SCR Variable Flow Rates

caveseeker7

AAARRRGGGHHH!!! :

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"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
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lizardland

I doubt it. ppO2 monitoring on a SCR is a reassurance, on CCR it is what drives the unit and is a primary system. You would definitely need at least two sensors, maybe three dependant on your views. If you are going to that amount of hassle then you might as well make the full conversion to CCR instead of polishing a turd.

On any CCR pure O2 is added so only relatively small slugs of gas are needed to maintain the loop. Trying to maintain the loop using diluent will need much bigger injections of gas. Bigger injections of gas may mean problems with loop volume and a lot of gas being vented off (i.e. wasted). Most SCR's don't have any way to manually add dil either. That said, some passive SCRs have variable addition rates.

As for how often I have to add O2 on my KISS, very rarely. It seems to be spot on for my normal pace in open water. A little bit more often when cave diving as I'm swimming harder. The O2 drop is very slow, I've dived my KISS with the O2 switched off completely, opening the tank valve only when needed and it isn't that often.

With the KISS, the only time you need to do any real manual control of ppO2 is when changing depth.

Go CCR.

As for whether or not the KISS is a fancy SCR... I get the comparison but it's wrong. SCR is nothing to do with whether or not gas is flowed continuously -- the RB80 and other passive SCRs (including saturation diver bail out systems) are semi-closed but only add gas when required.

What makes a SCR is that some gas is vented offboard regularly as part of the normal operation at a constant depth. On CCR it is not part of the normal operation. The KISS does not vent gas in normal operation at constant depth hence it is a CCR. Or put simply, if it bubbles it is SCR, if it doesn't it is CCR. What gas is injected and how is irrelevant.

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decoweenie

Me too, so never mind or we will be spending all day typing trivial craps...

RichClark

What I was trying to say, only much more eloquently put.

Ta!

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divetheworld

A load more here for those interested in the not-quite-bubble-less

http://homepage.ntlworld.com/brent.h...rebreather.htm

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PaulTG2

Hello again,

I think the answer to my questions are that:

Yes, it is safe to lower the flow rates in an SCR if you have an O2 monitor (preferably two (or three)) and a manual addition valve.

People do do this. There is a great spreadsheet to show you the effects (thank you Abowi for the link).

It was suggested that the PO2 drops very quickly in an SCR, but that is not so a mCCR (even with O2 off)

I think I'll expand the spreadsheet to include a graph that include loop volume and the speed at which the PO2 and FO2 drops with changes in O2 metabolic rate.

-- Paul

PaulTG2

Hello again,

I'm the newbie and I'm probably about to get ejected from the forum for this but...

In the end it seems any rebreather question posted anywhere involves going through all the SCR,mCCR,eCCR arguments including the caution that you'll die if you dive with "the other one".

I might just have to go with a home built SCR/mCCR/eCCR multi rebreather combination so that I can ask a question without offending everyone and starting the old arguments all over again. Of course there will be problems with that too.

Rational arguments can be made for the safety and effectiveness of each type of rebreather. I guess it is just human nature to want to justify why I spend X money on Y instead of Z.

Just like aviation.

-- Paul

gibbogeo

The reality is that if you are stuck in a current at surface with the wrong combination of gas and flow rate, you will get hypoxic.

To avoid it you will neet to have some sort of redundant oxygen monitor and a pure O2 adding system (it is faster to obtain the correct PPO2 than adding nitrox).

In other word you will need a manual controlled CCR. And there are no reasons to use it as a SCR.

So there is no reason to use the SCR with settings differents from the ones stated by the manufacturer.

Freef

Really? I get the best out of my Dolph by chosing the gas mixes and the flow rates. Sticking to the manufacturers recomendations is fine if that is what you want to do, but with a little bit of nore taking you can optimise every dive you do when SCR diving.

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David.

Diving the mahogany rebreather.

Freef

Paul,

You won't be chucked out for asking questions, however baiting me and my Dolph...

SCR, mCCR, eCCR all have different characteristics and design features, and a rebreather, ANY rebreather will kill you iff you take the ...., as will OC.

Ask away, you won't get bitten.

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David.

Diving the mahogany rebreather.