Mk15.5

My favoured rebreather for extreme depth dives.  I will not bore you with a teardown of my unit, but will just tell you about the modifications I have made.

I have removed the analogue electronics from the unit and replaced them with Juergensen Marine Hammerhead digital electronics.  This consists of a wrist unit and a electronic pod that sits where the battery pod used to be.  The power for the electronics comes from a single AA 1.5 to 3.6v battery that fits in a battery compartment in the wrist unit.  The solenoid is replaced with a new, much lower energy use unit.  I asked that my HH electronics be hardwired rather than use Fischer Plugs which was the alternative.  I wanted as much watertight integrity as possible.  Additionally, I filled the handset and pod with medical grade paraffin to help solve implosion problems.  (See below for more details.)

The only issue I have at the moment is that the battery compartment is not oil filled.  I am currently experimenting with doing that as well.  One of my major concerns is hydraulicing if I fill the compartment with oil and then screw down the sealing plug.

I have replaced the standard mouthpiece with a AP Valves mouthpiece (off the Inspiration) and lengthened slightly the inhale and exhale hoses.

I always use the best quality, fine-grained absorbent on major dives.  The extra expense is worth it.  I have had 9:40 out of the canister and felt it still had more time available, but one needs to qualify that statement  with a few other facts.

1. Most of the time on a big dive I am laying quietly on deco, producing minimal CO2.
2. Once at 6m, or even 9m on occasion I have air breaks every 30 minutes for 5 minutes.  I go OC to do this.  Once the air break is complete I go onto OC O2 for a minute or so to flush any air out of my lungs.  Then I go back onto the rebreather.  So the 9:40 time might actually be 9:00 in reality.
3. Once at 6m I flush the unit with O2 and operate the unit as an O2 rebreather.

The Mk15.5 breathes beautifully at any depth.  WOB has never been an issue for me.  Remember that when at extreme depth I am breathing a very high helium mixture though, which will reduce the gas density problem to a certain extent.

Condensation has not been an issue for me.  The cells all track each other accurately.  I am religious in replacing them every 12 months however.  By the end of a big dive the bottom felt in the scrubber will be soaked with…well, shall we call it “a liquid”?  The top one will just be damp.  I will most probably have some bubbling noises as I breath towards the end due some accumulated fluid in the exhale hose.  I usually just put up with that but could if I wanted too clear that into the scrubber area.  The scrubber sorb is always dry at the end of a dive.  At worst I have had just a slight dampness in spots, but that is unusual.

The Mk15.5 has two Iconel Spheres, each of 3lt capacity.  They can be pumped to 200bar comfortably.  They are “A clamp” design, but could be modified to DIN if I felt like doing it.  3 x 200 = 600 free litres of O2 which for me lasts about 10 hours.  Usually that is enough…just.  The diluent has the same capacity but on big dives I often use off-board gas.

I have modified the unit with a Whitey Valve on the left side.  The plumbing allows then either on-board or off-board gas to be used.  I use Swagelock fittings to allow for a quick connect/disconnect that allows no water into the system.  I also carry an adapter so that if a tank has a Scubapro type fitting (same as the Inspiration use) I can also connect to that tank using the adapter.  That allows me to dive with other divers who have more normal fittings.  On big dives I tend just to use off-board diluent and have my on-board diluent filled with bottom mix in case something goes wrong with the system in some way.

Mounted on the left side of my unit is a 3lt tank that holds air for wing inflation.  On the right side another 3lt tank with argon for drysuit inflation.  I have been innovative with the wing inflation gas on occasion however.  On very big dives the 3lt sphere of O2 is tight quantity wise.  With that in mind I have filled the wing inflation tank with O2, and then that is available to plug into my unit if required as additional Oxygen.  Bit unusual, but it works!

Once on deco at 6m or above I tend to plug an O2 tank into the diluent side of my unit and then as I metabolise the O2, instead of having to manually inject O2 into the loop, the ADV just allows me to draw in what is required.  A great system that allows me to continue dozing rather than O2 injections being required every now and again!  I never use the electronics for O2 injection at or above 6m, so I just set the set-point to 0.4 and the solenoid never fires.  Every now and again I will flush the unit with O2 at these shallow depths to keep the O2 as close to 100% as possible.  I usually do my deco calculations assuming an O2 content of 90 or 95%, just in case.

The VR3, by Delta P Technology is a very versatile and robust Trimix dive computer.  It has low power consumption and uses a single AA 1.5 – 3.6v battery.  One 3.6v battery lasts for ever.  I throw the battery away after a year just to make me feel better.  The computer is relatively easy to programme, but initially one does have to read the manual to understand how it works.  Get that, read the manual first, it is a lot easier, and yes, the latest version of the manual is better than the previous attempt.

The VR3 is versatile in that one can programme up to 10 gases that can be either “On” or “Off”.  If they are “On” then it will be assumed by the VR3 that you are going to switch to those gases during the dive at the MOD you have set for each one.  The TTS (Time to the surface) indicated during a dive reflects these gas switches.  However, the gas switch will not actually happen until you physically tell the computer the switch has taken place.  If you ignore a switch the TTS minutes will become “long”.

The computer gives deep stops that are two minutes in length.  This cannot be altered and until recently was not open for discussion (the impression I got) with Delta P.  The good news is that they are going to offer a version of the VPM-B algorithm sometime in 2005.  I also understand they will be modifying the VPM-B shallow stop profile to make it more conservative (read longer shallow stops than VPM-B gives).This is all good news in my opinion and demonstrates again that Delta P reacts to customers comments.  (Now, about my "On, Off, Bail" idea Delta P????)

