ISC posts test data

silent running

OK Alex, thanks in advance for digging up the 1994 NEDU document.

But there's some things I'm not sure I understand:

Whether testing for procurement or safety, the point of any CCR testing is a unit's suitability for use by a diver, which I would think implicitly includes safety. Why wouldn't the NEDU tests seek to emulate the actual WOB for the diver? Wouldn't any test that left off that .6 J/L pre-load internal human WOB be less accurate, less valid and thus less safe? It seems odd that the NEDU, which requires extensive manned testing, and in the Prism's case even included underwater stationary bike stress tests, would intentionally leave off that .6 J/L and thus use a lower, less realistic standard for a machine test.

Can we be sure that the 1994 NEDU breathing machine test protocol is the same as that used in the 2001 Prism NEDU tests? I seem to recall reading somewhere that these protocols had changed btw 94-01, I'll check.

Is the breathing machine used by the NEDU different in some crucial way from the one used for CE and not capable of being setup to pre-load the .6 J/L in the same way as the CE machine?

And which body position are those CE WOB chart figures for various RBs put out by CCR? Or is it some kind of averaged body position? -Andy

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NEDU test methods are described in NEDU TM 01-94, copy of which is on the Rubicon web site.

The language is quite involved, and spread out in multiple sections all of which have to be read, but it is principally the Chrome T used to tap the pressure, with a reference for hydrostatic purposes of the suprasternal notch or lung centroid depending on the test.

The fact it is additional resistive effort is clear from the calibration Lissajous, Figures 4-1, 4-2 and the surrounding calibration information, confirmed by statements in Sections 3 and 5 of the document. This emphasises again, why I always want to see the calibration Lissajou when anyone publishes any figures.

The reason Fig 4-4 is non-symmetric, or skewed, is not because of the elasticity of the hoses to the breathing machine as the report suggests, but a simple consequence of sine flow. Sine flow does not produce sine pressure, at all. If they run a sine pressure pattern, the Lissajou would have looked perfect.

I will post tomorrow a plot showing the WOB of a human for 40l/min RMV and 75l/min RMV against depth, so you can see the effect very clearly. It is late here now, and I am at home, so can't attach it immediately, but we have the curves in the office based on measurements we did.

Alex

silent running

Thanks again Alex for taking the time to try and explain things to a lame laymen like myself. -Andy

gtzavelas

Jakub, please let us know when will this be available - and the cost.
i assume we will have to send you back the old BOV to retrofit them, right?

jradomski

When I last spoke to jacob, they should be user replaceable.. Just purchase the new valves and remove the old ones..

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caveseeker7

So ... any word on tests and results of the Mini Meg?

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Free Whisky offer!

My colleagues and I have discussed publishing the WOB figures for the Inspo, Meg and Boris along with DLs figures.

The only thing we are missing is a Boris. If anyone has one in the UK that they can lend us for 2 to 3 weeks, please PM me. We will arrange for a courier to pick it up, I will take it personally to our test facility, and ensure it gets back to you in one piece, with some decent Whisky. Just let me know if you like Islay or Highland, or perhaps Armanac from Russia.

CCRB are very decent, I am sure is interested in honest figures under directly comparable conditions, so may well provide one, but if you can beat Kevin to an offer, then there is some whisky about ....

I should explain what these WOB numbers mean so you can understand why I am a sceptic until I see things in real life.

WOB is the area enclosed in the breathing resistance Lissajou curve measured in Joules per Litre (J/L).

A Joule is a unit of energy. As the X axis of the Lissajou is the breathing machine displacement in litres, then you can convert a J/L figure into an average breathing resistance figure by multiplying it by 10 and divide by 2 (because there is exhale and inhale). The reason for the figure 10 is Pascals into bar is 9.81 because it is related by the coefficient of gravity (from Newtons per metre), in case you wondered ... so I will use 10 rather than 9.81 because it is easier in the mind.

For example, a 3 J/L WOB means the average resistance is 30mbar including inhale and exhale, which means the average resistance is +/-15mbar. To take a second example, a 1 J/L WOB means then the average breathing resistance is 10mbar, i.e. +/-5mbar.

If the WOB is measured with a sine wave pressure flow, then the peak resistance is 1.57 times the average figure.

Take an example. The Boris claims 1.6 J/L WOB at 40msw, with an RMV (amount of gas you breathe per minute) of 75 litres. This means the peak resistance must be 1.6 * 10 / 2, i.e. 8mbar, and the peak is 8mbar * 1.57, or 12.3.

Plot WOB against depth such as in the example below, and you get a curve that looks like WOB = a *msw + b, i.e. it is straight-ish line that has a certain WOB at 0msw, such as 0.8 J/L, then increases linearly with depth.

This means that if you take the WOB figure claimed for 40msw figure and divide by 4, then the real WOB will be always be higher than that on the surface. The Boris figure of 1.6 J/L corresponds to a mean breathing resistance of 12.3 mbar, and 12.3 /4 is 3mbar. The windpipe of a person on the surface has a resistance of 0.6mbar (i.e. a WOB of 0.12 J/L). So the rebreather is claimed to have less than 2.4mbar resistance on the surface.

