valves are also available. Two types of cartridge disk-release  techniques are used. Standard release-type uses a slug  driven by explosive energy to rupture a segmented closure  disc. For high temperature or hermetically sealed units, a  direct explosive impact-type cartridge is used that applies  fragmentation impact to rupture a prestressed corrosion  resistant steel diaphragm. Most containers use conventional  metallic gasket seals that facilitate refurbishment following  discharge.  [Figure 17-14]  A wide range of diagnostics is utilized to verify the fire  extinguisher agent charge status. A simple visual indication  gauge is available, typically a helical bourdon-type indicator  that is vibration resistant.  [Figure 17-13]  A combination  gauge switch visually indicates actual container pressure  and also provides an electrical signal if container pressure is  lost, precluding the need for discharge indicators. A ground  checkable diaphragm-type low-pressure switch is commonly  used on hermetically sealed containers. The Kidde system  has a temperature compensated pressure switch that tracks  the container pressure variations with temperatures by using  a hermetically sealed reference chamber. Pressure Indication Two-Way Check Valve Discharge Indicators Two-way check valves are required in a two-shot system  to prevent the extinguisher agent from a reserve container  from backing up into the previous emptied main container.  Valves are supplied with either MS-33514 or MS-33656  fitting configurations. Discharge indicators provide immediate visual evidence  of container discharge on fire extinguishing systems. Two  kinds of indicators can be furnished: thermal and discharge.  Both types are designed for aircraft and skin mounting.  [Figure 17-15]  Figure 17-14.  Discharge valve (left) and cartridge, or squib (right). Figure 17-15.  Discharge indicators. The thermal discharge indicator is connected to the fire  container relief fitting and ejects a red disk to show when  container contents have dumped overboard due to excessive  heat. The agent discharges through the opening left when the  disk blows out. This gives the flight and maintenance crews  an indication that the fire extinguisher container needs to be  replaced before next flight. If the flight crew activates the fire extinguisher system, a  yellow disk is ejected from the skin of the aircraft fuselage.  This is an indication for the maintenance crew that the fire  extinguishing system was activated by the flight crew, and  the fire extinguishing container needs to be replaced before  next flight. The engine and APU fire switches are typically installed on  the center overhead panel or center console in the flight deck.  [Figure 17-16]  When an engine fire switch is activated, the  17-13 Fire Switch Yellow Disk Discharge Indicator Thermal Discharge Indicator (Red Disk)