Closed Circuit Dolphin Re-Breather
Written by Kerry McKenzie
Published with permission from
December 2002 J.W. Bech
This is a fully closed circuit
re-breather originally a Dršger Dolphin but
now just called the Dolphin MK 3There are no electronics used to control the
mixture on this unit as it is all done using a method called KISS GAS
INJECTION. A man by the name of Gordon Smith from Canada brought this type
of injection to the masses with the introduction of his KISS Re-breather
back in the early eighties. The Dolphin MK 3 Re-breather uses a similar
principal as Gordonís unit with some changes to suit the Dolphin breathing
loop and design.
Special thanks to Frank Feather
who put up with my ideas and machined all of the parts I required, without
complaint. He is now the proud owner of a second closed circuit Dolphin as a
result of the modifications done to the unit you now see.
I had been diving for years and
was always looking for something that bit different as I had dived all the
popular places around the area and living in Townsville on the edge of the
Coral Sea was able to dive in some magical spots. But you know how it is,
the grass is always greener somewhere and I was like most other divers I
know eager to find that exclusive spot or wait for that next model regulator
to hit the shop floor.
That was until I discovered
rebreathers in 1995 when Peter Ready came to Townsville and changed my
thinking completely about diving. And as a consequence, after much research,
seriously looking at the possibility of building a rebreather brought a Dršger
Dolphin in September 1998.
Being from and engineering
background I could see from the start that this unit was a tinkers delight.
But little did I realize that this would take me on a 4-year project and
radically change the unit from what it was then, to what it is now.
FROM SMALL BEGINNINGS
began with a standard package from Dršger,
a Dolphin complete with 32,40,50,and 60 percent jets but no cylinder. After
many variations of cylinders, valves and other pieces of engineered
equipment was not going to solve the problem. Semi closed circuit
re-breathers certainly have their place. Clearly the Dolphin was not going
to give me the versatility as I had first thought. I found that I could plan
an EAN60 mix for the days diving and find the weather would change allowing
a trip to the reef. This would change the diving depth and make the gas
mixture un-useable beyond certain depths. Or the opposite would occur and I
would find myself bleeding off heaps of gas using an EAN32 mix in 10 meters
The major problems were gas
mixing, and depth flexibility. I knew exactly how long I was able to spend
underwater so it didnít matter if I was an air hog or not, the resultant
times would be the same. The tank duration verses the size did not offer the
gas savings I thought it would.
When you live in a place like
Townsville its not so easy to just drop into your local dive store and get a
nitrox fill as you can in the larger cities. However I was lucky because we
do have a commercial diving facility in town, The Dive Bell. They were able
to mix the required nitrox fills for me.
I made some changes to the
re-breather to allow more flexibility of mix and depth by adding a metering
valve between the jets and the first stage regulator. But this only served
to prolong the tank duration by limiting the flow of gas to the re-breather,
but did nothing to address the ppo2 issues associated with depth and the gas
I dived this configuration for
about three months, but realized that no matter what I did I was not going
to solve the problems of flexibility, diving depth or gas duration. It was
at this point that I decided to bite the bullet and design a modification
for my Dolphin to go fully closed circuit.
THE CLOSED CIRCUIT MODIFICATION
To do this I brought 2x
2.6-liter alloy tanks with din valves 200bar rated. One with a left hand
valve and the other with a right so the On/Off knobs were both facing the
I designed and built 2x tank
mounts to bolt onto the shell of my Dolphin. The first ones were stainless
steel but later on I used aluminum and shortened scuba tank straps, along
with a proper carry handle on the back. The original handle will in time
crack your shell.
Below is a photo of the shell
setup for the two cylinders and the handle that comes in so handy.
Next I added 2x Poseidon first
stage regulators with din fittings and o2 cleaned both. They were Cyclon
5000 units and I used the first stages only. One for dilutant and the other
for 02 and oxygen cleaned one of the cylinders and labeled it accordingly.
I then designed the dilutant
addition by using the existing demand valve as it was already there so might
as well use it.... keep it simple principle. I found that the spring in the
bypass valve was designed for 16-bar line pressure and I was using 8.5 bar
so it was replaced with the spring from the demand valve of a US divers
Conshelf SE regulator and this worked extremely well. I could crack the
demand valve with ease no matter how fast or slow I descended. The contents
gauge for the dilutant is over the shoulder and clips to the D ring on right
hand side of the harness. I also took an air feed from the dilutant first
stage and brought the hose over the shoulder and across to the left hand
side and used an Air2 as a bailout and for BC inflation. This works very
well and has never needed to be changed since.
