Extendair better for flood recovery?

trob09

For me, it's not so clear. Example:

I have 1 penny in my front-right pocket. There are 4 pennies on the desk, I put them in my front right pocket. I have just dramatically increased the available cash in my front right pocket (400% increase). Sounds impressive, unless of course I have, say, $400 in my front left pocket. Then those 4 pennies are not so impressive relative to total cash.

What is missing here is relative measures - do the other factors in the loop (not just the scrubber can and 1 hose) make this measure relevant or is the change in WOB due to moisture insignificant relative to those other factors?

Again, I am not here to tear down the EAC, but claims of superiority demand empirical proof.

AD_ward9

Using the empirical data to get to the result you want without having to do an infinite number of experiments. If something can be calculated easily, then there is no point wasting tons of money on experiments. One does one set of experiments very carefully, gets the empirical data, then use well established laws of physics to calculate the other cases. The cost of these experiments is so high, I do not see lots of people on here repeating them and publishing their own results (alas).
You are right that there are a number of different experiments that can be done.

The particular experiment reported was to find the effect of flooding on a recovered rebreather. In most rebreather accidents, the mouthpiece falls out of the mouth (even though EN14143 requires a mouthpiece strap), and the unit floods. There is then a forensic need to work out from the flooded rebreather what state it was in before the flood.

However, we picked the 2 hour time because it is a good rebreather dive time. The loop is almost fully humidified and water is retained by the scrubber. This causes caking very similar indeed to that of immersing the scrubber in water. We measured the amount of retained water and it is less than the amount of water the scrubber generates by absorbing CO2 during a 2 hour dive.

The only drawbacks we are aware of are supply, cost and much lower tolerance for poorly designed scrubber gas flow.

The issues you raise are relevant, but is really up to vendors who use granular material to carry out these tests and publish the results to justify the safety of their systems under foreseeable conditions. We have published sufficient data and explain our views (and we accept there may be valid alternative views), that granular scrubbers do not seem to meet the safety requirements for SIL 4 operation in a diving rebreather.

Sorry, but I must disagree with the method you propose.

To compare the breathing resistance of scrubber A to scrubber B, we measure each scrubber, and eliminate all other parameters.

If you put scrubber A in rebreather X and then scrubber B in rebreather X, you introduce lots of other parameters that are simply noise. You get a result that is valid for rebreather X under the test conditions but nothing else.

The rebreather is a linear system.. If one component that is 80% of the total breathing resistance increases its resistance 4 fold, then the breathing resistance of the rebreather will increase by around four fold. If you want to go from this first approximation of the performance of the system to a much more exact number then you can do it either by calculation or by expensive measurements.

We have published a very details maths model of a rebreather, using which it is easy to put in the parameters of your loop, put in the resistance of the scrubber, and it tells you the total breathing resistance. The resistance of hoses etc, is very simple to measure and to model. There are only 4 gas laws involved.

Alex

trob09

Alex,

I'm not going to be drawn into a point-by-point pissing match. Half of my business involves statistical analysis and data projections. I am keenly aware of how data that 'proves' one thing can be mis-used for another.

You seem confident that your data is conclusive. From my perspective, there are holes, holes that you appear to be content to exclude from your analysis.

Fair enough. Best of luck to you.

Tim

AD_ward9

I do not understand your line of reasoning.

We have published absolute figures for the scrubber breathing resistance under specified conditions, not relative figures.

In comparing A and B, we compared the absolute figure of A against the absolute figure of B to get a relative figure.

In terms of using these figures to get the effect in a rebreather, it is a simple process: The maximum breathing resistance is defined in EN14143 (25mbar). It follows basic gas laws, so there is a hydrostatic element (eliminated in EN14143 depth measurements if Over the Shoulder CLs are used because it takes the measurement relative to the suprasternal notch instead of the lung centroid). So, at 5 ATA with air it is 25mbar max, at 1 ATA it is 5 mbar max, at 75l/min.

