Understanding Rebreathers - CMF Valves

[@DrHarryH]

ERROR IN TABLE at 7:31. The final column should be labelled P2/P1.

[@williamsweet7511]

I absolutely love your content. Your diving videos are amazing. Your teaching videos are very well done. Your explanations are concise and easy to understand. I bet your patients love you.

[@mijodo2008]

Schools in with one of the World's Best Cave Divers, thank you, KZbin. The University at our Fingertips. Thank you Dr. Harris. Cheers from Michael. Australia.

[@lolonav]

Not rebriether diver but definitely tecky thanks for this content wich open my eyes it all make sense

[@williamsweet7511]

do you think you could cover front mount counter lungs vs back mounted?

[@markusleitner3735]

Thank you for doing videos at this level on KZbin. Despite the hazards mccrs might have, I really like my classic kiss. Looking forward to more content!

[@shable1436]

Mind blown the technology and physics

[@nickjordan6344]

What a brilliant idea to use sonic choked flow to regulate mass flow.

[@justfly7730]

I actually learned about you from dive talks. I m not a diver, never dived even once but it's an amazing world. Just subscribed and thanks for all the things you ve given to the world.

[@hallytron]

Hi Richard, the table you pulled up at 7:31 was incorrect. The last column should be p2/P1(red), not the other way round

[@DrHarryH]

Quite right! Thanks for pointing that out. I'll pin a message.

[@tobiashartung856]

Hi Harry, that was a nice explanation. I'm just wondering, say I were to think about getting an mCCR and growing into diving deeper than 90m, i.e., outgrowing the 10bar IP, is there any downside to just getting the smallest orifice and cranking up the IP for the mass flow I need?

[@DrHarryH]

That is certainly one solution. It requires a bit of trial and error to get the oxygen delivery right for you as an individual, but the orifice that comes with the unit as standard works for most people with some minor IP adjustments. Remember all brands of 1st stage have a safe range to set the IP. Set it too high and you may increase the risk of 1st stage creep and even seat failure (I have experienced that latter once and the PO2 spikes very quickly indeed). With sufficient experience, you may find a needle vale solution as described a better option. I have dived mCCR to 140m using this technique, but I repeat, you want to have significant experience on the standard unit before attempting this. I prefer to dive an eCCR at these depths to be honest.

[@tobiashartung856]

@@DrHarryH Thanks for the detailed answer. I completely agree that you don't want to overdo it with this approach and it is inherently limited by the constant orifice and the size you can manufacture. As such, it is not a proper solution. My basic thought was mainly that the 10bar IP could be considered just about limiting for the depths that many people consider still tolerable risk (100m-120m ) range which would just be out of reach. Thus, closing off a Cyklon first stage for the higher IP tolerance and the smaller orifice could just about make it in there without needing a needle valve. Of course, it all depends on what IP you actually need with the smaller orifice, so it's also a bad idea to bank on it, buy a rebreather, just to then realize that the IP you need with the smaller orifice is still not quite high enough.

[@DrHarryH]

@@tobiashartung856 yes the old Poseidon 1st is easy to blank and I recall setting it around 13-14bar many years ago. Check out this for the unit that started me of way back! www.therebreathersite.nl/05_Reviews/evolution_of_a_rebreather.htm

[@tobiashartung856]

@@DrHarryH Thanks for the link! Very interesting indeed.

[@charlesroberson3805]

Hi Harry, nice explanation of CMF and it's cool to see the old homemade needle valves. I thought I would point out that the Fathom CCR has been using a needle valve with a blocked or uncompensated first stage for 6+ years now and the result is a very intuitive and easy to dive mCCR with none of the drawbacks of a fixed orifice. We also use a stronger spring in the first stage to increase the depth limit. I've used mine to explore Weeki Wachee at 125+ m.

[@DrHarryH]

Hey Charlie, yes I agree the needle valve solution is becoming much more widely accepted and plenty of deep dives being done on them now. I guess by the time you are doing these depths on mCCR you have the knowledge to drive the system. I do admire Kerry and the guys who started developing this over 20 years ago. Interested to hear if anyone is aware of an earlier use.

