one or two CL and scrubber dwell time

Drmike

Hi Im trying to get my head around this issue - can anyone help?

Ive heard it said that using 2 CLs will increase the dwell time in the scrubber but i dont get it.

Surely mass flow rate is constant scrubber volume is constant so what am I missing? why isnt gas velocity thru scrubber constant?

I get that having 2 cl gives slightly better wob but just how significant is that? Alex your design is single CL? Titan is single, MK16 single.

Chatting with RickS I know hes using only one CL in his sidemount and hes using that at 175m....

Any info pros cons appreciated

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paulraymaekers

here we go again mike, dwell time.... :-)

2 counterlungs, IF WELL POSITIONED, increase dwell time

one counterlung: only flow trough the scrubber half the total breathing time (double average gasspeed) eighter only during inhale (counterlung placed befor scrubber in loop) eighter only during exhale (counterlung placed after the scrubber in the loop)

dual counterlung, can take advantage of hydrostatic balancing in the system:
imagine both counterlungs are at the same height in the water: means waterpressure will try to level the gasvolumes in both lungs (lowest part of the gasvolume in both lungs at same height in the water)

so when you exhale, half of the expired gas will go trough the scrubber to the 'inhale-counterlung' the other half will stay in the 'exhale-counterlung', because waterpressure always trys to level the bottom of both counterlungs
meaning gasspeed trough the scrubber will be half of the single-lung system
inhale: same scenario: both lungs will be emptied at the same time, so half the gas comes from exhale lung, trough the scrubber now, half from the inhale lung, already scrubbed


now if the counterlungs are not at the same height, completely different story: one will be filled/emptied always first, (so no equal distribution between inhale and exhale) because waterpressure will always push the gas in the highest volume..

regards
paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Drmike

AHAH!!!

I get it!

Thanks Paul

Sooooo, maybe no point having two CL on a sidemount rebreather unless they are both at the same level?!..... Hmmmmm.......

Thanks again
M

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Marc T

On a sidemounted Rebreather you have a big hose ( usualy a Ray one which is very extensible ) on exhale side which have IMHO quite the same action +/- than a exhale CL.
Its perhaps a reason why there is few diference in this case.
Ill try in few day may sidemounted Rebreather with a exhale CL, but the reason is more to have a better water trap.
Ill let you know

Marc

paulraymaekers

mmhmm, perhaps a very small exhale counterlung, like the dolphin :-)

you can test the expandable volume of the big hose, while breating... will never be more than a glas of beer (and I mean a small pint!)

paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Marc T

Done some test with a dual OTS CL
On minimal volume and on exhale time there is a very small expansion of the exhale CL.
Then if you compress the exhale CL, no big change in the wob but if you compress the inhale CL you get far highest wob.

When breathing you have a far highest strengh on exhale than in inhale, so its rather easy to go through the scrubber on exhale time than on inhale.
The possibility of a temporary storage of a small amount of air in the exhale CL just decrease the pic pressure on exhale time ( it was Alex explanation on a previous thread, well what I understood ).

Marc

Drmike

So the exhale CL acts like an accumilator (or more accurately perhaps a pulse dampener) buffering the exhale gas and enabling it to flow through the loop during the whole breathing cycle instead of only during exhale time.

So I guess then that the required size of the exhale CL depends on the resistance in the rest of the loop? A loop with very low resistance would mean the exhale CL would not inflate?

Hmm

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Marc T

Yes an its also depends on the flow rate.
During my little experiment with OTS CL I clearly note that on a fast and complete exhale the exhale CL inflate more than on a slow but complete one.
Pulse dampener thats exactly that.

Marc