Question Running a rebreather on only dilluent

[wedivebc]

Well that's f'ing dumb. I wonder what the rationale is. One of the biggest benefits of ccrs is time and techniques to get your butt out of a pickle.

I can think of several reasons. First you should always have enough bailout to complete a safe ascent without resorting to survival mode skills. Second unless your decompression computer is tied into your cells. A diver using a CCR as a SCR rebreather will not have an accurate way to track inert gas loading. Third the more choices you have in an emergency, the longer it will take to decide on an appropriate response. I teach my students "when in doubt, bail out" . Once you are comfortable with that primary option for survival you can go on to learn other survival skills but I agree that SCR is best left to more advanced classes with more experienced CCR divers.

 

[nadwidny]

Does argon escape through the membrane just as well as nitrogen does? Or does it depend on membrane type? Any multi-membrane systems?

How about CO or CO2? There is more than just oxygen and nitrogen in the atmospheric input.

I can't say for the argon because it's not needed to be tested as per the standards for Medical O2 up here.

CO and CO2 are negligible.

 

[justinthedeeps]

I can't say for the argon because it's not needed to be tested as per the standards for Medical O2 up here.

CO and CO2 are negligible.

My understanding was that in some systems, to make even 32% nitrox from atmospheric air, twice as much air volume actually gets processed through the system, with any trace gases or impurities potentially doubling in concentration, unless there is a way for them to escape or be filtered out along with the nitrogen escaping.

When concentrating atmospheric air to achieve even higher oxygen mixes, anything else that follows along with the oxygen--rather than the nitrogen--will also be concentrated.

Question is, which traces and impurities are they, and how are they removed (or not)?

 

[Tracy]

My understanding was that in some systems, to make even 32% nitrox from atmospheric air, twice as much air volume actually gets processed through the system, with any trace gases or impurities potentially doubling in concentration, unless there is a way for them to escape or be filtered out along with the nitrogen escaping.

When concentrating atmospheric air to achieve even higher oxygen mixes, anything else that follows along the oxygen--rather than the nitrogen--will also be concentrated.

Question is, which traces and impurities are they, and how are removed (or not)?

Using concentrated Oxygen to make Nitrox basically comes out to nil for argon increase. Yes, you get 4-5% argon in your oxygen, but you are only adding 11% to the oxygen. 4-5% of 11% is basically nil. Add to that for open circuit, you are exhaling the gas, the argon percentage is constant, it isn't accumulating like when you use it for rebreathers. Concentrated oxygen is used in rebreathers in many places, it just requires awareness and diluent flushes.

 

[nadwidny]

My understanding was that in some systems, to make even 32% nitrox from atmospheric air, twice as much air volume actually gets processed through the system, with any trace gases or impurities potentially doubling in concentration, unless there is a way for them to escape or be filtered out along with the nitrogen escaping.

When concentrating atmospheric air to achieve even higher oxygen mixes, anything else that follows along with the oxygen--rather than the nitrogen--will also be concentrated.

Question is, which traces and impurities are they, and how are they removed (or not)?

Where are you getting "My understanding" from? If it's from a dive shop then it's wrong.

I'm just telling you what actual analysis shows. Sorry if it doesn't align with what you think it should be.

 

[justinthedeeps]

Where are you getting "My understanding" from? If it's from a dive shop then it's wrong.

I'm just telling you what actual analysis shows. Sorry if it doesn't align with what you think it should be.

I am just thinking through what happens when you concentrate gases over selective membranes etc
Does it concentrate any other traces and impurities? How are those removed? This is not opinion, this is matters of basic physics.
I am glad to hear that you test for this and the gas analysis looks good

 

[Tom_Ivan]

Lol. Someone woke up on the wrong side of the bed this morning

 

[belorn]

I am just thinking through what happens when you concentrate gases over selective membranes etc
Does it concentrate any other traces and impurities? How are those removed? This is not opinion, this is matters of basic physics.
I am glad to hear that you test for this and the gas analysis looks good

Membranes compressors don't remove trace gases. It just separate the feed gas into a nitrogen rich gas and a permeate. In diving the permeate then get used while the nitrogen rich gas get vented. The process can be used to create concentrated oxygen, but medical grade oxygen is cleaner and in general fairly cheap. Membrane filters are also a bit on the expensive side so in many situations it does not make economical sense.

For regular oc nitrox the trace gases are negligible unless the general air quality is already bad.

 

[justinthedeeps]

Membranes compressors don't remove trace gases. It just separate the feed gas into a nitrogen rich gas and a permeate. In diving the permeate then get used while the nitrogen rich gas get vented. The process can be used to create concentrated oxygen, but medical grade oxygen is cleaner and in general fairly cheap. Membrane filters are also a bit on the expensive side so in many situations it does not make economical sense.

For regular oc nitrox the trace gases are negligible unless the general air quality is already bad.

That's some reasoning that I mostly agree with.

'Negligible' is being used subjectively here. Is a five-fold enrichment still negligible? When is it not? Is 4-5% argon negligible? It sounds like a matter of opinion.

Fortunately the hopcalite in any decent filter system would convert away any enriched CO, should it ever occur. The most dangerous of possible contaminants.

Not so sure about CO2, I would guess this does get concentrated, though 5x of 400ppm is still 'just' 2000ppm (but a multiplicative ppCO2 effect at depth)

In the context of rebreathers particularly, the argon does matter, thanks to @Tracy for hinting at that.

 

[belorn]

That's some reasoning that I mostly agree with.

'Negligible' is being used subjectively here. Is a five-fold enrichment still negligible? When is it not? Is 4-5% argon negligible? It sounds like a matter of opinion.

Fortunately the hopcalite in any decent filter system would convert away any enriched CO, should it ever occur. The most dangerous of possible contaminants.

Not so sure about CO2, I would guess this does get concentrated, though 5x of 400ppm is still 'just' 2000ppm (but a multiplicative ppCO2 effect at depth)

In the context of rebreathers particularly, the argon does matter, thanks to @Tracy for hinting at that.

For 32% nitrox you get around 1.4% argon, or about 50% increase from regular air. The compressor will generally operate to create 40% nitrox and then dilute it with air to desired concentration. Since MOD decreases as nitrox richness increases, the increase in argon from membrane compressors generally balance itself out for OC. For dill purposes in rebreathers the nitrox could be used as a top up gas but I have yet to hear argon being an issue there. It is when people use membrane compressor to create as pure oxygen as possible that one see numbers like 4-5%.