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SF2 Rebreather Review

By Tom McCarthy

I’m from Long Island, NY.  I’ve been fortunate to be friends with several people along the way who have been way ahead of the curve with regards to rebreather diving.  Many of them, including myself, got into rebreather diving around the time where they were in the beginning stages of becoming main stream and less of a garage band / true explorers only device.


     That said, my first rebreather was the PRISM Topaz. A mechanical engineering masterpiece of its time, so much so that the PPO2 monitor was a needle gauge with a rotating knob to choose between which single sensor is being displayed. Not too long after, Billy Gambrill, then part owner of the infamous Jackson Blue area ‘Hole in the Wall House’ took rebreather diving back to the garage and created a sidemount rebreather utilizing the PRISM Topaz core. This unit was amazing in its own right but a bit too garage for most (myself included at the time). Years later I would come to discover another sidemount unit. The SF2.

What initially drew me to the SF2 was a sidemount unit that I could teach an authorized course on. What hooked me follows.


     When I first look at any rebreather, I want to know about its history. Rebreathers are not big corporate products. They are generally produced by a few eccentric divers who are too passionate to realize that their skill would likely be significantly more profitable doing almost anything else! With this in mind I thought it would be best to reach out to the source and let one of the co-creators of the SF2 do the talking…



History of the SF2: A letter from Horst Deiderichs- Co-Owner of SCUBAFORCE


“Where do we come from: In the early years of 2000 we were working on a rebreather system for our own exploration because the system that was on the market wasn't what we were looking for or not available for us.


The technical community in Germany was using PSCR systems during these days and we went into the same direction.


The result was the SF1, a PSCR unit. We tried out different materials for the unit, POM and stainless steel, for example.

In the end we decided upon carbon fiber tubes. They were durable and lightweight and allowed the unit to trim very well. The set up for the carbon fiber body was also the same that we have on the SF2 now. It had the added benefit of being very well isolated because there was no direct contact to cold water.


Being a PSCR meant that it was used with a stainless steel frame to mount huge tanks. The tanks were connected by a gas bridge. Different gases could be plugged in by a gas switch block using Swagelok connectors.


Because of this it was very heavy. Very difficult to use for wreck exploration in rough seas. In general we felt that a PSCR can cause quite a lot of serious gas problems. It might be ok for a well trained group of divers who are following very strict standards and procedures. To put it another way: They were good for cave explorations (and that is what PSCR are originally adopted for) where the logistics are totally different from open sea technical wreck diving.


From a business perspective it was also impossible to get a CE certification on a PSCR system in those days.  Without a CE you are not allowed to sell units in Europe. So the project was stopped.


We had tried several CCR after stopping the SF1 project. They had the benefit that they were much lighter and in normal operation, much more efficient from a gas use point of view.

We felt that over the shoulder loops weren’t as nice compared to a clean front of a PSCR. The trim on the units we tried was difficult as well – often requiring additional weights moved around to obtain perfect trim.


At a certain point we stepped back into it and the idea was to combine the advantages of a PSCR set rebreather with the advantages of a CCR system.

It took  a while and we had to overcome quite a lot of obstacles but based on the years of serious testing and changes we had we finally made it. The result was the SF2.


Beste Grüße,

 Horst Dederichs”



One rebreather, two hats


Along the way a man was called in to help create the new SF2. When presented with the idea of designing and building a new rebreather, his response, he has told me in a refined Dutch accent, was roughly “Ok, but I will not make a damn PSCR and it must be side-mountable!” Enter Tom Jaspers, the man the myth, the swimming Dutchman.

I was initially quite skeptical about the SF2. I felt that a unit that took on two many roles could not be good at either. My initial experience was in backmount. The rig trimmed out almost perfectly with no top end trim weights, it breathed great in all positions, it used shearwater electronics, had and uncluttered chest, and best of all it looked awesome.

The next day I was given a sidemount unit to try. Sidemounting a rebreather is an interesting thing. When you move the counterlung position to your side, odd things happen to your lateral buoyancy. As you exhale the unit wants to float. As you inhale it wants to sink. At first this is tricky and unnatural, after about 5 minutes you start to grasp it. After 20 hours you completely forget about it. Some folks choose to put a tail weight on the unit to help this. Personally, i’ve found that keeping the bottom bolt snaps tight does the trick. The sidemount unit itself breaths flawlessly while in trim. Head down has never been an issue for me but standing straight up and down does give the diver some degree of chipmunk cheeks. A small tradeoff to have a fully self contained unit on your side which is capable of doffing or handing off to another team member (should you be utilizing them as bailout rebreathers).



