Take this with a grain of salt. I currently dive a rEvo mCCR with a little over 400 hours on mine. I'm obviously biased as it's the only rebreather I've ever known. I still consider myself very green on rebreathers. This is my 3rd year on the rEvo just to give you some sort of a gauge of years vs. hours.
I did a bunch of research before buying. I tried several units on demos (Prism 2, KISS, rEvo) and most recently an SF2. I obviously don't have a vast ton of experience on different units.
I decided that at the time I liked a lot of the features on the rEvo better. For example, backmounted counterlungs, dual scrubbers, redundant pO2 monitoring with cells. I like diving and learning on a mCCR I feel It's kept me honest with the rule of always knowing your ppO2. I don't have to rely on a solenoid. I think learning to drive a rebreather manually has a lot of advantages for when and if the **** hits the proverbial fan.
People say the 5-6 cells and redundant ppO2 monitoring may be a marketing gimmicky but it's saved my ass a few times when I had a cell die on a trip (yes...I keep spare/backup cells but it's dead simple to just rotate a cell). I simply removed a cell from my secondary monitoring, put it on my Petrel and re-calibrated. No fuss, no drama and I didn't even make my dive buddies late getting in the water.
I'm an IT guy and I'm one of those people that thinks more information and data points are a GOOD thing.
Regarding flood tolerance..I've never flooded my unit although it's always pretty wet inside after 2+ hour dives. This is something that always concerned me compared to other units but it's never caused issues with any of my cell readings. If I have the opportunity between dives I ring out my shammys and clean any moisture/condensation out of the unit as best I can. If you're planning 6-8 hour dives this may be a problem. I truthfully don't know the answer to this as my longest dive-to-date on my rEvo has only been been a little over 4 hours.
It's not a big deal to me anymore and it's routine habit but it does suck to have to open the unit (Hint: Anytime you break the loop for whatever reason you're going to want to re-do your positive/negative tests.)
Let me correctly myself slightly, the one and only time I did "flood" it was while climbing up a boat ladder due to poor DSV management in heavy seas at the end of the day so I didn't lose any diving. This was when I was at something like 20 hours on the unit. The unit was probably still dive-able but it was end of the day so I didn't try finding out and didn't want to test that theory.
Since then I've lost exactly ONE dive since I've owned the rEvo and it was entirely my fault. I over-torqued the brass fitting that goes to the constant mass orifice and must have cracked the fitting. Stupid me. I was on a dive at 150' and kept noticing my ppO2 climb up steadily but not rapidly. I flushed the unit down several times. By the third time I knew something wasn't right and ascended, feathered my O2 on/off and ended the dive on the unit. No bailout, no major drama. Just knew something wasn't right.
Upon inspection the surface the fitting was cracked and allowed more O2 than I wanted to escape into the loop. This only happened because I took the orifice and check value off to clean them in my ultrasonic after I had discovered that I had been diving with a slightly clogged orifice for a while and my O2 flow rate was down (verified on a flow meter).
It's a simple part to find anywhere in the United States at Ace, Lowes, Home Depot, etc but try finding 1/8" NPT coupling on a remote island in Scotland (Orkney Islands, Scapa Flow). Turns out 1/8" BSP with ample teflon tape is close enough so that I could limp by for the rest of the week
So yeah those are the issues I've had with the rEvo. I'll break it down a little bit with a list
Pros
-Dual scrubbers (minimize chance of CO2 breakthrough and efficient sorb usage. I don't have RMS..another debate there..
-No "head" to send back for service. If your controller fails, run manually. See my notes under Cons.
-Super negative. If you dive a drysuit with thick/heavy undergarments you'll like this. If you have a wetsuit it will certainly be a con. If you dive somewhere in the middle then it could be a pro or con.
-Backmounted counterlungs supports cleaner chest profile like OC
-Easy for travel (It was important factor for me but I've taken it carry-on many times)
-Redundant electronics / 5-6 O2 cells (Gimmick? You tell me but it's saved dives for me.)
