Scrubber Question

Joseph Grech

Might be a silly question,

Why are scrubbers more efficient in shallower waters ? I always tought that CO2 builds up when oxygen is consumed by metabolisim. So if I am right scrubbers should be equally effective in greater depths as in the shallows, this beceause when diving CCR the physical pressure of O2 in the loop is maintained approx the same ( ppo2 ) at any level.

Someone told me that this is because at greater depts O2 and CO2 is more dense. Is this right ?
And if both are more dense should not the ppo2 level increase ???

I hope you guys can help

Joseph

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schford

The reason for this is that the deeper you dive the more molecules of gas will be in the loop and consequantly going through the scrubber. What this then means is that the reaction front in the scrubber eg the 'depth' of scrubber material being used to remove the co2 from what you exhale will vary with depth. Rhe more molucels of gas in the loop the greater the depth of scrubber material you need to remove them.

eg Say at 10m depth we might only use 1cm of scrubber material to remove the co2 but at say 50m depth we might use 5cm of scrubber material.

Hope that makes sense.

Stuart

Janos

Also,

Just to confuse things even more, I learnt at the Dive Show that the reaction front isn't a flat disk - it's more of an inverted cone shape - the lime is used up most at the edges of the cannister then along the central axis.

Janos

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schford

Janos,

I think that depends on the individual design of the scrubber units and is correct for cannister axials specifically.

Radial cannisters and radial donuts then I believe it is a different shaped reaction front...

Dead interesting stuff this!

Stuart

Joseph Grech

Stuart,

As you know, the amount of molecules of oxygen in the loop cannot increase with depth as it becomes hypoxic. As in the Kiss system you block the depth compensating first stage in order to have a constant flow.
It is true that the volume of dilutant increases with depth, but if CO2 builds up just with the consumed oxygen then it is irrelevant the depth, CO2 in the loop should be the same and not increase with depth providing diver does not work more hard.

Please let me know where I am wrong.

Joseph

Mdemon

I think the CO2 produced is the same regardless of depth, but the volume of gas in the loop is much greater at depth (more dil etc) so there is more gas flowing through the scrubber. Since the CO2 molecules have more gas to "hide" in, they get further through the scrubber before being scrubbed - hence the extended reaction front.

(Caveat: This is what I think but I am a newbie, so take what I say with a pinch of salt!)

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schford

Your right in that the number of molecules of O2 stay the same but there needs to be much more dilutent to ensure the lopp volume stays the same as depth increases..so the overal number of gas molecules in the loop increases greatly.

This then means that a deeper depth of scrubber material is needed to succesfully clean those Co2 molecules as the first unused scrubber material the Co2 comes into contact with probably will not be able to remove all the Co2, hence it is less effecient....

hoopa

i think the answer this guy is looking for is in the partial pressure % of a dive. As you know-- when you go deeper your o2% doubles each ata-- so at the surface you have 21%o2-- at 150ft your at 5.5 ata-- so your po2 at that point will be 1.15 or 5.5X the .21 of o2 you had at the surface--- so you now have 5.5times the molecules of o2/co2 ect in each breath-- so at that point your scrubber is having to remove 5.5times more co2 then it does at the surface.. hence faster use of the scrub.. Where this point will stop is the fact that your rebreather will have a 02 meter-- and lets say you have it set for 1.2 --- so at about 5.7 ata(188ft) your Rebreather will adjust the amount of o2 in the system to keep your unit at 1.2 p02--- so at that point you have hit the point of diminish and return(your going deeper yet your not scrubbing more c02 cause there is less in the system to start with at each breath)


disclaimer: lol-since i am also new to the Rebreather world-- if this is not 100% correct please come in and correct me any way you would like-- but i do think this explains why you would use the scrubber more at deeper and deeper depths.(flame suit now put on) lol

ps. if you get a chance read this artical that explains partial presure(its on this board and link will be below). The guy who wrote it should write all the dive manuals in the world-- very easy to read and understand.
http://www.rebreatherworld.com/artic...ticle&artid=22

schford

The number of molocules of CO2 that are present in each outward breath will be roughly the same regardless of depth. If we metabolise roughly 1L of O2 at 1ata then we metabolise that number of molocules regardless of depth.

When metabolised we then exhale roughly .97l of CO2 and x number of molocules regardless of depth.

The reason that the scrubber is less effecient is that those molucles of CO2 get lost in the increased numbers of molocules of gas in the loop due to the increase in gas and number of molocules to keep the loop at the same volume...

Stuart