Understanding Rebreathers - Part 1 

Thanks!

Excellent question, and one that rebreather manufacturers are working hard on solving. We use a time cut off as the first guide. There are other ways to monitor scrubber material and even the appearance of CO2 back in the breathing gas, but none of them are fool proof yet. Watch this space!

Look up the rEvo RMS system this really work to calculate remaining scrubber time

@@colmdoherty49 the rEVO system is a temperature based system that uses an algorithm to predict scrubber duration. A heat front moves through the scrubber material as the chemical reaction takes place, and this is detected by the thermistors. An algorithm that accounts for depth and temperature then predicts safe scrubber duration with an added degree of conservatism. So, like the APD temperature stick, it is using the physicochemical properties of the scrubber material to predict duration. These systems have been validated under various conditions and have certainly added to rebreather knowledge and safety if understood and used correctly. But, you do need to understand they are not giving us the best information which arguably be the diver's inhaled (should be zero) and end tidal CO2 (should be stable in the normal range). A scrubber using the temperature as a proxy can still be over breathed, and other non-scrubber based causes of high CO2 levels in the loop or the diver can still occur and go undetected.

Cheers Steve! Yes at least you have a popular unit. And you have spent some time diving with the occasional KISS expert too I believe! The Sidewinder is a good case in point though isn't it, as it takes a bit of trial and error to get sorted with your harness etc. Having other experts around can short cut that process.

Yep agree Harry. I am headed to Mexico in Nov to train and dive with Patrick and the Protec team... Skanda et al. Will be brilliant to see how they are set up and fully expect a heap of tweaking through that experience. Cheers again for the great overviews.

Sounds like a good choice!

I think KISS is ubiquitous throughout the southern U.S. and Mexico. Not a bad choice at all. I've seen a lot of ChOptima Rebreathers near me lately as well.

Yes, it's called a hybrid CCR or hCCR. Use the rebreather in a primarily manual mode but have the solenoid activate if the PO2 is falling too far e.g. at 0.40 (a "parachute system". In some ways, the best of both worlds. rEVO does a nice version of this.

@@DrHarryH thanks for the info that’s interesting to hear that there are duel systems that can be used

Yes you could, but actually it wouldn't help. Your oxygen saturation will generally be in the normal range with any PO2 above ~0.18. Including dangerously high PO2s. Importantly, we aren't really interested in oxygen saturation, but rather PO2 (to avoid CNS toxicity) and the fractional concentration of the inert gases (to manage our decompresion). We infer the latter by knowing our PO2 and keeping it as high as safely possible.

No. Doesn’t matter.

Pete is a great man!

Good questions. All of these possibilities are covered in rebreather training and there is a solution for all of them that may or may not enable the dive to be completed on the rebreather. Worst case scenario is bailing out onto the open circuit gas and completing the dive. But most scenarios like you describe can be resolved by either 'flying' the unit manually or in a semi-closed mode. The only times I have had to bailout and complete the whole dive on open circuit is following a catastrophic flood.

@@DrHarryH Thank you for the reply, I really appreciate it!!

Just regular tanks. The cave is very shallow and hard work; two factors which can make rebreather use risky.

I hope the Miflex hoses are OK for oxygen use because I have used them extensively! Not sure if they are actually rated for this application or not. There are reports of braided hoses obstructing due to a problem with the interior lining turning into some weird crumbly crystalline substance. I have one friend who wont use them at all because of this. Our group has not experienced this issue, and I don't know whether it happens with genuine Miflex or only the others. The polyether based lining is alleged to not suffer the problem. Lots of articles on the web about this.

@@DrHarryH thanks for the reply. I had seen about older miflex hoses having that issue you say about the interior lining but nothing on the newer ones.

Good question, they don't seem to have caught on. To quote Ashley Stewart from In depth "Galvanic sensors are essentially wet-cell batteries that generate a millivolt current proportional to the PO2 in the loop. Conversely, Poseidon’s solid state sensor uses luminescent quenching, wherein a red LED light excites the underside of a special polymer surface, which is covered with a hydrophobic membrane and exposed to the gas in the breathing loop. A digital color meter then measures the responding change in fluorescence, which is dependent on oxygen pressure, and an algorithm calculates the PO2." I don't think any other manufacturers have picked up the technology, and even Poseidon still uses the old fuel cell sensors in tandem. When we finally crack totally reliable long lasting oxygen sensors and CO2 detection (a whole other topic), rebreather safety will take a big step forward.

No. Like many technical divers I do all my filling for any mix in the shed. Two compressors, booster and bulk supplies of oxygen and helium at home. Heavily invested in this hobby!

no. it is a volatile gas, and they normally use only a fraction of it. otherwise the smallest spark will send you to the moon.

As long as you keep the oxygen fraction

Yes asthma may be a reason not to dive, but as usual, it "depends". Best thing to do is see a doctor trained in diving medicine to discuss your particular case and the possible risks. The DAN websites have some good information.

Thank you Sir for your reply .. i will definitely check the DAN websites ..

@@ketanmhatre6709 If you do end up diving, get insured through DAN. Keep it up to date. They're great people.

Actually it it more common to see mCCRs with a "parachute" system or failsafe, with electronics kicking in if the PO2 falls too low, commonly around 0.4. The rEVO is/was commonly run tis way for example.

rEvo do a hybrid.

Yes you absolutely can use a full face mask (FFM) with a rebreather, however it introduces a number of potential hazards as well as the possible benefits. Benefits include a warmer face, protection from drowning in the event of a seizure, communications, less jaw fatigue and so on. However the hazards include potential for CO2 accumulation (so you need a mouthpiece still), difficulty equalising and most importantly, the issue of managing bailout. It introduces a level of complexity that should not be taken lightly. This would be a good topic for an episode actually.

@@DrHarryH so I was thinking for a bailout just do it like you have a malfunction with the mask . Face mask regulator and I'm on my way up I'm confused at how much more bottom time you get to where I would be in danger of getting the Benz because I run out of air because of deco times it makes me nervous lol

So I just bought a ccr o2optima.chest mounted rebreather .any ideas on where I can take the class.im in va moving to nc

O2 only I believe, which limits their range.

Good question. A new set of 3 cells every 12 months (about AUD140 each) and the rest is much like other scuba gear...hoses and o rings as required. A few batteries for computers etc. Soda lime for each dive can add up (AUD $10-15/kg). If you regularly dive using helium based gases, these costs are easily offset in helium savings. If no helium used, it is a more expensive way to dive for sure.

Maybe better wording is “can you keep up with the increasing cost of wasted open circuit helium”? Bring on the rebreather!

No, not at all. Analogue gauges are fine.

expensive, requires specialized training and matinence, and they are very situational. And if something goes wrong, you have a very bad problem on your hands.

Use bailout and all these dangers (sorta) go away

$10,000+ USD for entry, fairly complex, and expensive to maintain. If you are doing recreational diving and bumping up against no deco limits (not uncommon), a single open circuit cylinder lasts about as long as you can underwater.

@@marcwanagas1234 are you referring to ocr?

I can go quite a long time on one cylinder open circuit. Usually do two cylinders per day. Depending on the depth, that can last me 3+ hours.