I own two VR3’s and have used them extensively over the years.  They are officially rated to 150m, but I have had them to 270m.  On that dive one flooded, but I suspect it was caused by contamination of the battery compartment O-rings or some such thing as I had just changed the battery on the one that had the problem.  For very deep dives (150m+) I treat the dive planning capabilities of the computer with suspicion.  On a number of occasions it has indicated on a simple 150m dive to drop some stage tanks for example, that the dive would be, 2 hours long, but even when all goes exactly to plan it will be 3 hours before I am out of the water.  Below 200m I will not use the computers other than as bottom timers.

I have used the VR3’s in anger during a bailout from a rebreather that happened at 181m in a cave, (see report) and this event really proved the capabilities of the computer.  The bailout gases were programmed into the VR3, but not turned on so that they did not effect TTS predictions if the dive had been routine.  Because of the bailout required, I was obligued to turn on gases as I switched diluents over many hours during the exit, while breathing semi-closed, or at times constant volume.  I was rather busy and the turning on of the gases added to my workload while doing all the other things.  As a result of that experience I made the following suggestion to DeltaP.

When programming the gases, instead of having the options “on” and “off”, have the options of “on”, “bail” and “off”.  Then, in the divers options menu (DVo,) you add the option to turn on, in one go, all the bailout gases that are currently set to “bail”.  That way you can have all sorts of gases in the unit, with the normal planned dive gases as the “on” gases.  If one needs to bailout then it is very simple.  Go to the DVo menu, turn the bailout gases “on” and now the normal planned dive gases AND the planned bailout gases would be on.  That would just leave the other “off” gases still off.  Suddenly, and very simply the VR3 now is setup for your bailout plan.  This change would reduce the workload significantly when in the situation I found myself.

They seemed to think it was a good idea, but as yet (Nov 2004) the idea has not been incorporated into the unit.  If you think it is a good idea write and tell them!  Maybe they are still working on developing the modification?

The long and the short of it is, I like the VR3 a lot.  As with any tool, there is always room for improvement, and the impression I get is the Delta P Technology strive to keep the VR3 as up to date as possible.  I recommend the VR3 as the only computer that is really suitable for the serious technical diver.

All my cave diving is done with me wearing a DUI CF200 drysuit.  I have used the same drysuit for many years and it has logged hundreds of often long dives.  It is very durable and I have only had to repair a leak once.  That leak was the result of trying to squeeze through an old rusted door in a mine.  I was being lazy and trying to get through without removing any stage tanks and the result was a pinhole leak that was simple to fix later.

The suit is warm and comfortable.  I wear DUI Thinsulate 200 underneath, with an appropriate number of layers depending on the water temperature and length of the dive.  With care and proper maintenance, the latex neck seals have also lasted for years.  Each day I am diving I use talcum powder on the seals.  When storing the suit, I again ensure the seals are well covered with pure talcum powder, and the suit rolled up in a cool dark place.  The zip I lubricate occasionally.

I bought an off the shelf drysuit and the fit was good.  The only modification I made was to the foot.  The size of the feet in the suit was too big for me by far (I wear a size 10 RockBoot).  The excess boot needed to be squeezed into the RockBoot (with my foot) and this was awkward.  I thus elected to cut off the feet and have new made to measure socks added.  This worked a treat and I am now very comfortable.  Of course, if I had ordered a custom made suit originally, this would never have been an issue.  The RockBoot are a great idea and make for safe clamouring over rocks etc at cave entrances.  One has to ensure that the fins used have a large enough foot size themselves to accommodate the RockBoot, but this is not a problem.

I added a Diverite Peevalve to the suit for those comfort requirements that seem to loom all too often on long dives.

The CF200 is an excellent drysuit in my opinion that is idealy suited to the "rough and tumble" of cave diving.

Diverite Peevalve or, as it is more correctly known, Urinator.  A wonderful gadget that allows bladder pressure to be relieved hands free!  There is no valve to turn on and off, just go when you need to.  It is easy to install and once connected up via a catheter condom use is even more simple than going at home so to speak.

The hardest part about the whole operation is getting the catheter condom off that sensitive part of your anatomy at the end of the day.  With certain US made brands, as Don Shirley of IANTD South Africa says, you could hang yourself from a tree and it still would not come off.

On a more serious note, some maintenance is required.  If the urinator is not flushed out with fresh water at the end of the day, or week of diving, in time the “salts” will cause it to no longer flow.  I found this out the hard way on a dive.  When I wanted to go, the body was willing but the Pee Valve was not.  The consequent backpressure, that built to an agonising degree as the 4-hour deco progressed…oh so slowly…was quite serious.  Serious to the extent that I know of others who have been put in hospital with damaged bladders when this sort of event has occurred.  I was lucky.  I was not wearing one of the US brand catheter condoms, so with time, and constant pressure I managed to get the condom to slowly, well what shall I say, slide off.  With the final release, the relief was both blissful, and for a short time, warm.  So, beware, and flush out the urinator on a regular basis.  I always test the device before I start a dive.  A free flowing dive is a happy dive I say!

Diverite have a maintenance kit available to service the urinator, replacing the rubber seals etc as required.  A good idea to do that every few years.

There, you did not realise I could say so much about a Pee Valve, did you?  Sorry about the lack of pictures on this subject.  Just use your imagination!!

More as I get time…..