Linearity check

jneves

Hi there, O2ptima addicts!
I saw the post of narked @ 90 today on old O2 cells needed for testing a new linearity check device they’re coming up with… Well, before it’s too late (I hate it when you have an idea and sit on it just to find out sometime later that someone released it before you did) I’d like to say that such a device already exists for us O2ptima divers but it is not yet commercially available . I set out to do a prototype a few months ago and have now reached the pre-final version. The idea was to prove the point and eventually tease the people that should and have the means to produce it (apparently that’s working too…and I will say no more…)
So here is how it works (and it does work!!!):
It is a Delrin lid for the O2ptima head. It serves several purposes.

• It protects the insides of the head when travelling
• It allows you to calibrate the cells without undue waste of O2 (you don’t need to flush the whole loop several times)
• It allows to pressurize above 1 ATA absolute so you can check linearity up to 1.8 or even 2 ATA!
• In doing so you can check all alarms are functioning properly ( DIVA vibration and LEDs as well as flashing handsets)

So now you've seen it!

Dive safe!

J Neves

Attached Images

Head lid.JPG (31.5 KB, 263 views)

Decodiver

Easy, Tiger!

John Routley and Brent have been working on their cell checker for a very long time.

I am sure John will come along himself and tell you how long, but I guess, a bit longer than you have been with your piece of kit for the O2ptima.

Now, that said, your device looks like a good piece of kit for the O2ptima user and of course has the benefits of a head-only calibration kit & as you can pressure it, it will give you checks on all your alarms, so well done.

Just remember you started a few months ago, well, in March, John showed me his finished product, which was just awaiting extensive testing.

Cheers,

Dave Cooper.

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jneves

Dave, I’m not trying to steal anyone’s work… or ideas… And thanks for your kind words. I am sure that John Routley and Brent are doing something that eventually is not unit specific (actually I don’t have a clue what they are doing obviously) while what I have done is specific for the O2ptima. Having said that, I am sure you know the idea is as old as the use of galvanic cells on CCR’s itself and countless have thought about it… and many attempts have been made to come up with something that works. So I can’t possibly claim to be the first. I first thought about it when I got my first Dolphin back in 1997 and did try to pot Dräger’s Oxygauge cell connector with good results (having just one cell not knowing if it is still good for your next dive is just not reassuring). When the O2ptima was launched I immediately decided that I would do something similar. I just never actually got to hands on development until December last year.

Cheers

J Neves

AD_ward9

The cell checker is a good idea, if you can control things carefully, particularly the rate you decompress the O2 cells, otherwise you damage them. The temperature, humitidy and rate of pressurisation is relevant.

A very interesting related fact, one of our engineers came across while trying to fathom why our PPO2 readings were not what he expected in air, was that the % O2 in air varies by about 20%. I kid you not.

In an article on LINK to metrology centre.net the author presents her research (1965-1985) on this theme. The partial density of O2 in air at sea level changes by 20% over the course of the year due to changes of temperature, humidity and atmospherical pressure. The figure below, taken from http://meteocenter.net/meteolib/o2_3.gif shows the change of O2 partial density (i.e. the parameter which an O2 sensor 'measures') through year (at January, April, August and October).

This may explain why some people with a reputation for doing things properly, such as John, have taken their time to produce an O2 cell calibrator.

The bottom line is that if you use a cell calibrator blast the cell up to an unknown temperature, at lowish but unknown humidity, at not accurately known pressure, you can get quite spectacular errors. It is better to understand what causes the change in O2 density, such as pressure, temperature, humidity, then measure all of those, and then calibrate the O2 sensor in air after compensating properly for those parameters. I do not suggest the compensation is simple: you need a computer at a practical level if you want better than 1% accuracy. After the cells are calibrated in air, one can check for ceiling errors by either applying a pressure test just to check the output of the O2 cells does get high enough, or doing some basic maths while it is being dived to work out if it has hit a ceiling.

Cheers,

Alex

NB: Thanks to Misha Soloviev for correlating the random changes in O2 sensors to something a lot more accurate than the phase of the moon.

NB2: There are translation sites that can translate the pages I referenced free of charge.

Attached Images

o2_3.gif (6.7 KB, 215 views)

SimonK

Hence the little card that comes with the analox O2 analiser listing expected PPO2 at different humidities and temperatures???

Simon

narked at 90

http://www.rebreatherworld.com/inspi...cker#post42352

Commercially completed version was available back then. Model specific to the inspo.
This is not yet dead, another unit specific model will be launched soon. It's likely to be first available at the NEC dive show along side something else equally as special as the below product.

Universal unit here -> PRODUCT
The universal unit above will be available in three variants;
a) Stand-alone basic unit
b) USB interfaced analytical logging. (95, XP and Vista software)
c) Combined Stand alone with USB output.

Isnt it nice to tease!


B & J.

Because we've lost too many friends already.

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devilgas

How does this lid attached to the extend air cannister. I see 2 latches on your lid. There is no latch, so ?

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