Bringing a 30yr Old Rebreather Up to Date

The Mark 15 is a great design but it is an old design. There is no other rebreather with the pedigree or history of this series and even by today's standards there are not many other units which can rival it for performance. For a rebreather first produced in 1968 it is still one of the most capable systems available.

But there are downsides and one of those is that 25-30yr old components may be well made but they are still 25-30yrs old. They will fail eventually. And when the electronics control pod failed on mine then I had no choice but to find a replacement. It says a lot when such an old design is still has control systems being produced by at least three manufacturers (Juergensen Marine, Laguna Research and Colkan). Colkan do a digital replacement for the basic analogue controller. It is by all accounts an extremely well made piece of equipment but it is a like for like swap, essentially a digital version of the analogue board which still has the same downsides i.e. fixed setpoint, same power hungry solenoid. The Laguna Research stuff looks very well thought out and it gets very good reviews but I couldn't get hold of the manufacturers. Kevin Juergensen of Juergensen Marine has been working on Mk15's for years and through a phone call and a few emails he tried to resurrect my old pod but after talking with him I decided the best way forward was to order a Hammerhead system. And pretty much good as his word, the system was ready within a week or so and not long after that was on my doorstep.

Things I really don't like about the Mk15: high draw solenoid that eats power and not easy to find a replacement for; a fixed setpoint that cannot be changed without opening the pod up; custom battery packs (kindly worked around by Carl from Sartek who made me a battery holder for 16 ordinary AA batteries); cables that are equally as hard to find as the solenoid; a switch that is just about impossible to reach with a drysuit on. The Hammerhead works round all of these. For a start, the whole thing is powered off a single AA battery. That's the controller, display, solenoid and HUD. All the cabling is replaced with Lumberg cables and connectors which are easier to replace if required. The controller's settings can be altered from the handset, the user can swap between setpoints or have the controller do it automatically and the solenoid draws very little power from the system. Add to that a very well designed HUD system and built in trimix decompression. The B-S-Ometer particularly made me smile. And the HH is now used in the Hammerhead rebreather, the O2ptima and is available for the Megalodon and Inspiration so there should be no problem with support and supply issues for the future.

Dave Sutton's teardown is the benchmark for descriptions of the Mk15 so I am not going to attempt to replicate that. But there were a few jobs I had to do as well as installing the Hammerhead so I took advantage of some late October sunshine to do a photo record. As Hank Hill in King of the Hill once replied to the question of when to change the oil in your truck: "Every 5000 miles... or whenever I get bored". The same could be said about taking apart rebreathers...

Step 1, gut the existing unit. Spheres and centre section have been set aside, the old electronics have been stripped out and sold as spares. Next thing, remove the old solenoid and fit the new one. The way I did it:
  1. Undo the compression fittings on the old solenoid.
  2. Undo the mounting bracket from the chassis.
  3. Remove the solenoid from the mounting bracket.
  4. Remove the Swagelok/NPT adapters and clean off the old tape.
  5. Tape the fittings and fit in the new solenoid.
  6. Fit the new solenoid in the mounting bracket.
  7. Fit the solenoid in place, hand tightening the Swagelok fittings.
  8. Screw the bracket in place.
  9. Fully tighten the Swagelok fittings.

Although the solenoid bracket is also one of the fixing points for the centre section you don't need to remove the centre section. I only did it because I'd let the unit sit just a little too long in the warm weather and the inside tasted... Well, it tasted. It shouldn't taste of anything so the time had come to Virkon everything. Lesson learned. Handling the centre section with the lung attached has to be one of the most terrifying experiences in working on a Mk15.

Next step, refit all this crap back into the chassis!
The centre section has been replaced just to get it out of the way. Did I mention I don't like handling it? Fitting a thin, irreplaceable rubber membrane past those stainless lugs is about 8.6 on the sphincter scale.

With the solenoid in place you can now fit the new pod. Unlike the old pod, the HH pod is just a junction box. You don't need access to it, everything is powered and controlled from the handset. Simply drop it in place of the old pod, fit the stainless bracket and screw the whole thing down. The HH pod is actually slightly smaller than an original pod so it rattles a little in the bracket. A wrap or two of rubber tape round it may happen in the future.

Juergensen Marine take your old Bendix cable and splice it into the new pod so once the pod is fitted all you have to do is connect it to your centre section with the original plug and socket. After that, hook the solenoid to the pod with the Lumberg fitting provided (yellow cable in the picture).

With the pod fitted and the centre section connected then it's time to fit the DIVA. This is a nifty little HUD unit with two modes. It will either flash to tell you if you are at setpoint or not or it will flash a light sequence at you to tell you the actual setpoint. Either mode is selectable from the handset. I've got mine on the green/good, red/bad mode as it is similar to how the original primary worked. An added feature is the DIVA has a vibrating alarm for situations that need serious attention. As it is fitted to your DSV then the unit rattles your teeth so, in theory, should make you pretty aware.

