Cis Lunar Mk 6

JonnyB

Hi Rich, thanks for all your responses.
One other thing on the same matter i would like to ask you, is how do you handle diving at altitude. Because if you would to handle that then you would be able to solve the inpure oxygen pretty much the same matter.

/Jonny

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dave t

Rich

I think I should say my original intention of bringing up the o2 variable thing was not to highlight the fact that it exists or the fact that some divers use variable percentages in their rebreathers. It was to highlight the fact that the white paper (or maybe it was the advertising blurb) says and I quote "forget about po2" Indicating (to me at least)the knowledge level of the divers needs only to be at basic sports level yet here we are already with a need for the "sports" diver to understand how to analyze and interpret the various processes of determining the o2 content of his rebreather cylinder and to further understand the reason for inputing the said result into the rebreather electronics and the impact it will have on his breathing gas. Or at the very least refuse to use the offered gas. I guess I am struggling a bit to understand exactly what the skill level needed is.

sorry I said I didnt have any more questions )

Dave

Ben Field

I was being nice! "You don't wanna see me when I'm angry" (Sorry, promised never to use movie quotes- damn!)

Yup, its pretty much always good here to (UK) but I would always check it (with an analyzer)

What surprised me was not your not knowing it but it not being on the list of factors for you to consider from the illustrious divers you listed, maybe I sounded alittle harsh but that was truly how shocked I was, its rather basic and has to make you (as the tech/R&D guy) question the other data you may or may not be lacking? Simply things that make a big difference are the worst-
Does your depth sensor recalibrate for Fresh and salt water, can it be set to different salinity?
What extremes of temprature can the system endure for both storage and use? (Can you remove it from an overnight stay in a frozen car boot and just turn it on and dive?)
Can the batteries and sensors be changed on a dive boat without removing little screws (that would suffer massive "fridge suck")
Can the unit cope with Dil than is "alittle" off air (21%) due to being pumped at a different temp/humidity/elevation etc?

(Just trying to list a few typical issues that revolve around the gas quality side of rebreathers- things which a manual rebreather diver can see and make decisions on but the Automatic rebreather has to cope with itself)

I'm really not "having a pop" at you or the Mk6, I'm (like others here) struggling with the marketting but the unit is without doubt a revolution in many ways and I am very interested in the tech side.

Unfortunatly as you are probably aware it is very difficult "online" to at first detect the difference between and an Expert and someone with access to Wikipedia, both informed.... but with a different quality and understanding of the data

I'd like to think the actual Rebreather divers aren't as illformed as that but I will never cease to be amazed at what "qualified" people do!

I hope the many wiser and more knowledgable people than I can help you, I'd like to think Rebreather World as a group is a usefull resource for people like yourselves, as long as you have someone sanity check whats on here!

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silent running

Hi Rich, as I dive a Prism which has a max electronic SP of 1.4, once the O2 gets below about 94% the unit will not be able to electronically maintain an SP of more than 1.3. So, if want to get a 1.3 SP with 94%, I put the SP at 1.4 to get a real PO2 of 1.3.

As for doing the electronics cal with the 94%, I cal the unit as normal, as if it's seeing 100%. Then I adjust the analog 2ndry calibration to reflect the real value of .94%. This way, the unit's electronics will maintain an actuall PO2 of 1.3, even though the electronics think it's holding 1.4, while the 2ndry will show the real PO2. The HUD will tell me if the SP is holding, as it is designed to do, but the actual PO2 will be seen on the 2ndry, as it is designed to do.

I can always check that it's 94% by looking at the 2ndry sensor readings in air-do they come back to .21? And obviously, one can always add more O2 with the manual add button to bring the PO2 up to say 1.6 on deco at 6M by monitoring the 2dry.

If the O2 is 88%, then the max SP the unit could maintain electronically would be 1.2 with the 1.4 setting.

Obviously, given the nature of the 2ndry needle gage, all this is not so exact and as we get lower and lower O2%, we must worry about what gas is making up the remaining %. And thankfully, so far I haven't seen less than 95% in the South Pacific... -Andy

Tino de Rijk

Nicolai,

Let’s not fight about definitions. “Calibration” and “validation” are quite broad terms, applicable to many applications and industries.
I just used a liberal interpretation of the terms to fit the difference between calibration in a CCR and the new way of dealing with cell behavior as used my Poseidon which I freely dubbed “validation”.
I have no desire to be scientifically of semantically correct.

