Diver Daves Mark-15 External Components Page 2

Mark-15 External Components

This is the rig laid on its' back, ready to dive. You can see the DSV (Dive/Surface valve), and the submersible pressure gauges for diluent and oxygen (with the black spiral wrap). On the bottom is the 'Utility' bottle, which is plumbed into the rig via a Swakelok QC-4 Quick Disconnect, and a 3-port Whitey Valve. More on this later. The bottle also feeds the drysuit, and it shown with an open circuit regulator attached as well. This bottle is not relied on as primary open circuit bailout.

The 'Butt Mount" is fabricated from two plates of Delrin, which are screwed to the bottom of the rig. They are then held in firm rigidity by two stainless steel rods, threaded on the ends for the nuts and washers that bolt the whole thing together. You can see that the radius of the mount seems to be too large for the bottle. The actual radius cut on the mount has two diameters: A major diameter and a minor diameter. The larger radius accepts a 66 ft/3 bottle (7" diameter), and the smaller radius is for 3.9" bottles (2 liter bottle is shown). This allows larger bottles to be mounted for deeper work. I've found that the 66 is really gross overkill, and use either a 2 or 3 liter bottle in this position almost exclusively. You can also see that care has been taken to position the mount so it does not interfere with the shutoff valves for the internal bottles.

Detail of the plumbing of the offboard bottle. The first stage, in this case, feeds the drysuit, the offboard diluent, and an open circuit regulator. In dives where substantial decompression is required, I remove the second stage and simply use stage-bottles for bailout gas supply. I have a small 'button' type pressure gauge on this bottle, for predive checks. Since consumption of this 'Utility' bottle during the dive is not something that requires monitoring, I have elected to not have an additional gauge hose attached. The 90 degree elbow on the feed-line allows the hoses to all be routed across the bottom of the rig for streamlining, yet I have sufficient length on the hose to allow a 'buddy plug' in case I'm trying to share gas in an emergency SCC mode with my (similarly equipped) buddy.

Here is a detail view of the back of the rig, showing the cutout wings that allow water to pass to the counterlung, which is located behind the holes in the chassis. The backplates are very simple stainless steel plates that merely clamp the wings to the chassis, and which have slots in them for the harness. The attachment screws pass through the fiberglass chassis and then thread into a second stainless steel plate that is bonded into the interior of the chassis. The backplates and wings are removed by simply unscrewing the (4) Phillips head machine screws and then lifting the assembly clear. The lower plate is mounted low enough on the chassis so as to allow access to the row of holes across the bottom for attachment of reels, lift-bags, etc. Visible at the lower right is the stainless steel pigtail that connects the offboard diluent to the rig.

Here is a view of the backing plates for attachment of the backplates and wing. The slots for the original military harness are obvious, as are the holes allowing water passage to the counterlung area. Also visible is the carrying handle on the top, a feature often added to allow easier on-boat handling of the rig.

On the left side of the rig is the manual diluent add valve (identical to the 02 add valve shown above), and the selector valve for diluent feed. This allows selection of a second diluent source, to allow gas-changes mid dive. This allows a descent on air diluent, a switch to Heliox based diluent at a precalculated depth (this makes a trimix in the loop), the reselection of air on ascent (with the loop then purged and re-filled, a nitrox decompression can begin), and then at 20 feet, the valve can be positioned at the mid-position, thus shutting off diluent and allowing a pure 02 operation to be conducted for final decompression. This works very well, and is a tried and tested system.

Looking at the back of the rig with the wings removed, you can see the pigtail for the offboard diluent, the diluent select valve, and the on/off switch for the main electronics (the silver colored selector in the top-middle of the photo). The location of the switch can be a bit hard to access when wings are added, so the following fix is available for retrofit.

This clever replacement switch has been fabricated with a Bendix connector to allow a remote-switch to be added. This switch can be hung with a snap from my harness, and allows me to turn the rig on and off even with a drysuit and thick underwear, and also with the wings inflated, something that's quite difficult to do with the original switch. This is a nice upgrade for the rig.

Bailout gas management has been the source of much discussion among rebreather divers, with philosophies ranging from 'pure alpinist', with little or no carried open circuit gas, to 'siege tactics' where a full selection of open circuit stage bottles are carried. For dives requiring little or no decompression, I have attached an Air-Source to the wings, fed from my onboard diluent bottle. I also have a regulator on my butt-mount. This gives two medium sized gas supplies, sufficient for direct ascent to surface. For dives requiring substantial decompression I also carry stage bottles. I use my drysuit as primary buoyancy control, the wings being used only on the surface. This can be done with ease on a rebreather, where total buoyancy change during the dive is minimal. Thus less weight can be carried, as positive buoyancy at the end of the dive is not as great a factor as on open circuit technical dives.

