1 point (high PO2) versus 2 point calibration

Gilles

Following a discussion and having learned something new, I thought I could suggest an alternative controller design that might be superior to the 2 point controllers that are out there.

A single point calibration at PO2 = 1 (or thereabouts) would be superior to the current 2 point cal systems because;

1) after setting the cal, one need to only let the cell relax in air (or dil) and see that it goes back near enough to .21 (or dil O2) to know that the cell (and O2 %) is ok.

2) less prone to error. Cell non-linearity (or O2 % errors) is hidden in 2 point cals, unless one really understands the relationship between cell mV and PO2 and you do lots of data logging and calculations (or mV tables).

3) would/should be easy to design an option to in-water cal at 3-6m at your PO2 of choice. Best a value somewhere above your typical bottom PO2

The only advantage a 2-point cal has is that it slightly mitigates against cell non-linearity versus the above mentioned 1 point cal. That mitigation however is lost from the extra susceptibility to error.

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Gilles
http://www.dirrebreather.com

paulraymaekers

just my 2c..

2 point calibration: a solution for a problem that is not there....

2 point calibration is needed when you have lineair behaviour and an offset

cells are quite lineair between 21% and 100% O2, and have nearly no offset!

so you do a 1 point calibration and check that calibration on another known point

regards
paul

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

heyydude

I concur with Paul. 2 Point Cal is unnecessary.

If your sensors are not linear out to 3 ATA with Zero = Zero, then you need to switch sensors or suppliers. We test the HH's to 3 ATA of O2 as part of our basic QC prior to shipping.

Kevin Juergensen
Juergensen Marine, Inc.

UWSojourner

I agee although, theoretically, it could correct for a system problem that should not be there.

I prefer 1-point-any-O2-content to 2-point calibration.

divetheworld

What he said.


Brent

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Gilles

Phew, good to see that I'm not the only one who thinks so.

So, are you guys building systems that utilize 1 point cals?

and

Can you set these so you can cal at any PO2 desired (including above 1)?

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Gilles
http://www.dirrebreather.com

jkaterenchuk

Gilles

The HH (used on the Optima, HH CCR and as a add-on) is one point and also the Inspiration Classic. I suspect the other AP CCR's but I do not own them so I am not sure.

John

Hanssing

The rEvo-dreams only calibrates at 100%.
Allthough not specifically stated in the manual, I allways calibrate / check calibration in 100% during assembly, and when assembly is done, I remove exhale-hose (P-connector - very easy), and breathe air into loop. Assembly is then done when all 4 cells steadily reads 0.21 -> Ie. I verified that my calibration makes sense.

This allso ensures auto-shutdown of the rEvo-dreams, after 15mins, and generally is better than leaving the system at 100% wich will shorten the cell-life.

Now if I were to use say a 95% gas, I would do the same, but exspect it to read 0.208*100/95 = 0.22. While diving I'd set the deco to run at a factor 95/100 lower PPO then read on the displays. No biggie.

Nicolai

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Gilles

Kinda boring when everyone is in agreement.

Perhaps I can stir some fury in the Meg forum.


I would like a single point cal system, that has options allowing in water PO2 cals up to 1.6 ATA.

I have a 2-point system and I find that it sucks. Have to be meticulous with the mV readings to check cells.

Dive safe

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Gilles
http://www.dirrebreather.com

Hanssing

Why?

At surface you have near constants pressure. You dont have your lungs on/offgassing into the loop. It would make no sense doing something as complicated as calibrating (with precision), in water, on existing rebreathers. Only when designed for it it "perhaps" makes sense. Ie. the 'POS Mk VI', where the loop is not flushed only the cell-chamber.

If calibrated at 100%@surface youre calibrating at 77% of normal scale (1.3). At the beginning of the dive you have the option to do yet another verification-point, diving pure O2 at 6m. before descending - I know zerogravity diving is advocating this method.

To summarize:

• Singlepoint calibration is sufficient, since the cells are linear nearly without zero-offset.
• Verification of cellcalibration is possible directly in air before dive.
• Immediately at the beginning of dive, verification of linarity to 1.6bar with pure O2 is a recommended practise by some. Primarily to detect currentlimited cells.
• During dive, full-dillflush if you want to verify readings.
• During deco at 6msw, O2-flush again to verify linarity to high readings.
• Furter cell maintenance includes a cell-chekker for detailed analysis on land.

CELLS ARE CRAP - by far the part in my rebreather I trust the least, besides my own abilitites

Still cant see why you want calibration during the dive - if so, then sign up for a PoS Mark VI

Regards
Nicolai

__________________
Woohooo - I can change my rEvo!
Its going to be bitchin' tricked out piece of gear