Why not redundant one-way valves?

Sutty

My point exactly!!
Its one of our few bits of equipment with no redundancy.

denzel

Nigelh,

That one way valve in your picture looks an awful lot like the overpressure relieve valve from the (east german) RG/U-FM.

Denz.

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jradomski

This would be like a pendulum type rebreather used for shallow water.. Keep this in mind if you use a .5% co2 as a max limit, that means at 90m (10 atas) you only need .05% to give you the same effect (PCO2).. This is why dead are spaces are so critical for a rebreather designed to be used under pressure..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

AD_ward9

Why is that? The gas volume displaced by the diver is the same in each case, so the scrubbing action of the gas flow is the the same (in fact, it is better because the gas is denser at depth).

The exhaled CO2 goes from 4% to 0.4% of exhaled gas in the example you gave, going deeper. The volume of gas going around the loop is the same. The number of molecules the scrubber has to deal with is the same. The more dense gas has a different water vapour capacity and flow characteristics through the scrubber, but with the 20% to 40% downrating for the depth, the scrubber still works.

Just interesting in your reasoning on why the dead spaces become more of an issue?

As to the issue on this thread, the solution is active respiratory monitoring. That tells you immediately on setting up the scrubber that the gas is not circulating properly, or if it fails to circulate at any time during the dive due to a mushroom valve failure. Respiratory monitoring is very simple to do, just measuring the pressure drop across the scrubber.
A decent scrubber life and health monitor needs those pressure sensors anyway plus the temperature stick, in my view, but if one is not in that camp, then it just requires there are two pressure sensors in a design. Pressure sensing is such a critical function, if deco is performed, they should be doubled up anyhow.

Another really simple method to run a respiratory monitor in a rebreather is to wind a Z trace of wire over the breathing bags and measure their inductance, to measure the bags inflating and deflating in correct sequence.

Cheers
Alex

Sutty

A simpler solution might be to have valves like some anaesthetic circuits, these have the valve flapper/disc visible under a clear plastic dome so you can see the valve working and see that it is seated correctly. I thought I'd do this if I ever make a homebuild, with 1 on each end of the DSV and visible during the dive (but probably misted up!).


Joe, the anaesthetic versions are definitely NOT pendulum systems, they have 2 long (4-8') hoses between the scrubber/valves and the patient, but the Y-piece is very close to the patient's airway.

Neil

Freef

When assembling, hose against cheek to check flow, when ready to go double hose over and try to breathe. Unless you are a beardie or are a senior diver the chances are your cheek will be smoother. If it isn't approach a manber of whichever sex you prefer and ask them to help you with your rubber hoses. Works every time.

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David.

Currently owner of two differently sized ankles.

Freef

Err, no it isn't I think? The following is ages old, but the deper you are the more dilute the CO2 is because of the gas density. [sorry about the spacing].

But before you settle down for a read, here are failure points on my Dolphin:
1] scrubber has to be packed by a pink chimp [me].
2] the P connectors only have one O ring
3] Hoses are exposed to possible damage while diving.
4] The pO2 meter is calibrated by said pink chimp.

I am more worried about a scrubber failure than I am a valve sticking. The valves get chaecked 3-4 times every assembly and dive, the scrubber is only packed once. I thought I had a CO2 hit once [false alarm], and I once gave myself one to see what it felt like [do NOT try that at home]. My eyes ached, a bit of tunnel vision, concentration lowered, breathing quickened and fely 'soupy'. Breathing air stopped the symptoms in about 30 seconds, but the headache lasted the rest of the day.

