I do not make/design rebreathers. I modify existing ones for my own use to reduce risk to an acceptable level to myself (i.e. as low as reasonably practicable). In the process it is fun and I learn a lot of interesting things.
The problem is the sensing technology which is designed to work on land at ambient pressure, but not in a rebreather.
Same thing with galvanic O2 Sensors.
Galvanic O2 Sensor as a technology is effective on the surface in ambient air, but less effective and reliable in a rebreather (with tragic consequences).
The problem with the CO2 Sensors is that a. some tend more than others to go off calibration even on normal use on land and require recalibration. These are unsuitable for rebreather use (i.e. you do not want a sensor which drifts off of its own volition in use, at least not easily and frequently), and b. all use infrared and a tiny mirror and require pre-heating and heating.
So, no CO2 sensor likes humidity...
The one I have used in the inhale counterlung works fine provided it is not stored while not used in a rebreather. So, if you insert before the beginning of the dive and remove it at the end of the dive, it is stable.
If you leave it in a rebreather after the dive, you will need to let it dry before using it again for about 24 hours, and it will work fine again. You cannot accelerate the drying process by blowing dry air over it (not even blowing dry air for 12 hours will accelerate the drying process). No re-calibration is necessary because it is stable (on occasion you need to make sure it is still accurate by blowing on it a known gas mix like 1% CO2 and 99% N).
Different brand of absorbents seem to affect the mirror surface destroying it (which makes you wander what absorbents do to your lungs). It works with Sofnolime.
That is on the inhale side, in the counterlung, which is one of the drier and more stable temperature wise and least aggressive environment.
On the exhale side it is not dry. The closer you get to the mouthpiece, the worst it gets. You need to measure the very last portion of the breath to measure the end-tidal arterial CO2 (which is what matters), so the further away you place it from the mouth, the least accurate the measurement it gets.
Basically, it is a technology which won't work reliably inside a rebreather loop on the exhale side.
On the inhale side, when it fails, it fails reading high, which is fail-safe, because say instead of bailng out at 0.5% pCO2, you'd be bailing out at 0.3% pCO2 (which is safer).
In any event, if zero CO2 is what you want in a rebreather, 0.3% pCO2 you are already heading towards the beginning of a slippery slope.
So, my solution (for fun, I am not in the rebreather business nor ever care to be) is not to place it inside the rebreather, but on the body where it is a less aggressive environment.
I suspect the break-through will come when you can swallow a pill and this will measure various physiological parameters and transmit them wirelessly to a wrist display.
The reality though is that no one has been able to design and build one such device ("End-tidal CO2 Monitor") and make it available to the general public, which means that rebreathers cannot protect the end-user from CO2 retention (and a host of other risks) - users can pass-out and drown without warning.
For recreational diving, go sidemount on OC - safer and good fun (more fun than rebreathers)!
I'm not sure the sometimes vitriolic responses on this thread are actually justified, when positions aren't that far apart. Personally I have been learning quite a bit which I think has to be good. Actually I am one of those almost 50, people with sufficient funds to consider purchasing one of these units, so I like to hear both sides of this argument before I commit one way or the other. I consider myself an experienced OC diver (within Rec limits) and I dive regularly (usually at least weekly when at home and two or three times a year on diving expeditions of some sort). I also make it a habit to practice skills regularly and to ensure I'm not "cheating" or kidding myself, get an instructor friend to "validate" my skills on a regular basis.
As may have been surmised I am interested in the Hollis Explorer. Now to test some of my understanding of the dangers the sCCR may present to me, which at the moment I look at this way:
- Arterial CO2 retention and measurement on the exhale side, which I understand in this context is primarily driven by the physiological impact of breathing gas denser than exists at seal level. Given density increases with depth and this unit is designed only to go to 40m, unless an individual is particularly susceptible to CO2 retention, it is less of an issue as the gas would not be dense enough to produce outcomes more dangerous than OC at this depth. In this I am presuming that OC is equally subject to this phenomena as CCR, given gas density seems to be the key issue (though I acknowledged that WOB in a CCR may be higher than OC).
- Inhale CO2 has has multiple ways of being assessed, CO2 sensor (though like one of the other posters I have heard that this may now be an optional extra), temp probe, scrubber cartridge duration timer and comprehensive and automated pre-dive checks including pre-breath and calibration. In addition unit cannot be breathed if scrubber is not inserted.
- Oxygen Tox in this unit is not likely to be any greater issue than Nitorx OC as MOD would have to be equally respected under either.
- three redundant O2 sensors subject to calibration testing pre dive.
- multiple ways to alert diver if error conditions exist including HUD (with vibrating mouth piece) and a system designed to tell you what to do, not what is wrong (which basically involves bail out via BOV or sling tank). I also understand that the unit has a light on the top of it to let a buddy know what is happening with it.
- All electronics are fully potted and simple BOV is standard and unit only uses a single nitrox gas. There is also a valve to ensure any moisture in the loop is exited from the unit on each exhale (though I'm not exactly sure how that works).
- I'm also told by the manufacturer that the unit is "fully CE certified", but to be fair haven't (yet) sighted a certificate or the standard to which its been accredited.
There are probably quite a few things I've missed from the design as I have not actually had the chance to play with an Explorer in detail, but when I look at the above, from a risk management perspective, while the risks are different from OC (some risks more and others less dangerous) I am not seeing that the unit is going to try and kill me. I do acknowledge that this is new technology and therefore by definition extra risk exists. However, the training required to be certified from what I understand, is far greater than OC (which I might add, having seen a few of my friends recently OC certified worries the heck out me with how little they know or are taught!).
For any interested in why I might pursue such a course of action I am a very keen photographer (alas not that talented) and wreck diver. In addition the dryness of the OC gas I find quite debilitating. I consider myself very diligent and detail oriented, and the fail safes in the above unit and the thinking behind its design, in combination, lead me to believe the risks are acceptable.
OK, so no flames, and I fully acknowledge I am a novice in the world of CCR (and sCCR), but I am interested in any and all critiques on the above and any ways the unit might try to kill me that I haven't yet understood.