-Easy access to counterlungs for cleaning/disinfection. Hose out, air dry.
-Easy breakdown (bias here but I've watched other people breakdown their units and my maintenance takes slightly less time.)
-Easy build process (bias here again but I've watched other build their units. I think the rEvo has a relatively simple build
Cons
-Absolutely no flood tolerance. None. If you have anything larger than a cup of water enter the loop, you should bailout.
-WOB. This truly never bothered me. It comes with territory of most backmounted lung lung units. I fix this "issue" with copious amounts of Helium in my diluent.
-Super negative in water. In a wetsuit I'm a boat anchor. I dive a stainless steel mini. Maybe the titanium unit would be better. If the majority of your diving in in a thin wetsuit you're going to be super negative.
-****** rEvoDreams for secondary monitoring. When they work..they work great. I admittedly was a fan of them for a while. For reason they decided to solder the battery in the new rEvodream P5s. Stupid. I've had piezo switches die and I've had one flood on me at 200ft. This is an easy fix. Buy a Shearwater NERD and be done with rEvoDreams.
-Apple philosophy of not selling the end-user parts and telling you there are no user serviceable parts inside the case. rEvo and the service center do not want you opening the unit up. They tell you everything must be done by rEvo service center.
-Counterlungs..again. I've been up until 1AM aquasealing pinholes in my counterlungs with the entire case off.
-Ironically the reason I bought into the rEvo was the supposed philosophy of everything is replaceable by the user and there is no "head" to send back for service. Try replacing or buying counterlungs or buying or replacing the O2 supply lines/bulkheads inside the case..
So where does that leave us? I'm shopping for my next rebreather as we speak. I just demoed the SF2 on Saturday and did a leisurely 60 minute dive in one at depths of 20-50'. Still better than a pool.
Overall I still have lots of questions but here is what I really liked about the SF2..
Flood tolerance. For me this is a huge advantage. I'm starting to gradually do longer dives and think about the future. I really want my next unit to have some sort of flood tolerance. True it's never been an issue on my rEvo but if I ever have the opportunity to be 4-5+ hours in the back of a cave and/or facing 4-5+ hours of decompression in the ocean, you can bet i want something with flood tolerance and the ability to de-water the loop.
Easier to reach valves (for me). I have short t-rex hands. The tanks on the SF2 sit closer to my back. I can reach them very easily without any contortion necessary in thick undergarments. This was awesome for me but this is not unique to the SF2. I can reach Prism2 valves easier too and I imagine JJ and other rebreathers with "cans."
Trims out nicely. I had no issues trimming one out. I didn't have to do anything special. The counterlung on the bottom seems to lend itself well to keeping you horizontal in the water. Some rEvo divers suffer with this and have to resort to placing 8lbs of lead on the top of the unit and/or switching to lighter fins. I think my trim in the rEvo is not bad but I've seen some rEvo dives that look like sea horses.
WOB was no better or worse. Honestly this was a none issue for me coming from the rEvo and I was diving air diluent. I almost never have air diluent in my rEvo
Cells stayed very dry. The cells were dryer than my rEvo but it was only a one hour dive. I was reading about a potential issue about Cell #2 on SF2 occasionally getting wet. Maybe it's a none issue now?
Nice MAVs. I really liked the placement and ease of access.
I'm not sold on the counterlung up my a*s but it's not exactly a deal breaker either. Considering I can actually BUY them and replace them myself.
Going back to the rEvo. For me the rEvo is like a first love/child. It's all I've known for rebreathers so there is obvious bias there. I still don't think it's a bad choice and I'm still not discounting getting another one as my next rebreather.
Like with everything there doesn't seem to be a one-size-fits-all approach to rebreathers. You have people with very strong opinions who are convinced that the glorified humidifier on their back that they just spend $10K on is the best unit on the market.
