Please explain?

[peter k]

OK. I get the closed circuit bit and I get the open circuit bit, but, how does a semi-closed work. I've never seen one or even tried to understand one. I would have thought the circuit is either open or closed. Where does the in between bit come from and how does it work?

Peter

[jasondrake]

I'm no expert but here goes. There are two types of scr - active and passive. Active are the most common so I'll start there. A valve with a precise sized orifice, a constant mass flow valve, allows a constant bleed of fresh gas into the loop at a given rate. The gas is usually nitrox and no separate oxygen/diluent gases are used. As you breathe from the loop the oxygen fraction decreases and the constant flow allows enough nitrox in to replace it. An exhast valve is set to let a precice amount of gas to be vented from the loop to get rid of all that inert gas that is being injected. The actual gas breathed will have a lower oxygen fraction (and so a higher inert gas fraction) than the injected gas. So a bottle of 30% nitrox might give an FO2 of 23% or something. This can give a big gas saving over OC and can of course be used for trimix. The orifice used is set for the gas used and the dcesired FO2, and the diver is limited to a given depth range. There is no deco advantage over OC and FO2 is often variable making calculations a little tricky.
Passive scrs are similar but the counterlung is a bellows system with limited volume. As you breathe you trigger the injection of fresh gas in keeping with your breathing rate/tidal volume. As metabolism of O2 is closely linked to breathing rate this works fairly well. There are many more advantages to passive systems including a more stable FO2 and a much greater gas saving. But they're more comlicated to build. See the Halycon website for their RB80 passive do-da.
While scrs seem to lose many advantages of ccr (deco/gas efficiency/bubble free) they can run with no electronics at all making them robust and reliable. Various militaries use em as do some deep cave divers.

[peter k]

Okay thanks. I understood most of that but what does Fo2 stand for. I'll probably kick myself when told, but it's just the abbreviation I don't get.

Peter

[jasondrake]

FO2 stands for the inspired fraction of oxygen. So if I sit here breather the air around me, my inspired Fo2 is about 21%. It's just a way to differentiate between the mix in say a bottle and the mix you actually breathe from the rebreather loop. I think. There's hundreds of websites on rebreasthers including scr's which can explain it much better.

[Dr Stevil]

<font color='#0000FF'>AIUI, FO2 = &quot;Fraction&quot; of O2 which is (to be pedantic) technically an inaccurate description as it is (IME) mostly used when talking about the O2 content of your mix and setting your nitrox 'puta levels, which of course are partial pressures and are therefore percentages/ratios rather than fractions
Chee-az
steve

[peter k]

Thanks people. You've made a dim lad slightly cleverer.

Peter

[jasondrake]

You must be as bored as I am. I was going to start a conversation on whether percentages are fractions etc. , but then I had a word with myself!

[Dr Stevil]

<font color='#0000FF'>Bored?, no, just a pedantic old git with too much internet access &nbsp;

But it would depend on the intention of using the word fraction, literally it means &quot;part of a whole number&quot; so 25% is still that, but if in a &quot;works&quot; situation I was using partial pressures and describing them as fractions I would expect someone to point out that they are in fact proportions, &nbsp;&quot;we increased the proportion of oxygen in the experiment by applying a ppO2 of 1.4bar to samples x, y &amp; z....&quot;

Anyhoo, lets not go any further down that route, unless tomorrow is terminally boring &nbsp;


Chee-az
steve