Craftsmanship


The first thing I noticed about the SF2 was the machining. The hose connectors and DSV’s are beautifully done; the head tolerances are super tight; the electronics bay is ingeniously laid out to reduce size; the carbon fiber makes the rig super light for transport; the tank brackets are simple and rugged; and the ingeniousness of the sidemount-backmount conversion cannot be overstated. 



Electronics


One of my favorite things about the SF2 was that it utilized shearwater electronics. Anyone who has been in technical diving for a while understands what I mean. Shearwater has been the shining light of manufacturers for some time now. Their customer service is miles above everyone. They’re constantly innovating but they do so through real world input and not innovation for the sake of innovation. Their software updates are generally akin to Christmas morning because they somehow manage to add new features into old hardware at every turn. So why wouldn’t I want to use shearwater as a primary controller? I find that often companies create their own electronics systems which are plagued with issues. A company may only produce a few hundred units and at the end of the day electronics are only a small piece of their larger rebreather development. Personally i’d rather go with a company whose entire business consists of computers. Anyone who has ever used a car based GPS suite understands this. I’d much rather have a place to stick my iphone with Google Maps running. Few would disagree with me when I say that Shearwater is the Apple of diving right now.

One nice thing about the Shearwater controller system is that the handset can be removed via a wet connector and sent in on its own for service, should it be necessary.

The backup electronics options are even better. You get a blank Fischer cable and that’s it. But Tom, shouldn’t it come with something more permanent? Anyone who has been around for a while understands the plight of dive computer obsolescence. The Fischer connector, in my opinion, is the best way to prevent this. Don’t like your current backup? Go buy something else and plug it in! Did the new Shearwater Seagull/Albatross/Penguin/Osprey/Ostrich/Emu came out and you’re one of those people who compulsively camped out at the dive store to be the first to have it? Then this is a great option for you.

The rebreather itself is a standard ECCR and comes fitted with triple Oxygen sensors nestled in a separate and sealed compartment. The connectors are super sturdy gold plated coaxial style attachments which all but eliminate the constant issues that come with the more common plastic molex connectors.

The solenoid is run off a standard store bought 9V battery, while the shearwater has a separate battery. In the event that the 9V dies, you still have two backup PPO2 monitors which will allow you to simply fly the unit by manually adding oxygen (the primary method most instructors will make you practice during your entire course). This gives the user more control over PPO2’s rather than the alternate method of defaulting the electronic oxygen addition to a low set point.  The computer board is potted in a hardened epoxy and prevents total loss should the electronics bay become flooded.


Scrubber


The SF2’s scrubber is another well made piece. It features a delrin body with all stainless steel hardware. It is a standard Axial flow setup and holds roughly 2.4kg/5.3lbs of absorbent. It’s rated for a 3 hours in cold water with a pretty conservative buffer built in for safety.


Counterlung


The SF2’s counterlung is nicely protected behind the user. It is a single bellows design (similar to that of the RB80). It’s located in the bottom 1/3rd of the body tube and has a standard user replaceable drysuit style OPV. The counterlung itself is also quite easily user replaceable in a pinch. One of the benefits of a rigid tube surrounding it is that you don’t have concerns with regards to abrasion or inadvertently crushing loop gas out in tight restrictions. A great feature for wreck and cave divers.


Body tubes


The crux of the sidemount/backmount convertibility lies in the carbon fiber body tube. In order to switch between the two styles, the user will have two distinct base tubes as well as a sidemount and backmount DSV/Loop hose setup. On the backmounted version, the cylinders are attached via quick release brackets to the base tube. The backplate and wing are also bolted onto a vertical flat bar (with standard backplate bolt sizing and spacing) which is attached to the same. The sidemount unit base replaces these mounting brackets with a simple hose clamp that allows for the lower attachment bolt snaps to be put in place. Swapping between the two is a process that can take as little as 5-10 minutes depending upon how you set up your gas supply hardware.