With the center section removed and laid aside, the balance of the chassis is exposed for further study. Details of the external trim weights are visible, as is the red internal trim weight originally fitted at the factory. The backing plate for the upper backplate is seen as well, along with the acorn nuts that prevent the threads from the installed backplate from damaging the counterlung. All that remains in the chassis is the pneumatics system and the electronics system.
Starting with the 02 portion of the pneumatics system, the 02 is lead from the regulator, though a filter, and to a T fitting, which distributes the oxygen to the solenoid and the manual add valve. This photo shows the filter (top) and the T (bottom) where the flow is divided to feed the manual add valve and the solenoid.
This is the interior view of the manual 02 add valve. This is a Whitey toggle-valve with a custom 'paddle' substituting for the original toggle. You can see the feed line and the exit line, which then will go back towards the injector manifold where all of the gases that are to be injected join before being routed to the injector nozzle.
This view is looking from the area of the removed center section, and shows the next two components on the 02 side. Supplied 02 is fed through a restricting orifice to a 50cc accumulator, visible as the silver tube in the photo. A 50cc bolus of Oxygen is then dumped into the injection manifold when the solenoid is triggered. Using this system, a metered amount of 02 can be accurately injected into the manifold, without needing to accurately time the solenoid pulse-duration. The red item behind the mounting tab is the solenoid itself.
Here is the heart of the automatic injection system, the solenoid. When it is triggered by the electronics, it allows the 50cc accumulator to dump the metered quantity of 02 into the manifold, and thus to the injector port. You can see the accumulator behind the bracket, and the cluster of tubes just under the electrical connectors is the actual injection manifold. In this view the center section is installed in the chassis.
The solenoid is connected to the primary electronics with a Bendix connector. In this photo the Bendix has been removed for inspection. Bendix connectors are waterproof to almost infinite depth even when open faced to the ocean. A flooded connector will very likely still function, as the contacts are gold, and in any case will not allow water to pass into either the cable or the component to which it is attached. To say that these MILSPEC connectors are expensive is an understatement. They are also unparalleled for reliability.
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While we are on the topic of Bendix connections, they are color coded to show when they are tightened, and when they are not. The red band showing on this connector shows that it is plugged in, but has not yet been tightened by hand to close it. This particular Bendix is the connection between the primary display and the main electronics pod, but all are of the same design.
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In contrast to the above, this Bendix is 'Black', or showing safe for dive. It is little details like this that set the military design of the Mark-15 apart from civilian designs. Are these details essential? Certainly not. But they do show an attention to design detail that is far beyond the norm.
Having explored the 02 side of the system, here is the diluent side. Without a solenoid, the diluent system is slightly simpler. Here you can see the interior of the manual add valve, identical to the 02 side. In addition, you can see the back of the Whitey diluent selection valve, which allows selection of either the on-board or off-board gas as diluent. The entry of the pigtail from the offboard supply can be seen in the middle of the photo, feeding one side of the valve. On the other side is the supply from the on-board regulator. The exit from the valve bifurcates, feeding the manual add valve on one side, and then through that the injection manifold. The other branch feeds the Schraeder valve for automatic diluent addition and demand breathing.
The pneumatic system feeds gas to the center section by Cajon (Swagelok) face-seals, which are an O-ring sealed connector. On the right you can see the face-seal that runs from the injection manifold to the injection port. Through this line are added manually added diluent, manually added 02, and solenoid added 02. On the other side of the center section, a line feeds the Schraeder valve through a second face-seal fitting. Also seen in this photo is the open Bendix receptacle for the analog secondary display, which directly reads the voltage of the sensors for display as raw PP02.
Here is the other face-seal for feeding the Schraeder valve. You can see the diluent filter at the extreme left, and then the branch in the diluent line where gas is taken to the manual add valve on one hand and to the Schraeder valve on the other. Also seen is the main Bendix cable, that leads the sensor output to the main electronics pod. The red component at bottom center is the original main on/off switch. The braided stainless steel flex-hose is the diluent high pressure gauge line to the divers diluent gauge