I also tried hypoxia as well, I felt tired and lethargic, again quickly recovered on air, but had a couple of mins on 50% just in case.
Here on Zorg, we abducted some humans to test your resistance to CO2 and the efficiency of our patented CO2 grabbing demon chamber.We took a human and connected a hose to them. The hose supplies gas and has one-way valves. The exit of the hose goes into a box. Inside this box are 1000s of little demons. These demons adore CO2. They will grab a passing molecule of CO2 and hang onto it for the rest of their lives. They can only hold one each. After the CO2 demon box there is another box withdifferent demons inside - these count the number of O2 molecules you have used and replaces them.We observed that humans when in a steady state consume the same amount of O2 per breath, regardless of the pressure we subjected them to. When given 100 molecules of our gas, they would use 4 molecules of our oxygen and turn this into 3 molecules of CO2 and 1 molecule of water vapour.So in the test, with 100 molecules of gas in the loop. The human push/pulled this through the box with the CO2 demons in it. Every breath, 3 lucky demons grab a CO2 molecule each and are happy for the rest of their lives. We repeated this for many of your earth hours, pushing 100 molecules of gas through the CO2 box at a nice steady rate - the happy demon front line progressed linearly through the CO2 demon box until eventually they are all happy. At that point, the loop gas has some CO2 in it and we observed that the humans started to show signs of unease, panic and general ill-feeling. They eventually died a rather uncomfortabledeath.To continue our experiments, we abducted more humans and carried on, this time we subjected them to a pressure of 2 bar. This is the same as being under 10 metres of your water. There is now 200 molecules of gas in the loop, but the human still only uses 4 molecules of O2 and turns these into 3 molecules of CO2 and 1 water vapour. Each breathe pushes 200 molecules through the CO2 demon chamber, so the demons have to work fasterto grab the CO2 molecules and die happy. Sometimes a front-line demon misses, but the 2nd line catches it OK. This carries on and eventually all the demons are happy, then as above, the human dies painfully and horribly from CO2 poisoning.We needed to do more experiments, so we continued with our abduction programme. Now we're testing to 90m. There are now 1000 molecules of gas in the loop, but as observed before, then humans still only take 4 molecules of O2 out and metabolises these into 3 of CO2 and one of water with each breath, However, the poor CO2 demons now have 1000 molecules of gas going through their chamber like a hurricane, and in those 1000molecules there are still only 3 molecules of CO2! It's now very hard for the demons to catch a CO2 molecule and hang on to it! The front-line demons have a really hard time catching the CO2 molecules and a lot more pass further down the line to be caught by the latter ones. Eventually, the front-line demons are full, but still the latter ones need to work to catch the CO2 and there will come a stage where there aren't enough latter ones who can catch the CO2 fast enough, so some will get through. Eventually so many will get through that the human starts to notice it and dies horribly as before - even when there are still some unhappy and empty CO2 demons left.Continuing our experiments with more abducted humans, we test again at 90m, but then we decide to ascend the human to some depth where the number of molecules in the loop is much less, so each breath the CO2 demons have more of a chance to catch the CO2 molecules left.Eventually, after 100's of trials, killing a great many humans every time, (And you should have seen our abduction budget! Off the scale!) we have come up with some rules for keeping humans alive and maximising the happiness of the CO2 demons. Our rules are many, long and complex but to simplify them for you humans we have reduced them to 3 simple rules..Rule 1: You have 3 hours maximum.Rule 2: For subsequent dives deeper than 20m: You must leave the bottom when the total time breathed through the system reaches 140 minutes.Rule 3: For subsequent dives deeper than 50m: You must leave the bottomwhen the total time breathed from the system reaches 100 minutes.

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David.

Currently owner of two differently sized ankles.

RonMicjan

this meant that he didnt do a pre dive check, or a prebreathe on the rig before the dive. Bypassing proper checks is a good way to become a statistic. All parts on a rebreather (or anything, for that matter) will eventually fail, its only proper maintainance that keeps things from descending into entropy. Does this diver now do all his checks, or still not learned the lesson?

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jaap

I discussed this with a guy working with development of military Rebreather and WOB generally increases the more one way valves one introduces. But there can also be other interesting effects from adding an extra valve into the loop. Depending on its location relative to the gas injection point/s one can get quite different mixing of the gas in the loop.

One not so related thing that could be interesting is to have an opv valve with dubble valves. Like on some drysuits made for working in contaminated waters. It would problay be easy to add and adjust on many units without altering to much compared to more valves on the DSV.

I have not had any troubles with the valves in my DSV and I check them prior to every dive. But I have had some suspected minor OPV leaks.

AD_ward9

If I can help with some numbers...

1. The breathing resistance increases by the 4th power of the diameter of a hose. Going from 24mm to 36mm internal bore reduces the resistance by a factor of 5.

2. Below are pictures of a hose we checked to see how much the resistance can be reduced by for a particular and quite deep application. The answer at the depth we designed to comply (just) with EN14143, is 10mbar. From one hose, to one breathing bag. I cannot say any more about the application without asking the client's permission to release information, but for the present debate the exact depth is immaterial because for another design depth, a designer would reduce the hose size so they comply with EN14143 at the design depth, and make the Rebreather easier to use.

You see it is possible to put in 4 valves, but breathing resistance requirements would mean making the hoses bigger.

3. Breathing resistance is not the sum of hoses, scrubber and hydrostatic resistance: it is less, because the resistance is measured with peak flow, and counterlungs integrate flow and take off the peaks.

4. You cannot breathe from the hose: it does not compress. You can breathe from the counterlung at the end of the hose, if it has gas in it (if not, ADV comes in).

Cheers,

Alex

Attached Images

	Resize of Rebreather hose and MP simulation model.png (123.2 KB, 64 views)
	Resize of pressure plot.png (217.3 KB, 66 views)