation causes perforating charge  1204  to explode. 
     In one embodiment, the polarity of the diodes  1008 ,  134 / 810 , and  132 / 810  are chosen so that alternating positive and negative voltages on the power line  1002  are required to detonate alternate perforating guns. That is, a negative voltage on the power line  1002  is required to detonate perforating charge  1012  as dictated by diode  1008 , a positive voltage on the power line  1002  is required to detonate perforating charge  1106  as dictated by diode  134 / 810 , and a negative voltage on the power line  1002  is required to detonate perforating charge  1204  as dictated by diode  132 / 810 . 
     In one embodiment, the perforating system  122  is controlled by software in the form of a computer program on a computer readable media  1305 , such as a CD, a DVD, a portable hard drive or other portable memory, as shown in  FIG. 13 . In one embodiment, a processor  1310 , which may be the same as or included in the firing panel  106  or may be located with the perforation apparatus  122 , reads the computer program from the computer readable media  1305  through an input/output device  1315  and stores it in a memory  1320  where it is prepared for execution through compiling and linking, if necessary, and then executed. In one embodiment, the system accepts inputs through an input/output device  1315 , such as a keyboard or keypad, and provides outputs through an input/output device  1315 , such as a monitor or printer. In one embodiment, the system stores the results of calculations in memory  1320  or modifies such calculations that already exist in memory  1320 . 
     In one embodiment, the results of calculations that reside in memory  1320  are made available through a network  1325  to a remote real time operating center  1330 . In one embodiment, the remote real time operating center  1330  makes the results of calculations available through a network  1335  to help in the planning of oil wells  1340  or in the drilling of oil wells  1340 . 
     The word “coupled” herein means a direct connection or an indirect connection. 
     The text above describes one or more specific embodiments of a broader invention. The invention also is carried out in a variety of alternate embodiments and thus is not limited to those described here. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.

Summary:
A first end of a conductive spring is embedded in a wall of a large chamber of a piston housing. The spring is held in tension by a second end of the spring being pinned against a bead contact by a trigger pin. The diameter of the piston and a tensile breaking strength of the trigger pin are selected so that the trigger pin is breakable and the tension in the spring is releasable upon the presence of a predetermined pressure difference between a pressure on the contact side of the piston and a pressure on the pinning side of the piston. Release of tension in the spring closes an electrical circuit.