Important features when choosing a rebreather

[tbone1004]

What are the compromises in sidemount rebreathers?

it's a LONG list, but they are all different.
Sidewinder is not a sidemount unit, this pisses Edd off every time I tell him it's not, but it really isn't. It's a backmount unit that fell onto your side, but it is still integrated to your unit and primarily on your back.
The KISS Sidekick and SF2 are very different than the Liberty and are true sidemount units, but again it's a long list of compromises. There are threads on the compromises from all of these units, but if you have one in particular I'll list what I believe they are, but it's too long to list them for each.

These units should only be considered IMO if you actually need them and not as a lifestyle choice like OC Sidemount.

 

[rjack321]

What are the compromises in sidemount rebreathers?

Just a few

• WOB is highly positionally dependent, and rigging dependent
• Even "CE" rated SM CCRs probably lied about WOB in at least some of the required testing orientations in order to pass (CE is self policed)
• You end up just as fat chest to back as with a BM unit if you actually carry redundant dil/BO (at least with every SM unit besides the liberty)
• Often no place to put the required O2 bottle that is actually streamlined and/or ends up requiring a 20yo, out of hydro, $1500 non-DOT approved sphere as an O2 bottle - and someone willing to fill it
• Flood tolerance mediocre to non-existent depending on the unit
• Ability to dewater decent to non-existent
• Ability to plug in off-board O2 usually has to be figured out by the user
• Depending on the unit, your thigh is going to be continually pushing it into an A frame (because they are longer than an AL80) which is a good way to get ratcheted and stuck
• No good place for suit gas bottles that doesnt completely mess with either the CCR or the BO or both
• No way to carry the CCR and 3+ BO gases without being a Christmas tree

Unless you NEED a SM CCR just don't.

 

[GF99/99]

it's a LONG list, but they are all different.
Sidewinder is not a sidemount unit, this pisses Edd off every time I tell him it's not, but it really isn't. It's a backmount unit that fell onto your side, but it is still integrated to your unit and primarily on your back.
The KISS Sidekick and SF2 are very different than the Liberty and are true sidemount units, but again it's a long list of compromises. There are threads on the compromises from all of these units, but if you have one in particular I'll list what I believe they are, but it's too long to list them for each.

These units should only be considered IMO if you actually need them and not as a lifestyle choice like OC Sidemount.

Just a few

• WOB is highly positionally dependent, and rigging dependent
• Even "CE" rated SM CCRs probably lied about WOB in at least some of the required testing orientations in order to pass (CE is self policed)
• You end up just as fat chest to back as with a BM unit if you actually carry redundant dil/BO (at least with every SM unit besides the liberty)
• Often no place to put the required O2 bottle that is actually streamlined and/or ends up requiring a 20yo, out of hydro, $1500 non-DOT approved sphere as an O2 bottle - and someone willing to fill it
• Flood tolerance mediocre to non-existent depending on the unit
• Ability to dewater decent to non-existent
• Ability to plug in off-board O2 usually has to be figured out by the user
• Depending on the unit, your thigh is going to be continually pushing it into an A frame (because they are longer than an AL80) which is a good way to get ratcheted and stuck
• No good place for suit gas bottles that doesnt completely mess with either the CCR or the BO or both
• No way to carry the CCR and 3+ BO gases without being a Christmas tree

Unless you NEED a SM CCR just don't.

Spot on!

Also to add a curve ball into the mix you also have the chest mounted variants. And of course these units come with there own compromises as well. And similar to how tbone1004 describes the sidewinder I would not really call these "sidemount" rebreathers, they are chest mounted rebreathers that you use sidemount equipment with.

 

[rjack321]

Also to add a curve ball into the mix you also have the chest mounted variants. And of course these units come with there own compromises as well. And similar to how tbone1004 describes the sidewinder I would not really call these "sidemount" rebreathers, they are chest mounted rebreathers that you use sidemount equipment with.

By the same token, there are basically 2.5 variants to backmounted CCRs & BO

1) Those with backmounted dil/bo (lp50s are the most popular currently but some use AL40s)
2) The divers who use conventional stages and just let them hang however
3) Backmount CCR (or chest mount) with sidemounted BO.

My informal guess is 15% of BM CCR divers are using backmounted dil/BO. 30% are just using traditional stages. And a little over half of BM CCR divers (~55%) sidemount their BO

 

[rddvet]

but I abhor CMF with fixed first stages from the PITA factor of the first stage itself.

what reasoning? The reliance on a special reg in case it fails? I havent had any major complaints about the blocked first stage on the fathom other than they’re apeks regs (which I now despise) so we have had two hpmseat failures. Not a big deal though. Just grab any reg in the spares box, move one hose and one OPV to the new reg and you’re good. I’ve dove the fathom without a blocked first and it wasn’t a big deal, though I’m sure going down to 200 ft it could be annoying. For my wife’s rig we got the halcyon diaphragm reg and bought the blocking plug from tecme.

 

[tbone1004]

what reasoning? The reliance on a special reg in case it fails? I havent had any major complaints about the blocked first stage on the fathom other than they’re apeks regs (which I now despise) so we have had two hpmseat failures. Not a big deal though. Just grab any reg in the spares box, move one hose and one OPV to the new reg and you’re good. I’ve dove the fathom without a blocked first and it wasn’t a big deal, though I’m sure going down to 200 ft it could be annoying. For my wife’s rig we got the halcyon diaphragm reg and bought the blocking plug from tecme.

with the CMF it means you have to have a blocked first stage which restricts you to a handful of regulators and if you need to swap one out you have to make sure you have the IP of the bad one recorded, set the IP on the replacement, pop the environmental seals off and put the plugs in then assemble everything back. They're then still depth limited and they can't sustain consciousness so you can still go hypoxic in a high work load environment or if your metabolism drops on deco for whatever reason it's really annoying to maintain minloop. With a needle valve it's difference since you can just use a normal first stage. The KISS needle valve that doesn't officially exist is the best one out there since it has a CMF, then a clicky needle valve, then another CMF so you don't need a blocked first stage and it's almost depth independent, but they are only available to a select few and are horrifically expensive.
The needle valves make the mCCR's a lot less annoying, but the CMF's are a righteous PITA. You've met me, I'm a big dude, and my metabolic consumption when I'm on deco is vastly different than when I'm kicking and the O2 addition from the CMF is basically irrelevant when I start moving compared to what my muscles are burning, especially if I'm diving wet and burning a lot more to stay warm.

 

[tbone1004]

@rjack321 you forgot about the custom valve and sphere holder that are annoying to get. I have one, it works or me, but I agree that it's definitely not what I would recommend

 

[Wibble]

Do you ‘dynamically' adjust a needle valve during a dive? How do you calibrate it in the water?

 

[LFMarm]

Thanks a lot for your suggestions! It really sounds like SM CCR is not the way to go. I will then consider BM CCR and SM the bailout bottles

These units should only be considered IMO if you actually need them and not as a lifestyle choice like OC Sidemount.

Unless you NEED a SM CCR just don't.

 

[tbone1004]

Do you ‘dynamically' adjust a needle valve during a dive? How do you calibrate it in the water?

with the clicky ones you can typically just remember the number of clicks from "off" that you need for deco and then adjust as your ppO2 starts driving. Drifting up, turn it back a click, drifting down then turn it up a click. If you have a blocked first stage then it is not depth dependent, just workload dependent and the Fathom uses this approach albeit without a clicky needle. When you have a normal needle, the process is the same just without the detents to tell you where it is located.