SCR Variable Flow Rates

PaulTG2

Hello,

I'm just getting into rebreathers. I've been reading quite a bit of technical information about rebreathers and have a question regarding the real world flow rates for SCR rebreathers that use adjustable flow valves with manual insertion ability (such as the Azimuth for example).

The flow rates suggested in the manuals assumve a very high O2 metabolic rate -- usually in the 3-5 lpm range. This seems extremely conservative and wasteful.

Asumming that you have an PO2 monitor (which seems an obvious requirement for me) and have the ability to manually add gas, it seems reasonable to slowly adjust the flow rates down until you have something closer to your actual O2 metabolic rate. This would be using the same principle that the KISS CCR uses but in an SCR environment.

Questions: Is this safe?

Question: If people do this then was are the actual flow rates typically used above 130'? Pure logic would suggest, for example, a 32% NITROX flow rate of around 4.0lpm instead of the 15+ suggested in the manuals.

Question: How fast does PO2 drop when you start working harder? How often, for example, does someone have to hit the ADD O2 in a KISS rebreather?

What is reality, assuming you want to be safe but not wasting tons of gas?

Thank you!

-- Paul

caveseeker7

The success of the KISS rebreathers.

Adjusting the gas flow closer to your metabolism, and increasing the flow rate if needed, has been done. Do do it safely, IMHO, you should have more than a single sensor and readout. You will also need a valve that is easier to adjust than the Azi's yet can't be changed by accident. The question then is why not either convert the unit to a manually controlled CCR or do what a lot of people have done over the past few years and buy a KISS.

With a conversion you'll have all the SCR bit safe for the valve, the new valve and 3 sensors, housing, cables, displays ... almost the same as the stock unit. On a KISS it'll be about the same, largely because it is about the same.

While donning my flame proof suit, mCCRs are just SCRs with a really, really slow flow rate afforded by pure O2. They have a continous gas addition, and when the loop is full the OPV vents gas.

In case of the conversion it will involve some fiddling*, more likely than not you'll end up using a KISS or like valve anyway so why not save the effort and money**, get a KISS, get properly trained on it and just go diving.

You'll get the CCR advantages, lower gas consumption and optimal mix for the depth.
Can an adjustable flow rate SCR be done safely? Sure, but what's the point?

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Add-ited: By the way, welcome to Rebreather World.
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* fiddling may be part of the fun for you
** if you're looking at the Azi at an MSRP of $4500 (plus modifications)

__________________
Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

decoweenie

This is same as saying the YBOD is essentially a SCR with occasional bursts of pure O2...

What is the difference between a) flowing only O2 in really, really slow flow rate, and b) injecting the same amount of O2 in bursts ?

SCR flows diluent (which also contains O2), not pure O2!

abowie

Have a play with this Excel spreadsheet http://www.davidteubner.com/images/SCR%20sim.xls .

Should give you an idea of how quickly things can come unstuck with an SCR.

__________________
Andrew Bowie

Rebreather-friendly Buddy

Freef

I use various mix and jet combinations through my Drager Dolphin. vO2 [metabolic oxygen consumption] rates vary with the individual and workload. By using a pO2 meter, noting it's readings against depth, and knowing the cylinder fO2 and flow rate you can determine your vO2 thus:



Loop fO2 is derived from the usual pO2/depth.

If you take the following example:

Cylinder fO2 = 60%

Flow rate = 5.8 litres/min

Depth = 12m

pO2 = 1.12 bar

From the depth and pO2 it can be determined that the loop fO2 is 50.9%

Putting these figures in the formula thus:



Will give a vO2 of 1.07 L/min.

My typical vO2 is 0.9-1.0, with a 'working' peak of 1.35.

When determining the particular tank fO2 and flow rate combination peak workload must be taken into account to prevent hypoxia. While at depth life will be sustained on a pO2 that would give a hypoxic fO2 on the surface, it must be remembered that when using nitrox the reduced level of O2 will result in a higher level of N2, which may lead to DCS.

Other formulae can be used to calculate desired loop and cylinder fO2's from a known vO2, I have an article being looked over at the moment pending submission to the library here.

__________________
David.

Diving the mahogany rebreather.

RichClark

Not really a flame, I just don't agree

How does this really differ from any CCR, the fact that the injection is either manual or electronic is irrelevant, isn't it?

Rich

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The more I learn about women, the more I want to go diving... just don't tell my wife I said that

To taste something a little different, try http://www.thechillikitchen.co.uk

Freef

The YBOD and other eCCR's aren't SCR in the slightest, as they do not have a continual flow of gas to the loop, but have the metered addition of O2, which is the difference between SCR and CCR. Pure O2 will be metabolised, so even mCCRs shouldn't bubble.

For a luddite like me, the mCCR [slow and continual flow method with passive electronics] is preferable as I don't trust electronics that much [I'm a mechanical engineer, so anything I can't hit with a hammer is magic]. The continual slow flow with occasional pops of O2 to bring the numbers up is one way of getting the right loop pO2, the electronic measuring and solonoid is another, they both do the same job, just in different ways.

The only disadvantage of mCCR that I can see is if you have a problem that prevents manual addition of O2.

Some O2 SCR's [usually military] exist, with the corresponding low flow rates and depth restrictions. It is possible to get a 100% O2 jet for the Dolphin-only if you are an instructor though.

__________________
David.

Diving the mahogany rebreather.

caveseeker7

Really? WOW!

As you so nicely underlined, I mentioned that difference.
My point is that the gas addition technology is basically the same,
a little hole that feeds gas continously into the loop. If you open
the tank valves on both types of rebreathers they empty even if
you don't use them, venting gas through the OPV. In other words,
the gas either flows or it doesn't, if it is metabolized or not doesn't
matter any more than if it's sufficient. If your Evo works
like that you might want download V2.1.17, it's supposed to add
O2 only as needed to maintain the setpoint.

The bits and pieces are about the same, bit more so when the SCR
has an O2 monitor fitted. So the step up from a cmf SCR to a mCCR
isn't really a big one in many ways, but the advantages of the mCCR
are considerable.

Adding a setpoint controller, power supply and solenoid valve does
make things a bit more complicated and expensive, and it does change
the operating principle quite a bit.

__________________
Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

Freef

Interesting spreadsheet, and it certainly shows how SCR's can cause problems if you aren't careful.

[EDIT] I just tried increasing the vO2 for a given tank and flow rate, and the loop fO2 went up? I know vO2 as metabolic O2 consumption, are you calling it soemthing else?

SCR's aren't any more dangerous than OC or CCR though, as with any diving circuit choice you can kill yourself. SCR gas planning can be a bit more involved than choosing the correct dilutent for CCR. The only problems that I have had with SCR gasses while diving [apart from my cock up in the 'incidents' section] is having a lower vO2 than planned and having to dive slightly shallower than planned to prevent hyperoxia, but 2m shallower to drop below 1.3 bar isnt that bad as I can still be below 1.4 bar at the bottom, but the beeping of the pO2 alarm is annoying.

__________________
David.

Diving the mahogany rebreather.

Freef

But setting a KISS type Rebreather usually means the flow rate of O2 is below the vO2 of the user, requiring manual top up, so no gas is wasted.

__________________
David.

Diving the mahogany rebreather.