As to your four scenario’s: the first three are clear and obvious and are indeed used as such in the MK-VI as far as we can tell from the white paper and Rich’ and Nigel’s explanations.

The fourth one (mixed mode, both solenoids active, creating a predetermined reference mix) however is not simple at all to pull off. The main point is that for the first three (only one solenoid on), the injection does not have to be very precise.
As long as the solenoid opens long enough to fully flush the little calibration chamber of the 1st cell within the MK-VI, it is fine, and you have a KNOWN gas to use as a reference, if combined with the ambient pressure coming from the pressure transducer, which these days are very accurate.

However, to get a reference MIX of two gasses is a whole different ballgame.
Now also both solenoids need to be calibrated, i.e. need to inject a very precise and very small amount of gas in order to get an accurate mix to calibrate (not validate) against. This effectively means it will have to have a very accurate flowrate, regardless of temperature, ambient pressure, intermediate pressure etc. If a solenoid is even just a very little bit off, you get a different mix than expected in the calibration chamber.

It would in fact have to be far more accurate than the mass flow orifices in MCCR’s, where it really doesn’t matter if it injects 0.50 or 0.52 or 0.48 liter per minute: acceptable margins are quite wide.

The only way known to me to do that is by measuring it against a flowmeter, which you can not easily do while diving.
In order to do this in a reliable way, you would have to have an in-dive way of constantly checking the precise flowrate of the solenoid – yet another level of calibration and validation, this time for the solenoids, to add to the equation and complexity.

So, for all practical reasons, I don’t see this as an easy thing to pull off, unless I’m overlooking something obvious and/or the Poseidon boys have come up with something revolutionary to do that.

Ciao,

Tino.

Tino de Rijk

Rich,

First of all it is usually not by choice we use low-% O2. It just happens to be of lower purity in many parts of the world, and then there is usually also no other choice locally.
Especially with the rise of PSA (Pressure Swing Adsorption) systems as a cheap way of making O2, you typically end up with purity in the low nineties, with the remainder being N2 (no problem) and Argon (a bit of a problem). PSA systems typically produce up to 95% O2 at best.

However, you are also right in that the effect of it in a CCR is not that high, as long as the impurity is in the low to high ninety %.
This is because the impure O2 is mixed with usually pure air (in terms of the O2 % being around 21% that is, not in terms of pollution).

I made a small spreadsheet to illustrate that, for whoever is interested.
In fact, the deeper you go, the less impact the impurity has.
With an O2 impurity of 94% at 20 meters depth it means that with an expected setpoint of 1.3 you in fact breathe gas with a PO2 setpoint of 1.26: so quite close indeed.
At 30 meters this is 1.27 and at 40 meters 1.29. This is logical, as a relative bigger part of the breathed mix exists of diluent at these bigger depths.
And with a setpoint of 0.7 at 6 meters you in fact breath gas with a PO2 of 0.86 instead if the assumed 0.70.
I think most MCCR divers would kill for keeping such a close match to actual intended PO2 setpoint...

As a result, the risk of hyperoxia is in fact diminished; however the risk of DCS increases somewhat due to the higher fraction of N2 than anticipated on by the DCAP algorithm.
This is however also not very big, and might in fact fall in the conservatism level that is no doubt already applied.
After all, this is a recreational model, so the deco-algorithm needs to cater for old, fat, no-condition, beer-drinking, sea-sick divers on medication – guys like me, as to speak… J (Rich, you’re also not getting younger….J).

For those interested: I attached a little Excell spreadsheet to play with. It is ZIP'ed, as Rebreather World does not accept Excell-type attachments.

Ciao,

Tino.

Attached Files

impact of O2 impurity on PO2.zip (3.9 KB, 27 views)

deepreef

Hi Jonny,

First, to answer your question: Same way as any modern dive computer: ambient pressure at surface.

But again, the question doesn't seem to be the *discovery* of having non-pure oxygen, so much as the feature to allow diving in spite of it.