Just an extremely floppy bag with a 36mm tube going into it, inflated and deflated in the water produces 2mbar, and if the bag have covers this can increase to 7mbar. The counterlungs are the biggest contributes to WOB. These figures assume there is no hydrostatic imbalance, using an optimal loop volume is optimal.

You can now see why any figure below 2.1 J/L at 40msw, for an RMV of 75l/min, needs independent verification. Boris figure stands out as needing an independent check.

Moving to EN14143:2003, this specifies a maximum Work Of Breathing (WOB) limit of 2.75 J/L (Joules per Litre) at 75lpm RMV, 3 litre tidal volume, 40msw depth (actually, 5 bar absolute). This means the average resistance allowed is 13.75mbar (i.e. 2.75 * 10 / 2), or a peak with sine pressure of 21.6mbar.

There is a difference between sine pressure and sine flow. A sine flow of 1mbar, is a sine pressure of 1.2mbar. EN14143 states sine flow, which means the Lissajou is skewed, and takes slightly less area than for sine pressure. The figures above are for sine pressure, because it is a resistance being measured. The difference between the two, give a 20% extra headroom to the various figures quoted above: I described in very simple maths so it could be followed. The bottom line is, do the very simple maths, then add in 20%.

We are pretty sure the DL O.R. unit has a WOB which is not worse than the Boris: in fact, it must be better, as we have a 36mm bore to all tubes, huge floppy mushrooms, two extremely flexible single layer counterlungs very close to the perfect theoretical form with lowest elastance, and use EACs which have a third the resistance of a 36mm hole. We have measured many rebreathers, and never seen any come close to the DL figure: the DL unit has the lowest WOB by far to anything we have ever measured. The figures are below, for the diver vertical position, in air.

So let us continue this by taking the Boris and we measure it under these same EN14143 conditions. We have figures here for the Inspo, Meg and Dolphin, and will publish those at the same time. So, just need a Boris for 3 weeks. PM me please ..

To keep things absolutely fair, we offer the same for anyone who claims a figure underneath the curve below. Our commercial rebreather manages less than this curve, but normal sports units will have a hard time. I look forward to being proved wrong (again, being wrong is more educational than being right IMHO ).

Attached Images

DL Sport Rebreather WOB in air.png (24.6 KB, 142 views)

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I promised to expand on the difference between NEDU and EN14143 WOB figures.

The attached file shows the calibration of a breathing simulator, with an open end (i.e. the self work of breathing, intended to emulate a human physiology), and the various Lissajous one gets using the EN14143 and the NEDU TM N01-94 calibration orifices.

For simplicity, all Lissajous go the same way around in the attached doc.

You can compare the Lissajou figures with those in the NEDU TM N01-94 document, available from the Rubicon site. You will notice the EN14143 figure is pretty well spot on, but the plots of the NEDU figures are out by the WOB of the breathing machine for the higher pressures (though oddly, the very lowest RMVs they include the human WOB, probably because the tubes into their mannequin are long enough to come to same sort of number). This confirms what comes out when one studies the text very carefully, that the US Navy measure the WOB of the equipment itself, but EN14143 measures the WOB of a human using the equipment. NEDU have then come out with new proposals to reduce the WOB limits with depth, because humans can withstand less external loading at depth: obviously, they have their own WOB to deal with as well. If you add up the diver's own WOB to the WOB of the equipment under test, you find the new NEDU proposals for rebreather WOB and the existing EN14143 are much the same thing.

So, be careful about comparing US WOB figures to European figures. This is why I think publishing the results of the main rebreathers measured both the European and US ways, with all calibration data so you can see the US result as well, is the proper way to do things.

Alex

Attached Images

BM_calibration_080411.pdf (90.1 KB, 18 views)

paulraymaekers

hello Alex, can you explain me in a few words why in some tests you take into account the WOB of the machine, when the pressure/volume curve is taken right at the input of the mouthpiece?
I assume if you mesure the pressure diff exactly at the mouthpiece, then the machine does not play any role?

regards
paul

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In EN14143:2003 it states clearly in Section 6.3.2 that WOB and respiratory pressure has to be measured against the displaced volume, i.e. the piston in the breathing machine. That is one point is in the mouthpiece and the other the end of the piston volume inside the breathing machine. This correlates completely with the EN14143 calibration Lissajou, which gives 25mbar positive pressure under those circumstances, when the dimensions of the breathing machine match a human (dead volume, internal resistance etc).

In contrast, NEDU measure from the mouthpiece to a mannequin volume, and the breathing machine is remote. The whole into a hollow mannequin from a mouthpiece T is not at all like a person, more like a fixed reference volume. If you look at Fig 4-6 for example in NEDU Technical Manual 01-94, it shows the reference point clearly simply as a box in the water, not connected to the breathing simulator. The result is that at high flow rates, NEDU measure the equipment, whereas EN14143 measure the equipment as it would be from the lung of a human: much better, but much stricter if done properly.

Alex