Are SCR safer than CCR?

[Icarusflies]

Hello all;

OK, this is my situation. I have been diving with OC and I recently received the call of the dark side, rebreathers!!!

I want to buy a rebreather but I put my safety above all and I don't mind starting with a SCR and later go the a CCR.

This is my question: Are SCR safer than CCR?

 

[caveseeker7]

In some ways, yes.

Commercially availabe SCRs use pre-mixed gas(es) whereas CC mixed gas rebreathers use pure O2 and a diluent that are being mixed to maintain a chosen setpoint, either mechanicly or electronically (there are self-mixing SCRs but those are military rebreathers). Hence CCRs have more parts that can fail, and a loop gas that varies to a higher extend than SCRs. SCRs contain no electronics whatsoever in stock form, though it is advisable to add a pO2 monitor which many if not most people do. Either in form of a simple pO2 gauge or an O2 cell-integrated deco computer.

SCRs fall into two categories, active gas addition units that use a form of constant mass flow orifice to add gas or passive gas addition units that are keyed by the respiratory minute volume (breathing rate) of the diver. Dräger Dolphin and Ray, OMG Azimuth and Submatix ST are among the former. The Halcyon RB80 and various "inspired" units belong to the latter group.

The cmf units add gas at a rate higher than the metabolic rate of the diver and expell the difference through an over pressure valve, hence the loop is semi-closed. The orifice can either be fixed (Dräger and Submatix) or adjustable (OMG), both need to have the flow rate matched to the O2 content of the pre-mixed gas (usually nitrox). The higher the O2 content, the lower the flow rate, the longer the duration but the shallower the maximum operating depth (limited by pO2 to avoid O2 toxiety). Fixed orifices are available Nitrox32, 40, 50 and 60 for the Dräger Dolphin. The OMG Azimuth uses an adjustable valve that allows any flow rate matching any gas between Nitrox32 and Nitrox60. So your dives are limited in MOD and time by the gas you use. As the injection rate is a fixed flow, the loop content depends on the diver's metabolism, as well as the ambient pressure. That can vary vastly from the cylinder content, hence many diver's preference for O2 cell-integrated deco computers. Of all rebreathers, cmf SCRs are the least efficient, the gas extension is roughly four times that of OC. Especially the orifice equiped units can easily get blocked as the orifices are tiny. If that happens the O2 content in the loop will drop and become hypoxic. The main reason why I believe a pO2 monitor should be standard and is mandatory for the safe use of cmf SCRs. First rule of rebreathing, always know your pO2! From a NDL/deco point of view these units are also the least eficient, the pO2 in the loop can drop considerably during excertion so the pN2 will increase. They are less efficient in that respect as using the same nitrox mix OC. Except for the Azimuth AF (which is larger, heavier and at least as expensive as some CCRs) they are not designed to be used with multiple mixes and/or He, adding further restrictions on MOD and time.

Passive addition SCRs available these days have two bellows which are filled with exhaled gas. The smaller one's content is expelled through a dum valve, the larger one's content rebreathed. The ratio between the two is fixed in all available units (though can be changed when disassembled) and usually about 1:10. Actual efficiency is given as 1:8 in those units, so gas will last about eight times as long as on OC. The mix in the loop is more stable than on cmf SCR as the gas addition is keyed to the respiratory rate of the diver. If the gas from the smaller bellows flows back into the loop during a failure, no new gas is added and the loop becomes hyppoxic. They are not more efficient regarding NDL/deco than OC on the same mix, but are generally closer to OC. As the current crop of pSCRs was designed for deep cave exploration most all have rather large scrubbers and optional gas switch bocks that allow the use of multiple nitrox and He mixes. Dive and deco gas choices are planned as on OC.

So depending on the diving you do, cmf SCRs may well be not worth considering as the advantages over OC are few and limits are plenty. While hyperoxia (O2 poisoning) is not more an issue than on OC, hypoxia (O2 starvation) and hypercapnia (CO2 poisoning) are shared dangers with CC rebreathers. In addition, most components making up the rebreather are the same, counterlungs, breathing hoses, scrubber, DSV, all the needed O-rings and connectors. Thus from a maintainance point of view SCRs and CCRs are quite close, as are pre- and post dive requirements.

The highly variable loop content of a gas mixing CCR is their biggest advantage, giving them enormous gas efficiency (independent of depth) and some deco advantages. On the other hand that setpoint needs to be maintained, manually or electronically, or the oxias can and will cause the divers death.

 

[Jason Ooi]

If you ready go for rebreather no doubt go CCR

 

[caveseeker7]

A final cnsideration you'll need to make is the availability of units, training and spares.

The Drägers are fairly easily available, as is training for them. Spares too, but unless you have a stocking dealer close by you'll be mail ordering from Germany (which isn't an issue and usually easier and quicker than even most dealers).