We measured at 90l/min at 1 ATA, 13mbar. So divide over the whole cycle due to the CL averaging, and we have 6.5mbar at 1ATA for a rebreather with everything else optimal. If you have wider scrubber, just divide by the ratio of the surfaces area.

Now increase the breathing resistance in the scrubber by adding water, to 40mbar, so now the ideal rebreather using that scrubber is now 20mbar at 1 ATA and does not comply with EN14143 safety requirements at a depth of just a few metres (actually 1.2 ATA, i.e. by the time you get to 7ft).

We can probably debate this a long time. The only point I am making is granular material increases the WOB dramatically when it is packed dry and then becomes damp, though any means. This phenomenon is large enough that it behoves any manufacturer using it to justify the safety when it is exhausted and when wet with the amount of water generated during a dive.

If you wish to have further measurements, or disagree with the method, then please do the further experiments and post the results. We are happy to both defend our numbers and debate things more when you have your own numbers, whether from experiment or from calculation.

I accept our data is not conclusive for all conditions, but give a good basis for a first order approximation that covers most conditions. We do genuinely welcome the publication of other experiments and would reproduce any experiment that indicated our figures our not sufficient or accurate under those conditions, but I need either the data or the calculation to justify that.

Alex

Drmike

Alex, thanks for taking the time and effort to post the data you have and how you interpreted it. Interesting stuff.

BTW you wrote "a used granular radial scrubber (2 or 3 different time periods), can be determined by simple arithmetic based on its geometry and the figures in the report."

Can you tell me what the relationship is?

I have noticed the increase in breathing resistance through 'clumping' of used sorb at the end of dives where the scrubber is a solid caked lump at the end

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sven

Alex,

Thanks for the detailed information. I guess the problem is that most of us are challenged when it comes to translating the technical parameters (eg. increase by 30mbar) into something that we can compare to our experience. Most of us have no "feeling" for what the numbers actually mean from a user's perspective. Is there any way of expressing the test results in terms that allow simple users a comparison to the WOB on their equipment, or, OC gear? Something like 'flooded scrubber on unit <insert your favourite unit name here> breathes "three times as hard" ' (or whatever the ratio is)?


Cheerio from sunny (?) Sydney!

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silent running

Hi Alex, thanks very much for all the good data to chew on. But I've got a few questions:

Can you explain this bit above about the OTC CLs and the "suprasternal notch" and EN 14143 depth measurements being eliminated with OTS CLs?

Do you know how mbars compare to kPA units? I'm not even sure what kPA are, joules maybe?

And you mention in your test that you had 80% humidity in the canister/breathing machine. But isn't the humudity in a CCR loop very close to 100% and wouldn't this have the effect of increasing the WOB and thus effect measured resistive loads and is it necessarilly a linear increase for both EAC and granular as humidity increases?

And while I understand the care taken in your study to islolate the various factors in WOB and the modular nature of the test protocol, I can't help but wonder if you really could just change the maths and plug in a radial. If I'm understanding the basic factors/maths, we would need to know the length of the grain boundry, the overall volume of the scrubber and then factor in the gas flow path, which will be dependent on the proportions. Doesn't seem so simple to me.

And back to Tim's 4 cent analogy, if a same sized radial starts out with a lower WOB to begin with, is that 4 fold increase in WOB due to moisture really that noticeable? To make it more complicated, in my experience with the Prism in-out flow, the sorb clumps up around the inlet tube, roughly evenly top to bottom, which might suggest that exhale resistance could increase a bit and the inhale side might not. Then we have the gas tending to take the path of least resistance and moving around the clumped sorb, which given shorter grain boundry of a radial, would make any WOB increase even less. I myself have not noticed any increase in WOB on my radial until the very end of a long push, but by way of full disclosure, I have been accused of being insensitive by several girlfriends.