Weighting and trim is important on all rebreathers, and in the case of the Mark-15 deserves special mention. The original mission of the unit was for explosive ordinance disposal, where the diver was essentially 'standing up', thus although the rig is neutrally buoyant, the trim tends to be 'head up'. Add a small butt-mounted bottle and it get's worse. I solved this issue by slicing a 6 pound hip-weight in half and bolting it to the top of the rig, half on each side. This trims the rig out perfectly. I dive the system with a mere 8 pounds of weights on my hips using a Viking drysuit and winter weight underwear. You can see the padeyes I added to allow the rig to be tied down on the boat.

Back to Main Mark-15 Page

	The 'Butt Mount" is fabricated from two plates of Delrin, which are screwed to the bottom of the rig. They are then held in firm rigidity by two stainless steel rods, threaded on the ends for the nuts and washers that bolt the whole thing together. You can see that the radius of the mount seems to be too large for the bottle. The actual radius cut on the mount has two diameters: A major diameter and a minor diameter. The larger radius accepts a 66 ft/3 bottle (7" diameter), and the smaller radius is for 3.9" bottles (2 liter bottle is shown). This allows larger bottles to be mounted for deeper work. I've found that the 66 is really gross overkill, and use either a 2 or 3 liter bottle in this position almost exclusively. You can also see that care has been taken to position the mount so it does not interfere with the shutoff valves for the internal bottles.
	Detail of the plumbing of the offboard bottle. The first stage, in this case, feeds the drysuit, the offboard diluent, and an open circuit regulator. In dives where substantial decompression is required, I remove the second stage and simply use stage-bottles for bailout gas supply. I have a small 'button' type pressure gauge on this bottle, for predive checks. Since consumption of this 'Utility' bottle during the dive is not something that requires monitoring, I have elected to not have an additional gauge hose attached. The 90 degree elbow on the feed-line allows the hoses to all be routed across the bottom of the rig for streamlining, yet I have sufficient length on the hose to allow a 'buddy plug' in case I'm trying to share gas in an emergency SCC mode with my (similarly equipped) buddy.
	Here is a detail view of the back of the rig, showing the cutout wings that allow water to pass to the counterlung, which is located behind the holes in the chassis. The backplates are very simple stainless steel plates that merely clamp the wings to the chassis, and which have slots in them for the harness. The attachment screws pass through the fiberglass chassis and then thread into a second stainless steel plate that is bonded into the interior of the chassis. The backplates and wings are removed by simply unscrewing the (4) Phillips head machine screws and then lifting the assembly clear. The lower plate is mounted low enough on the chassis so as to allow access to the row of holes across the bottom for attachment of reels, lift-bags, etc. Visible at the lower right is the stainless steel pigtail that connects the offboard diluent to the rig.
	Here is a view of the backing plates for attachment of the backplates and wing. The slots for the original military harness are obvious, as are the holes allowing water passage to the counterlung area. Also visible is the carrying handle on the top, a feature often added to allow easier on-boat handling of the rig.
	On the left side of the rig is the manual diluent add valve (identical to the 02 add valve shown above), and the selector valve for diluent feed. This allows selection of a second diluent source, to allow gas-changes mid dive. This allows a descent on air diluent, a switch to Heliox based diluent at a precalculated depth (this makes a trimix in the loop), the reselection of air on ascent (with the loop then purged and re-filled, a nitrox decompression can begin), and then at 20 feet, the valve can be positioned at the mid-position, thus shutting off diluent and allowing a pure 02 operation to be conducted for final decompression. This works very well, and is a tried and tested system.
	Looking at the back of the rig with the wings removed, you can see the pigtail for the offboard diluent, the diluent select valve, and the on/off switch for the main electronics (the silver colored selector in the top-middle of the photo). The location of the switch can be a bit hard to access when wings are added, so the following fix is available for retrofit.
	This clever replacement switch has been fabricated with a Bendix connector to allow a remote-switch to be added. This switch can be hung with a snap from my harness, and allows me to turn the rig on and off even with a drysuit and thick underwear, and also with the wings inflated, something that's quite difficult to do with the original switch. This is a nice upgrade for the rig.
	Bailout gas management has been the source of much discussion among rebreather divers, with philosophies ranging from 'pure alpinist', with little or no carried open circuit gas, to 'siege tactics' where a full selection of open circuit stage bottles are carried. For dives requiring little or no decompression, I have attached an Air-Source to the wings, fed from my onboard diluent bottle. I also have a regulator on my butt-mount. This gives two medium sized gas supplies, sufficient for direct ascent to surface. For dives requiring substantial decompression I also carry stage bottles. I use my drysuit as primary buoyancy control, the wings being used only on the surface. This can be done with ease on a rebreather, where total buoyancy change during the dive is minimal. Thus less weight can be carried, as positive buoyancy at the end of the dive is not as great a factor as on open circuit technical dives.