I designed the o2 injection
using a Clippard Minimatics two-port upstream valve and o2 cleaned it. To
this I added a Swage lock needle valve and instead of using brass fittings
to plumb it into the bypass valve, ported it directly through the body of
the Clippard valve block. This made for a nice neat package enabling good
control of the injection of oxygen into the breathing loop. The needle valve
had a flow rate of 0-5 liters per minute and I found that controlling the
flow rate of oxygen was very easy.
The two main parts now used are
the Clippard Minimatics MJV-2 manual bypass valve and the Swagelock fine
metering valve B-SM2-S2-A
I put this between the Poseidon
first stage regulator and the Dosing block. My dive buddy Frank Feather
machined a small flow restrictor and plumbed it into the dosing block.
The Flow rate at 8.5 bar IP
pressure was around 8 liters per minute; this was ample flow for a manual
rebreather. It would take approx one minute to fully flush the loop, so I
would have plenty of time to fly the rig on the tank valve if for some
reason the manual Oxygen addition valve started to leak past the seat.
If you look on the left is the
automatic dilutant addition, and on the right is the oxygen injection point.
Nice and simple backup,
remembering to keep it simple.
The only thing left to do was
plug the hole in the Oxygen regulator so it would have a fixed pressure and
not compensate for depth changes.
This was achieved by making a
simple brass plug to replace the outer cover and works fine. You can see the
modified regulator on the right used to deliver oxygen to the breathing
Access to the loop is made very
easy due to having no BC in the way while trying to do pre-dive checks, also
assembly and dismantling is made so much easier.
Oxygen contents gauge is over
the left shoulder and lies next to the Air2 and is easy to see.
Oxygen addition is by needle
valve adding just enough to equal the divers inspired metabolic rate and a
manual addition or by-pass valve is used to balance the loop Ppo2. After
many hours underwater I find that this only needs to be done once or twice
over the course of a dive and most dives are usually around 90-120 minutes.
Injecting the oxygen into the exhalation side of the loop means it is mixed
completely by the time it reaches the breathing bag. As a result of this the
ppo2 is ďveryĒ stable.
Tank duration is 10-12 hours
regardless of depth so I can take the rig diving for a weekend and never
have to worry about tank fills and of course there is "No Mixing" before
I added 2 Dršger
Oxy Gauges to the rig and have them programmed to low setting .5 and high
setting 1.6 and I dive the rig around .9 - 1.2 most of the time. It is easy
to see what is happening with the mix and if you think there might be a
problem with one of the sensors it is simple to flush the loop and watch the
gauges track as the ppo2 rises, if they donít track evenly you know you have
a problem and can take steps to correct it.
The B.C. modification
The final modification was to
remove the BC from the shell completely and replace it with a Sea Quest
Black Diamond Buoyancy compensator.
The Black Diamond lends itself
so well to a re-breather because of its comfort and positioning of the
weight pockets for tank trimming are actually in line with the counter
lungs. So balancing the unit was a breeze. With 50 pounds of lift, adding
additional tanks for deep wreck dives is not a problem. The original Dolphin
BC was struggling to get your head out of the water on the surface and
replacing it was a wise move.
It is a real pleasure to dive
the unit and balance is excellent.
The B.C. is attached to the
shell by a simple ďAĒ frame and it takes less than 30 seconds to fit or
remove, this makes last minute checks in a small boat that much easier.
I would recommend this B.C. for
the Dolphin Re-breather as nothing else comes close for comfort or balance.
This concludes the
modifications to the Dolphin MK 3 re-breather and now I am just happy to
dive the unit.
I have well over 100 hours up
on this last configuration and the second Dolphin, Franks is identical and
behaves and dives the same as this unit.
For the last 6 months I have
had so much enjoyment from the interaction with marine life especially when
diving with a second re-breather and no open circuit divers are about. Now I
am seriously thinking of going back into underwater photography, as the fish
just take absolutely no notice of you at all.
I am now a CCR convert and will
never change the unit back to SCR.
anyone is interested in the work I have done I can be contacted on email,
firstname.lastname@example.org or you can go to the web site
Kerry, thank you very much for your contribution. I found that your input in
the SCR and CCR designers and (re) builders of the Dolphin unit is most
valuable. Many people asked your advise, and I am happy to be in the
circumstances to publish this article. I wish you luck with future
developments, and hope you will keep us all informed