Also connected is the very nice Sartek secondary. Carl at Sartek replaced the old Bendix secondary port on my centre section with a Lumberg. His secondary is not dissimilar to how the KISS and various homebuilt displays work: circuit board, panel meter, trim pot to calibrate and a battery. A very simple digital alternative to the old analogue secondary. The secondary came on a long cable and the excess was taken up by a couple of wraps round the pod.
I always hated trying to fit the Bendix plugs as they were a pain to get aligned. On the Lumbergs I've marked a black line just to make life easier. I've never claimed to be clever and anything that makes life simple helps!
I use R22DHO cells on my Mk15 rather than the D10's. These are the same shape as R22D's as used in the KISS and Inspiration and unfortunately are too big to fit in the sensor ports. They need to have the tip of the cell removed precisely... I do mine by tying them down and chopping them with a Chinese meat cleaver, one crack and the tip is gone.

Normally they go cell face pointed to the centre of the sensor holder but they are narrower than the port so need either packed out with some o-rings or left to rattle. I don't like either so, inspired by the Laguna Research sensor holder, I modified mine. OK, it's nothing impressive, I just removed the rubber grommets for holding the D10 sensors in place to give me a bigger hole.
The cells are then fitted sideways and fit perfectly. In fact, it is a nice, tight fit, the cells do not move at all once in place. The assembly is finished off by some rubber inner tubing just to hold everything in place. On the surface they seem to work fine in this orientation and I'll see how they go underwater.
Here are the cells in place with the scrubber fitted. Before there was just enough room to get the scrubber on and quite a few times I'd clipped the top of a cells or a wire and pulled a banana plug out of the board. As can be seen from the photo, there is now a lot of clearance between the cells and the inside of the scrubber. They could even be fitted with molexes now if I needed to replace a cell and didn't have the chance to solder the wires on.
This is my "reassure Dave Sutton" picture. When I bought the unit, Dave was quite concerned that the top spider was (a) not the right one (it was a bottom spider) and (b) slightly damaged, so would not retain the top pad properly. Dave recommended getting someone to weld something up from stainless wire but being a homebuilder at heart my solution was to cut a spider from the side of an old sofnalime keg and use it to retain the wire spider. Similarity to a Nazca spider is entirely coincidental and due to my incompetence with a pair of shears.
The unit with the spheres in place. The oxygen regulator is original and still retains the stainless hoses. On the diluent side I needed spare ports for my wing and offboard connection so I had to go with another regulator. A lot of people have used Poseidon 1st stages so so have I. The long hose is the connection to the BOV which is in turn connected to my offboard by a QC.

I am not entirely happy with the set up, not least because I've lost the stainless hose advantage. I do have the option of adding offboard to the loop via the BOV so I am planning to go back to stainless hoses and no offboard connection.
Here is the unit with the case fitted. The unit came without the standard latchess on the side and is instead secured by a velcro strap at the bottom. I have to say this is a very secure modification. I have seen more than a few YBOD owners come back on the boat with their latches popped or the shell loose. The velcro strap has yet to let me down.
Here is my hi-tech argon strap mounting: a couple of loops of inner tube threaded through a couple of stainless D's.
Finally, the BOV and loop hoses are fitted. The BOV is a Golem model which is well made. I prefer it to my old Jetsam unit, it breathes better, feels better in the mouth and the OC bailout switch is very smooth. I always found the Jetsam version a bit clumsy and a bit big, the Golem BOV has the 2nd stage on the bottom and the switch on the front.

The DIVA has been fixed in place with a piece of inner tube looped round it. It was the simplest (i.e. cheapest) option I could find and seems secure enough. Again, it needs to be seen in the water to see if it all works.

Also visible is the Enrique Alvarez split backplate, not overly comfortable on the surface but very comfortable and very secure underwater. The wing is an old Zeagle I had knocking around. I made some reinforced eyes in the cordura bag at the top of the wing to fit the top plate, at the bottom I extended the existing straps and made them into loops which I threaded around the bottom plate. It works well and has the added benefit of not having a favbric centre panel so the perforated counterlung plate is completely open to the water. How important is that when it is sat on top of the back of your drysuit anyway? I don't know, it can't hurt though.

So that is the installation of the Hammerhead controller. It probably took longer to unpack the set from the box and rummage round in the polystyrene packing flakes to make sure I had everything than it did to fit. I am very impressed at just how painless an installation it was. The only frustrating part was not realising that the HH requires the cells be wired the reverse of the analogue pod. I did spend quite a while trying to work out why the display was reading zero. All in all I would say it takes a whopping ten minutes to convert a stock unit to a Hammerhead. Tools needed: a flat blade screwdriver, a Phillips screwdriver, some spanners or an adjustable wrench or two. Rough estimate, it's a one beer job.