[@mikeller_nz]

Thank you very much - very clear and concise explanations. One slightly nitpicky comment: On the OmniSwivel (made for KISS) CMF / manual add block the O2 inlet and common outlet seem to be swapped, with the common outlet being in the middle (at least this is the case on the unit that I own).

[@DrHarryH]

Must be made differently to this one I guess

[@davidMax78]

Hello Dr Harris, What do you think about the movie just came out from Ron Howard ? Accurate to you ? Thks

[@DrHarryH]

I actually haven't seen it yet as i have been away. I have heard it is pretty correct but obviously a bit dramatised.

[@davidMax78]

@@DrHarryH yes actualy normal for Hollywood movie. Thank you very much for your response Sir ! Greatings from France

[@davidedebenedictis]

The clearest explanation ever seen on this subject. Question: with an unlocked regulator and needle valve, how many times should the valve be operated during depth changes? I have been using a kiss for quite some time and do medium to deep dives. I find the orifice to be very delicate, but I also don't want to be adjusting the needle all the time. Is it very sensitive? Never seen one in person.

[@DrHarryH]

In my normal depth range (say down to 40m) I would say I don't really adjust it. I haven't done a lot of diving past 100m on this valve but I think the adjustments are minor. Send Charlie Roberson at Fathom Dive Systems an email and ask him.

[@davidedebenedictis]

@@DrHarryH thanks, much appreciated

[@PaulAnthonyDuttonUk]

Do they do one like the semi which opens every three breaths? That seems even simpler because it’s a delivery not dependent on gas analysis yet better correlated to metabolic effort(if that’s a thing)..

[@DrHarryH]

CMF is used in some semi closed systems as well, but that is different to a system that is keyed to respiratory volumes.

[@wesleymaples1928]

Thank you for making these videos. I have begun the journey of tech diving and all signs are pointing to a rebreather. These videos are helping me decide on which unit I would like to look deeper into based on their individual attributes rather than your affiliation with a particular brand. That in itself is greatly appreciated. Please keep up the awesome work. Love your content.

[@hummaid1]

Hello, thanks for cmf description So rebreather has depth limited to 60 meter with special cap to first stage regulator but for more than this depth what is the solution maintain CMF?

[@matthewwyjad]

Fascinating. That is a very cool design. I love simple things. 0.003 is pretty small. Definitely incentive to keep dust out of the system. I was watching another ccr video recently where some dirt clogged a solenoid and jammed it open. It hit 1.9ppo2 before it was noticed. He was ok though.

[@johannmattis4842]

First human being who took the effort to explain in a way, so I understand all critical details 👌

[@jasonfoster9118]

Looks like I just found my next money pit. I just want to get into underwater photography and I love the idea of the inherent dependability of the KISS system. There are dozens of deep holes in the local creeks I'd love to be able to just hang out in and watch the wildlife. Maybe I can sell tips to tournament fishermen on the lake lol.

[@sergiescandell]

Thanks Richard, very useful information for rebreather divers. So glad to have your knowledge available for all of us. I hope to see more.

[@CubanazoJB]

I’m getting slowly in this and can’t appreciate enough the explanation!! Thank you…

[@cavediver2579]

Just setting up a needle valve on my sidewinder tonight. I watched this episode twice. I had been frustrated that I was not hardly seeing a change in the flow with the needle but I also realized I didn’t want that much flow change. The video gave the why behind the how.

[@mikea5745]

That's a brilliant design Thank you so much for making these videos!

[@bmg7485]

Thank you so much Richard for this clear and simple explication of a really tricky probleme 🙂

[@ProfileP246]

Love watching your vids very concise and on point with no added on drivel, thank you!

[@sebastien2487]

Great video : crystal clear explanation 👌🏻 thank you

[@hummaid1]

I beleive the maximum depth for manual rebreather is 50 meters, because CMF, and using deompsated first stage for O2. In case I want to go deeper to 100 meters or more what is the solution and procedure? Thanks for your vedio.