OMG has a US distributor in Nevada, but I don't know about the availability of units and parts. OMG also has a new unit, the UBS40, based on but smaller, lighter and less expensive than the Azimuth. Training in the US is available.

Submatix as well as most of the pSCRs are from Germany, without US distribution or dealers, and AFAIK without US based instructors. You'll have to come and visit.

The RB80 is made in the US, and both instructors are based there. But training is by GUE, very high quality if you're willing to put up with it. With their high standards come lots of requirements and a very high price tag (of both unit and training), you'll need to take DIR-F, Tech-1 and Tech-2 to even be eligible for RB training. They try hard to keep the riff-raff off their rigs. :mooner:

On the CCR end, PRISM, Kiss, Meg and Optima are made in the US or Canada, the Inspo and Evo have US and Canadian distribution. The MK15.5 is made in Australia without US representation, the Ouroboros in the UK also without a US distributor.

Give Doug Ebersole here at SB hauler, he's in FL, and an instructor on the Dräger, OMG and Kiss units. If you tend towards CCRs, conatct Curt Bowen from Advanced Diver Magazine, he has another CCR tryout coming up with Inspo/Evo, Kiss, Meg and Optima CCRs.

 

[Padipro]

I'm ordering my Meg the first part of January. Damn I can't wait.

 

[wedivebc]

The Drägers are fairly easily available, as is training for them. Spares too, but unless you have a stocking dealer close by you'll be mail ordering from Germany (which isn't an issue and usually easier and quicker than even most dealers).

Doesn't Aqualung USA carry Draeger parts?

 

[caveseeker7]

That seems to be the problem.

 

[Icarusflies]

Thank you for taking the time to explain, it is very usefull.

The reason why I was thinking about SCR vs CCR is was O2 poisoning. I don't know much about rebreathers so forgive me if I don't make sence in what I am about to write. I am worried about not seeing a problem comming like in the case of O2 poisoning at any given depth due to an electronic malfunccion for CCR. With SCR's we can feel the progresive effects of Hypoxia or Hypercapnia and react to them which are the main risks of SCR....am I making sence??? Plus you only depend of your common sence, no electronics. That is the main reason why I am considring SCR's. In other words the risks of SCR's are pretty close to what I am used to with Nitrox diving...???

Happy politically correct holidays

 

[caveseeker7]

CCRs have multiple O2 sensors and monitors for redundancy.
Monitor your pO2 and you will be fine. Don't and sooner or later you'll have an accident.
Personally I find hypercapnia more worrysome, as there is no monitoring available yet.
And it can sneak up on and leave incapacitated before it kills you.
It really depends on the individual and on the dive, much like nitrogen narcosis.
And it's a danger that SCRs share with CCRs. If you're on a CCR and feel something
is wrong you look at your gauge(s). If it's not pO2 it's pCO2.

Anyway, I was briefly tossing that decision around, too. In the end I decided on CC
as SC just didn't seem to be worth the hassle, and the intresting units and training
were not available where I lived.

I've got trained on a PRISM which I choose mostly for the simple to dive electronics
and the excellent scrubber design. As those rigs have a long waiting list I got trained
on and bought the Sport Kiss in the meantime. Great little rebreather, very easy to
take apart and assemble, very easy to dive as it's manually controlled.
Three sensors, connected by three cables to three pO2 displays, each with it's own
battery, so there is redundancy in the displays. It hasn't failed me yet.

The operating principle isn't all that much different than that of cmf SCRs. It also uses
an orifice to add gas to the loop, O2, at a rate below the diver's metabolism. To keep
the setpoint, you manually inject additional O2. Diluent is added by an automatic valve
as needed, and can be manually added with the standard BOV. That's an OC 2nd stage
integrated into the dive/surface valve. A quarter turn on the knob and you shut off
the loop and are on OC. Doesn't get simpler and quicker than that.

The Sport Kiss was designed with recreational depths in mind, and the factory MOD
is 50 meters/165 ft. That's all I need right now so it suits me well in that respect.

Personally I believe in most cases a unit like the Sport or Classic Kiss, the COPIS Meg,
the new rEvo, or the manually controlled CC version of the Submatix are better solutions
than the cmf-SCRs. Many Dolphin users end up modifying their units by closing the loop and adding the missing bits like the KISS valve and pO2 monitors. The big pSCR units
tend to be more capable due to their big scrubbers, but then there are eCCRs I still
would prefer, like said PRISM or a MK15.5.
Just MHO.

Hence my recommendation to meet Doug ("debersole" I believe) and have a look at
his SC and CCRs. You should really analyze your diving, your wants and needs.
Doug has the experience to help you there.

 

[DiveTub]

I went down the SCR first path, what a waste of time and money that was. Just pick yourself up a ECCR and get some good training, don't waste your money piss farting about with SCR's

Cheers
Chris