Here we see the center section with the cover removed. The actual scrubber cannister is still in place, and you can see the green sensor bay in the center of the scrubber. Also visible is the foam water-absorbent pad in the inside of the lid. This is designed to soak up a reasonable amount of drool, condensation, and other water that may collect in the center section.
	Here is the scrubber removed for filling. You can see the gas-path holes, and under those you can just barely see the Porex barrier that filters out any dust, and provides a 'water barrier' of sorts. The Porex is not waterproof, but it does slow the rate of water entry into the scrubber itself if any does accumulate in the center section.
	Peering down through the center of the scrubber, we can see the three sensors held in their brackets at a 120 degree spacing from each other. Having the faces of the sensors on different orientations assists in preventing blockage of the sensor faces by water vapor. Sensors are located out of the main gas-path, so they are exposed to less than the total volume of water vapor present, yet are in a position where each breath passes a sample of gas over their faces. This makes the Mark-15 one of the most resistant designs to sensor impairment due to condensation.
	Looking down into the center section with the scrubber removed, one can see more details of the sensor array, the water-absorbent pads, and the stainless steel grate that holds the absorbent pads in place. The exhalation port is at the leftmost side. It is important to be able to visualize the gas-path of the system. Look carefully at the interior diameter of the scrubber bay of the center section and you will notice that it is not the full diameter of the center section. Close inspection will reveal about a 1/2 inch gap between the outside wall of the center section and the scrubber bay. This is the path through which gas flows from the scrubber (on this side) to the counterlung (on the other side of the center section. So: The gas-path is from the exhale hose, into the port shown here, through the scrubber, and then through the annular port to the other side of the center section where the counterlung diaphragm resides. From that space the gas if passed back to the inhale hose.
	Here is a detail of the interior of the center section with the scrubber removed. There is an additional water-absorbent pad located here, and you can see the label indicating the maintenance history of the unit exposed for viewing.
	Located so as to port gas to the interior of the scrubber is the exhalation port, which is fed from the divers exhaust hose. In this photograph, the absorbent pad shown in the previous photograph has been removed, and the interior of the center section is now plainly visible.
	The center section is held to brackets on the chassis by a few machine screws with lock-washers. Here the screws have been removed and the center section lifted from the chassis. You can see one of the loops where the sphere retention O-Rings are attached, and on the extreme left one of the brackets that attaches the center section of the chassis. The main Bendix connection to the electronics has been disconnected and is shown, as well as the face-seal that brings diluent  to the Schraeder valve (auto add valve). Each of these components will be described in detail below.  The rubber counterlung diaphragm is visible on the top of the photo.
	I have now removed the center section and flipped it over on it's back to reveal the counterlung. This 'counterlung' is really little more than a large diaphragm, as found on a king-sized second stage regulator. In fact, it -is- the diaphragm of a king-sized second stage regulator, as when it is drawn down hard by the diver collapsing it by breathing from a counterlung with insufficient volume to fill the divers lungs, it presses against the Schraeder valve, which adds gas exactly like any normal SCUBA second stage. The concealed internal position of this diaphragm is one of the main factors leading to the reliability of the Mark-15. As you can see, there is little if any possibility of damaging the counterlung during a dive.
	This is the interior view of the exhaust valve. The center button is adjustable in height, and it's this that triggers the addition of diluent from the Schraeder valve.
	And here is the other side of the exhaust valve. Note the tag showing date of overhaul. This type of logging of work is essential to maintaining a complex system of this nature.
	Like all components in the design, the counterlung attachment band is of the highest quality MILSPEC material available. This is an aircraft quality component, and is as good as it gets.
	The counterlung being removed, we can now see the side of the center section that is normally against the divers back. The Schraeder valve (automatic add valve) is visible in the very middle of the bottom of the sensor bay, and the wires leading from the horseshoe board to the Bendix connectors are visible as well.
	A close detail of the horseshoe board. This board was originally potted with an epoxy, and the wires leading from it were silver, and subject to corrosion. In this custom board, the wires have been replaced and a new board installed. The board is potted into place with crystal-clear potting compound. The entire assembly is waterproof, yet is visible for inspection against water-intrusion, etc. The Schraeder valve for the auto-addition is clearly visible in the center of the assembly.
	Here's the Schraeder valve button in detail. This pushes nothing more than a stainless steel tire-valve, which is the 'regulator' mechanism for the auto addition. While this system at first glance seems crude, it has many advantages. First, it's extremely simple. Second, the Schraeder is an 'upstream' valve, meaning that you need to push the valve core towards the gas-inlet flow. This means that the valve will lock tighter in the event of a high pressure leak in the diluent first stage, and not freeflow. Since it's a 3000 PSI valve, it can hold the actual sphere pressure if need be. The high cracking force is not an issue with the large mechanical advantage of the extremely large diameter of the counterlung.  Elegantly simple and absolutely reliable in every way.
	Here is the injection port, where gas is added to the center section. Manually added 02, solenoid added 02, and diluent all feed this mixing nozzle. The nozzle is designed to swirl the injected gas with the gas in the counterlung for fast mixing of the two. Multiple holes prevent a clogging failure at this critical point.
	The last component of the interior of the center section that we'll examine is the inhalation port, which feeds the inlet hose to the diver. Here you can see the annular slot where gas passes from the scrubber (mounted on the other side of the center section) to the counterlung side, where it can then pass to the diver when he breathes.