There are several ways it can be discovered when the diver doesn't already know about it; but the question is, what are the maximum threshold tolerances (in this case, of oxygen impurity) that are allowed (either in terms of an acceptable level of skewed sensor calibration, or in terms of having the diver explicitly tell the electronics what the FO2 of the "oxygen" is) before the system allows the diver to continue of the dive.

Aloha,
Rich

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deepreef

I think that was in the brochure (definitely not the whitepaper). And yes, I cringed when I read that for the first time also (at DEMA). But in Poseidon's defense, I think the point (maybe a bit lost in translation??) was that the system design is intended to be more reliable than a "well-trained" rebreather diver, and as such it may not be as important that the diver monitors the PO2, as much as it is important that the diver reliably responds to an abort signal correctly.

There are two basic ways the system can fail:

1) It doesn't send the diver to the surface on OC when it should;

2) It does send the diver to the surface on OC when it should not.

From my perspective, #1 will kill divers, and #2 will simply annoy divers (thereby hurting the reputation of the rebreather). Thus, #1 is by far the more important of the two.

Next, we have to determine how rarely the system fails #1, compared to how rarely a well-trained diver fails #1. Obviously, a well-trained CCR diver will succumb to failure #1 less often/less likely than a poorly-trained CCR diver will. But if the system matches or exceeds the reliability of the well-trained CCR diver, then the playing field is leveled.

Of course, there are a MILLION ways the diver can fail, even when the system doesn't (i.e., all the alarms work as they should, but the diver fails to respond to them). But the same is true for OC scuba, driving on the freeway, and just about any other activity any of us ever does.


Not true at all -- the diver only needs to know this stuff if the diver wants to *intentionally* dive with oxygen that is not within tolerances of proximity to 100%. The basic sports level diver will simply get a "Do Not Dive" alarm, and will be pissed off, and will send Poseidon a nasty letter -- but will at least be alive.

Now, it may well be that there are parts of the world where sufficiently pure O2 is simply not available -- in which case the diver cannot dive. Period. (At least not safely). But this is true of ALL rebreathers -- the difference is that the MK-VI is capable of *determining* that the O2 is out of tolerance, and is thus capable of alerting an otherwise naive diver of the problem.

Aloha,
Rich

P.S. Back to work for now -- more later....

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Database Coordinator for Natural Sciences
and Associate Zoologist in Ichthyology
Department of Natural Sciences, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
email: deepreef@bishopmuseum.org
http://hbs.bishopmuseum.org/staff/pylerichard.html

dave t

Ok excellent explanation Rich (and it makes sense as well ) except, and I agree about the million ways a rebreather diver can fail but under the current thinking at least he was taught (we hope) the escape routes in the first place and the rebreather is configured to allow the escape techniques. Under the Mk6 regime he is not taught them as the rebreather will not support them. I understand that for some the only way to dive a rebreather safely is not go in the water but there is a large proportion of Rebreather diver present and future who can respond very well to warnings and ways of getting out the sh1t without losing their diving and making the best of what rebreather diving can offer

As for OC divers we can say "if your mouth is full of water its not working" but as we all know its much more complicated than that with rebreathers.

Dave

Hanssing

Agreed, however it was my understanding that the "flushes", are continous trickle-flows while the validation is ongoing?
Otherwise the breathing turbulence would dilute the chamber?


If this stategy is to be used, the system could get the relative flows at surface, and with compensated 1st.-stages the flows would maintain the relative sizes, or perhaps at least a estimated flow based upon absolute pressure. SO I dont think it has to be acurate, if the flwo is meassured in known conditions (ie. at surface with validated cells). Perhaps this idea is not used, and perhaps it not realistic to implement....
But *IF* the flows are trickling, then you would see a mixed steady-state cell response to both flows active. If the gas-composition is known, then you could have a 3rd. gas-mix to use...

HOwever I'm guilty of only glancing over the whitepaper, and thinking, cool they haev 3 gasses to test with....
I'll read the paper agin, and probably denounce myself an idiot

Update:

So clearly It's not a flow, but a specific volume. It still "could" work. But I dont think so.
The paper only descibes dil and o2 burst. The added complesity only to check for currentlimiting - well go figure.
I'm stoopid - But I declare my novel idea mine, and free for all to use....


Nicolai

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