I’m hoping that this thread can be for a productive discussion between those who want to debate which design differences between eCCR's and mCCR's may contribute to reducing accident rates. This thread is not intended to be a debate about weather training and skill are the only factor in mortality rates…that’s been beaten to death and I think we all understand the point, training and vigilance are at the very least the number one contributing factors to keeping you alive under water and yes, all rebreathers can kill you. I do believe, even though I’m starting this thread, that it's best to continue emphasizing that the biggest factor effecting safety in any kind of rebreather is diver skill, so please, lets leave that argument to other threads.

I wish to expand on some of the points that have come up in other similar threads lately. I believe there is value in attempting to identify the possible contributing factors to the reduced mortality rate specifically due to design differences between mCCR’s and eCCR’s. I believe this could lead to improvements in eCCR design or style of operation that could some effect on mortality rates.


I've only used a Kiss in a pool and am by no means an authority on it's function, but it is the mCCR that i am most familiar with so that is my main mCCR to compare with the eCCR’s I’m familiar.

From limited talking around, it seems that an orifice that is well matched to metabolic rate/o2 consumption results in a very stable PO2 through much of the dive, requiring manual injection mostly on ascent. So, since addition is so rare maybe it's not the regular checking of PO2 and adjusting that leads to fewer fatalities. Certainly the mindset could be the major factor and I think it is also possible that it's the constant flow orifice or maybe the BOV. Also, it may not be any one thing but rather the cumulative effect of a wide range of factors.

The main factors that i can see that set the KISS apart from an eCCR are:

- use of a BOV (though some eCCR's have them too)
- a constant flow orifice that requires some "topping off" by the diver but provides a minimum of life supporting o2...basically a positive feedback loop for oxygen addition as apposed to the negative feedback loop of the eCCR
- No electronic controller driven solenoid…less dependent on electronics to govern gas balance. (it would seem that orifice cloggin is a lot less likely than electronic malfunction, so it appears the balance of risk between the two favors the constant orifice)
- the diver must be engaged, especially on ascent where po2 drops the fastest.
-not the best build quality, but apparently more than adequate in the ways that count.

A BOV may turn out to be one of the deciding factors. Perhaps the use of a constant flow orifice and the diver for additional injection works better because it's a kind of positive feedback loop (based on cumulative causation) for O2 addition preventing hypoxia as apposed to the negative feedback loop of an electronic controller driven solenoid. Interestingly, hyperoxia does not seem to be at all common on the KISS, responsibility for avoiding that is left entirely up to the diver.

Perhaps there could be a hybrid of the eCCR and mCCR that would be the best of both worlds. Certianly adding a BOV to eCCR’s is becoming more common.

While the sample group will remain very small for quite some time to come, making any kind of comparison of limited value, the sample group of living divers and the fatality group are growing and watching accident reports for things like weather or not there was a BOV on the rig or weather the diver was known for “flying manually” will be of interest to me. (A standardized accident report would be a very good thing for us to create and promote to track contributing factors. At this point we are very in the dark with respect to fact driven accident analysis.)

There is one difference with the KISS which I’m most apt to point to with respect to it’s potential effect on mortality rates, it’s the constant flow orifice. Incorporating some version of it into the eCCR may turn out to have the biggest effect, diver skill not withstanding, in accident rates. Since I’m drawn mostly to eCCR’s for their apparent advantages, I’m tempted to think of ways to hybridize the o2 injection system with mCCR’s, of course this may just be going further in the direction of complicating things, if so, it won't likely proove worth it.

Perhaps the positive feedback system of the constant flow orifice with a solenoid assist would be helpful, closely matching the metabolic rate of consumption of the diver but can be “topped up” by the electronic controller if the diver fails to respond to a precipitous drop in PO2 (a version of flying an eCCR manually) while at the same time showing warnings through the HUD, Handset and Buzzer (provding the advantage of additional warning of the typical eCCR).

Or maybe simply using all the bells and whistles of an eCCR like a HUD, temp stick (vision), PO2 warnings, buzzers with the one acception of leaving out an electronic controlled solenoid, instead incorporating a KISS style constant flow orifice. Maybe this would be the best of both worlds, requiring the diver to add oxygen beyond the minimum required to sustain life while reducing the risks associated with a electronics injection malfunction.

Your thoughts?