Why not redundant one-way valves?

Sutty

Just a thought which has been nagging at me since before I bought a Rebreather....

Why do we not have redundant one-way valves, ie 4 rather than 2? You could have 1 inhale next to the mouthpiece and one at the scrubber head, same on exhale side.

I find it hard to believe it would increase WOB significantly, and I understand there has been at least 1 death from deformed valves.

I suspect if my kiss inhale valve was stuck open a significant part of each exhale would go back to the inhale counterlung (especially if I was tilted right side down) and get re-breathed, potentially leading to hypoxia/hypercapnia. Same applies to other makes of Rebreather.


Any thoughts anyone?...

Neil

decoweenie

The death you mentioned was on a passive SCR. Since the valve was deformed, there was no dumping thus resulting in no new gas added.

So after a while, the loop became hypoxic and the diver passed out.

As far the mushroom valve on a CCR, it isn't quite the same.

On an eCCR, the electronic will keep the PO2 at set-point. On mCCR, the orifice will continue to add O2. But the critical thing is the diver has 3 O2 sensors to monitor the loop.

The chance of the loop becoming hypoxic is very unlikely.

However, a deformed mushroom valve on the DSV will create a hypercapnic loop condition. And as you have learned in your CCR course, the symptoms are different than hypoxic. And it is more likely that the diver will recognize the problem.

Add 2 more valves to make 4 falls under the same idea of having 6 O2 sensors monitoring the loop. Where is the optimum balance before deminishing return ?

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Sutty

I agree the loop is unlikely to become hypoxic - but the problem is that you won't really be breathing from it!
If we take worst-case scenario of all exhaled gas going back to inhale counterlung, the volume of a kiss inhale hose (23") is 500ml, so particluarly taking into account the dead space of DSV, mouth, trachea, etc (at least 120ml), you will be breathing out into a dead space and back in fom it, there will be virtually no alveolar ventilation. You will therefore become hyperapnic and hypoxic quite quickly. The dead space is attached to the loop but gas is not circulating. Your PO2 guage will be happy though as the cells won't be hypoxic!

decoweenie

Sorry, IMHO, there isn't enough volume to breath normal full breaths as you have stated.

And as soon as I can't breath normal full breaths, I would know something isn't right.

__________________
"...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..."

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Genesis

I think there is a point to this from an engineering point of view. This situation could also be detected with a mass-flow detector, but those tend to be rather power-hungry beasts and you don't want the heat-producing kind in an enriched atmosphere either

caveseeker7

You may underestimate the WOB influence that one-way valves have.
Every manufatcurer I talked said that there is a very fine balance between soft, easy to breath and less flexible, better sealing, harder to breath mushroom valves.

In regards to the pSCR, the Tourill MK1.5 has dual valves to prevent gas from the inner bellows to flow back to the loop.

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jradomski

Hi neil,

If you have ever seen testing of the WOB on rebreathers, you wouldn't even have to ask this question.. Its something that many divers are not aware of..

On many designs the greatest contribution to WOB IS the DSV, and the flapper/mushroom valves are a significant part of this..

Most people do not realize how much changing the DSV can alter the characteristics of the breathing loop.

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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.

narked at 90

i was chatting to a rebreather manufacture last year about this and that, and WOB came up and after all the testing on the unit the thing that had the biggest influence on WOB was the humble mushroom valve. hard to believe but true.
kindest regards john routley



www.narkedat90.co.uk

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Sutty

500ml is the volume of a single hose - I measured it before posting, no reason why you would be unable to take a full breath as the deformable part of the circuit (counterlungs) is still attached, just with a huge dead-space.

Caveseeker/Joe: I can accept that the design of a valve can have a huge affect on WOB, but once you have it right would having 2 make so much difference? Resistance to gas flow is Hugely affected by diameter (ie 16X (if I remember right) the resistance by halving diameter of tube (admittedly these are not simple tubes), so a small diameter valve will cause problems, but 2 large diameter ones much less.

jradomski

The other issue is that the valves would really have to be positioned right next tro each other.. haveing the valves too far apart are no better than having just one valve.. The further they are apart the greater the dead air spaces..

With flapper valves in the DSV, and proper training, a failed flapper valve is nothing to worry about.. Its time to abort the dive, but with the proper actions the loop is still usable..

__________________
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.