[@DrHarryH]

Hi, the maximum depth (from the perspective of oxygen addition) depends on the intermediate pressure (IP) of the 1st stage regulator when it is 'decompensated'. For example, if you want to dive to 100m (11 ATA), you need to set the IP to to at least 12 bar. The alternative solutions are to use a bypass so you can add oxygen using an add button at any depth beyond that, or use a needle valve and a normal (compensated) 1st stage.

[@JohnMaxGriffin]

Thanks Harry, great explanation of this topic

[@looloolama]

Why not use a much smaller orifice with an appropriately higher IP to give you a CMF usable all diving depths (Say 30 bar)? Seems like an easy way to remove the needle valve from your workload. Does higher gas density impede sonic flow? Higher risk of blocking the little hole? Cylinder pressure drops below IP too soon? Thermal expansion/contraction of the smaller hole (adiabatic/environmental)?

[@DrHarryH]

Hi, the complex physics of sonic flow is way above my pay grade. But there are some practical issues in simply increasing the IP higher and higher. Namely stress on the 1st stage components and the small orifice is certainly prone to blockage (this is not uncommon using an orifice). The needle is not difficult to use seems to lower the risk of a blockage.

[@EvaYFan]

Hello Harris, does the needle valve have certain marks on it as each time you twist it lets you know the exact amount of O2 you’re letting in into the system? Or it doesn’t necessarily need to be as precise as your metabolic rate? Thanks!!!!!

[@DrHarryH]

I have a mark on mine which is equal to 800mls flow on the surface. From then it is just a matter of adjusting open or closed during the dive. If the first stage is compensated (like a normal regulator), I also like to have a shut off valve in case the flow gets too fast at any point. I also like the shut off because I very worried about not turning my oxygen cylinder on before the dive, so this way I can turn it on and not worry about losing gas before I dive. Open valve, give a blast of oxygen into the loop, then enter the water.

[@ilikehiking]

Great explanation! very interesting.

[@h.r.hufnstuf4171]

Reminds me a little of rocket engines

[@joelnorton9742]

I opted for full eccr. However i do like palagian needle valve for o2 "c mf". If i build a unit or create a submarine control unit i will utilize the needle valve,

[@joelnorton9742]

O2 shutoff, and manual o2 addition seems like the best for deeper diving. I'm glad i fly manually as much as possible with my unit taking parachute mode if i didn't keep up.

[@joelnorton9742]

If i had a cmf valve of course.

[@mitchelljack1590]

Very interesting, though also very confusing to my brain 😂

[@brois841]

At a very high level: most first stages on the market are depth compensating, meaning as surrounding pressure increases, so does the intermediate pressure - IP. Because mCCRs are constantly pumping oxygen into the loop the first stage is converted to be non-compensating, meaning it has a max pressure and that's it. Its pressure gradient decreases as you descend, until surrounding and IP are essentially equal and flow stops. This is why mCCRs are depth limited from the factory. They're typically limited to

[@tvelocidad]

I really love your content but I do feel like I need to go back to school.

[@MrHeroicDemon]

Hey loved you at divetalk, and also used your info and what you all did to save those people, into my "Culminating" exam, and referenced you, divetalk, and the team of divers. (with some headline articles reviewed by doctors) Because of me nerding about different subjects of science in Biology, My teacher loved what I brought up with high levels of curiosity. I used the factor of the metabolism rate you suggested from the rescue. Just genius, amazing. Thank you, and great video ! The education channels need more of this all the time ! I watched all these channels already, and all videos. I'm almost done DiveTalk. (Brady creator of numberphile and other channels like perodictable videos/deepsky/sixysymbols) Smartereveryday, vertasium, mark rober, william osman, michael reeves, i did a thing, physicsgirl, and many many more. Tom Scott. ChubbyEmu, Nile Red and blue, Stevemold, computerphile, many more. I can't count them all. Some on the Law side as well.

[@graemedicks6938]

Hi Richard, Thanks for your well explained, video, and knowledge. This is essential for anyone who intentends to go the rebreather route.