EACs look like an attractive alternative to a conventional axial for several reasons, including your findings above relating to the stable WOB throughout the duration. But I doubt that an EAC which takes up the same volume as a properly propotioned, well insulated radial could match it's duration. Then we have the sorb vs. EAC portability issue, the final deal breaker for me... -Andy

Sorry to throw all this at you in one post.... Thanks, -Andy

n2diving

It's a difficult situation... publish very detailed technical data that takes a fair amount of time for someone to plow through and interpret, thus limiting the audience to those people with the aptitude to do so.... or publish broad generalizations that are easy to understand but tend to sweep aside the details and special cases...

Stepping back and looking at the big picture, it seems that Alex has said WOB characteristics for granular and EAC are different when the absorbent gets wet as a result of floods. The EAC will have signifcantly better WOB than granular. How much better? Significantly better. That seems clear and accurate enough for my purpose, which is to go diving with my CCR.

Alex has further asserted that WOB on granular increases over time with use and moisture is released by the normal chemical reaction. He's produced the data to support this assertion. How much is the increase in WOB? Significant increase.

So practically, what does this mean? The EAC outperforms granular when wet, well we already knew that although Alex's data provides more documentation. The way granular divers deal with WOB issues is how it's always been done, don't exceed recommended durations and use fresh absorbent before deep dives. As been pointed out many times, there are more reasons than just CO2 absorbent capacity to change granular.

AD_ward9

The resistance is a function of the cubic resistance of the material, because flow through granules is very turbulent.

This means that one can take a reading on any axial scrubber and calculate the resistance per cubic cm of granules, call that Z. The resistance for another axial under the same conditions is the Z*L/2.pi.r^2.
r is half the diameter, and L is the length.

For a radial, assume the flow is from the centre to the outside. The resistance is the sum of the resistance of tubes, with wall 1cm thick in this case, flattened out. The tube circumference increases as 2.pi.D, so if the depth of the radiual is L, then the resistance of the first tube is Z/2.pi.D.L.
Note as there is no squared terms, the resistance of the whole radial scrubber is half the bed depth, times Z/2.pi.D.L where D is the the diameter of the point at which half the bed depth occurs.

Of course, pore size of the container etc makes a bit of a difference, but compared to the scrubber granule resistance, not much.

Note the same sums do not work for EACs because flow resistance through an orifice and through a tube is the same, until the tube is more than 40 times the orifice diameter (at which point the resistance of the tube is 10% more than the orifice). The difference in handling the two cases is that flow through the EAC is laminar whereas through granules it is turbulent.

Alex

AD_ward9

EN14143 measures hydrostatic pressure relative to the suprasternal notch (a point on your skin where your two top ribs join in the middle). This is wrong.

NORSOK U101, on which a lot of EN14143 is based, uses the correct measurement: hydrostatic pressure is measured relative to the lung centroid.

The result is that EN14143 is highly biased towards OTS CLs, for no valid reason. It also is highly penalising for BMCLs. With a hydrostatic measurement relative to the suprasternal notch, the centroid of an OTS CL can be the suprasternal notch, so there is no WOB penalty from diver position. In reality, OTS CLs do have a WOB penalty because their centroid is not the lung centroid.
There are conversion tools on the web.

Pascals are pressure. Joules are energy. mbars are pressure. ATM is 1.013 bars. PPO2 for example should be in Pa or ATM, but often quoted in bars without correcting for the difference.
It is not 100% because there is condensation in the counterlungs etc, and adding gas, even O2, dries the atmosphere because O2 has no water in it.
I just posted an answer to Dr Mike on the same topic. Using the method described is a very good first approximation (i.e. to within 10% of the right number). Measuring it has an error also (around 10% when all is done, due to differences in temperature, humidity, hose routing, hose diameter, turbulence at the base of the scrubber etc).
No, it is not noticeable. It just kills you.

That is the nub of the issue. WOB is not measured at all well by human lungs. The first thing the diver knows is when they start to get a CO2 hit. That can be too late.
See earlier I posted figures showing over 5 hours from an EAC under EN14143 conditions. That would be quite a challenge for any granular system.

Cheers,
Alex