Extendair

ScubaDadMiami

Please clarify. Does this mean that when using Helium mixes the duration is longer at depth than in shallower water? Or, does this only mean that there is a higher duration at depth using Helium in comparison to non-Helium at depth?

Please define "with depth." 100 feet, 200 feet, etc?

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Howard Packer
IANTD CCR Instructor
Miami Beach, Florida
CCRDiveTraining.com

paulraymaekers

is this the case for 5atm, 4°, 40l/min, 1.6l/min CO2 ??

I can not believe you only have 18% when say 3 atm, 20l/min, 25° and 0.8l/min CO2: the reaction-front is a small area, means that you use your scrubber easely up to 70% before breaktrough

regards
paul

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www.rEvo-rebreathers.com

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

AD_ward9

It means when you switch to helium at a given depth, the scrubber lasts longer than at the same depth on air.
Breathing machine in a chamber.
Alex

Claudio

Why?

AD_ward9

I can report phenomena: I am engineer. I do not know why the phenomena occur: for that you need a scientist.

To determine why it occurs, the scientist would do the same as an engineer does when debugging something with unknown cause, namely:
1. List all the possible hypothesese.
2. Prove or Disprove each one in turn, by mathematical or empirical means.

Some may waffle on about He having much better diffusion, smaller molecular diameters, less density etc, others might explain how CO2 is intimately mixed with the helium so get carried into the matrix better. Others might suggest it is due to a temperature difference between the two gases, even though on the face of it that would weight against helium rather than for it. So, help yourself to the theories. It does not really matter. What matters is the result.
Cheers,
Alex

Claudio

Ok, Alex
I am in agreement, but...
only with EAC or also with granules?
Claudio

AD_ward9

Both probably, but certainly on the EAC (that is, a scrubber at depth n, on helium lasts longer than at the same depth on air. Scrubber life still reduces with increasing depth, by a factor that depends on dwell time and flow rates).

Note: This He/Air conclusion is different from when we first tested the EACs, from doing the experiment a lot more carefully. We have invested a lot into new test facilities, rather than several yeara ago when using other people's facilities where one has to trust that when the mass flow meter says 1.6l/min of CO2, that is what it is doing at all depths, not just on average. A flowmeter calibrated in air can change the flow rate by a surprising amount at depth.

Our latest CO2 injection system has no flowmeter on CO2: to get greater accuracy, uses variable orifice valves calibrated by measuring pressure increase in the chamber with a very accurate sensor and waiting a long time (along with temperature and humidity compensation), then does the test sequence, then checks the cal again. We take a small bleed off once a min to check the PPCO2 is that expected, but that is just a check not a control. We have moved completely over to fully computerised test sequences (computer controls pressure, flow rate, breathing rate, all together).

The hard numbers will be published in November.

Alex

jaap

OK I will waffle some then

About the He/Air duration difference of the EA. Do you have any means to measure potential shifts between turbulent/laminar flow inside the testing loop?

If I recall correctly I read an article about potential CO2 retention in OC diving and the effect of depth, gas and flow conditions. As depth/gasdensity increases the shifting point is moved inside the human airways so that CO2 retention is increasing with depth. If a low density gas (He-rich) is used this effect is minimized. Maybe its old news.

The diffusivity of CO2 would also be higher in a He-rich gas compared to air but I doubt that the effect would be that big. But It would probalbly help the scrubber quite alot if the effect is significant.

How big is the He/Air difference?

paulraymaekers

see:
http://www.rebreatherworld.com/gener...pth+dependency

from text 45 and on..

paul

__________________
www.rEvo-rebreathers.com

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

AD_ward9

There is a big difference between turbulent (orifice type) flow and laminar flow in terms of flow resistance, and we perform measurements of the ratio of helium resistance to air resistance to determine how much of the flow is turbulent and how much is laminar.
This is correct. For extremely deep dives in the future, breathing assistance is necessary, but by extremely deep, I mean more than 350m. At these depths, all diving is on heliox, progressing to heliox with a shot of hydrogen, a whiff of nitrogen etc - keeping the END at not below 1ATM is good practice IMHO, even for these sorts of depths. I say, "in the future" because health and safety is getting tighter, and dives in the future to these depths, as opposed to those in the past, will no doubt have breathing assistance.
The theoretical range for the flow resistance of He/Air is 2.5 to 7.0. The actual numbers depend on the proportion of laminar flow to orifice flow. A tube whose length is less than 40 times its diameter, acts like an orifice. You have to measure the He/Air ratio for every configuration of each rebreather to get the exact numbers. An expensive business. Flow resistance does NOT increase linearly with depth, in either helium or air. That is, if you measure the breathing resistance of a rebreather at increasing depth, the resistance increases as a quadratic function (i.e., there is a constant multiplied by depth to the power of 2, as one of the parameters).

Incidentally on a comment posted earlier on this thread of the "1%" plastic in the EAC: we are not privy to the exact number - the number is very low indeed from knowledge of what the plastic is and tests on the EAC material: it is extremely weak when unpacked, and is too weak to machine. The EAC material is highly porous, which the pure plastic is not. The mechanical properties change considerably as the material is used. If you calculate how much CO2 is absorbed, then the active material must be close to 100%. All this points to there being very little room for the plastic, which acts just like a binder. As regards the comment that the Ca(OH)2 content being 99%: the active component is very high indeed - we have no other way of explaining the long scrubber life. The exact formulation is a commercial secret of Micropore.

Cheers,

Alex