Oxygen Analysis (Nitrox)

[JimC]

sheck33 once bubbled...



That a gas mix will never settle out like a mudd/water mix is true but gasses WILL mix themselves due to the Brownian motion of the gas molecules. It will just take a while depending on the gasses involved.

Darn commas. ut: Let me rephrase, since we said the same thing:

Gasses will never settle out.
Gesses will mix them selves.

And it dousn't take very long...

Unless your mixing cold.. real cold...
Like -186C cold...

 

[DivePartner1]

NetDoc once bubbled...
If yu calibrated on room tamperature air (most likely) and then measured hot air from a newly filled tank, the measurement variation could be significant. Redo the analysis.

I think MikeF answered this but can this variation be more signficant that a percentage point or so?

 

[JDostal]

I don't remember the brand of the o2 analyzer that my friend bought - I just use his as we always dive together...

I know it's fairly cheap to build one yourself. You pretty much just need to get the o2 sensor (the expensive part) and then just wire a voltmeter together....i've found the plans on the net, but don't have the links handy anymore.

 

[The Iceni]

Omicron once bubbled...
I don't remember the brand of the o2 analyzer that my friend bought - I just use his as we always dive together....

Most UK shops and divers mix by partial pressures because we like to adjust our mixes for the proposed depth ,although 32% is the most frequent mix I have seen used. Most UK Nitrox divers tend to have their own analizers. I have an Oxyspy but wish I'd waited until Analox started to market their cute piece of kit the O2 EII.

See

http://www.amoxtec.com/site/content_o2e2.php

http://www.analox.net/pages/pdf/Mini O2DII.pdf

As for my Oxyspy;- a bit bulky

http://www.abysmal.com/pages/images/OxySpy-Eng.pdf

As for the mix varying after a fill, I have never seen it vary sigificantly after a proper analysis immediately after a PP fill. I suppose it could happen.

 

[Genesis]

Did the guy doing the analysis at the shop blow off a bit of gas from the valve before doing the analysis? If so, and if he allowed the tanks to cool (or filled slowly in a water bath) then the analysis is probably ok.

It is possible to trap a (very small) amount of HP gas in the valve when it is shut off and the whip detached. If you hook the analyzer up directly (and immediately) you can get a bad number, because the gas in the valve is confined (and 3000 psi to 14.7psi allows a LOT of expansion of that "small" amount of gas!)

Also, as was pointed out, the sensors are heat sensitive (they're also PRESSURE sensitive - they're basically a small fuel cell)

In any event a 90' profile should be fine if you got a cut mix from 36%; you can go to 94' on 36% and not violate a PO2 of 1.4. I'd just stay out of the corner on the NDL, which is a good idea anyway.

BTW, if you want my opinion on analyzers, build one. There's a kit called the "YDI" one that I built, and it calibrated within 0.3% on pure O2 when I got done putting it together. That's better than the one in the LDS that I frequent (an expensive - commercial - Minox unit!)

Here's a link - $90 and an hour of your time will put you in business (plus whatever fittings you want to use for sampling)

http://www.oxygenanalyzer.com/YOUDOIT.htm

 

[The Iceni]

Genesis once bubbled...
Did the guy doing the analysis at the shop blow off a bit of gas from the valve before doing the analysis?

I'm "that guy"!

Ah, Caveat emptor! We have to sign "the book" to confirm that we have analized the mix and we know the MOD.

I always analize from the cylinder and blow off a certain amount of gas (but not too much!) so there is no risk whatsoever of whip "contamination".

As for building one's own DIY meters, my time is worth more than £100 per hour!

 

[Genesis]

but the shop does the analysis - in front of you.

I typically check it anyway when I get home. Why? Once I saw the guy not cal the meter before plugging it in to the tee, and he cold-started the meter in front of me. It turned out that what he marked on the tank and read to me as I signed the log was almost 2% higher than what was really in the tank.

That's not way, way off, but its enough to raise an eyebrow or two...

 

[Spectre]

Genesis once bubbled...
It turned out that what he marked on the tank and read to me as I signed the log was almost 2% higher than what was really in the tank.

I have in the past, re-analysed a tank in front of them. Of course, he knows I know he's a moron so I don't really care what he thinks now...

Hey... it's my life and my signature.

 

[Genesis]

Its my tail if I'm wrong.... If I have any reason to be suspicious I plug my analyzer in before I get the tank wet.

 

[Michelle Analox]

Many thanks to Dr Paul Thomas for the lovely comment on our new O2EII Oxygen Analyzer.

We will be at the NEC Dive Show (1110) and Dema (3700), it would be great to meet you all and show you the EII.

If you want any more info, please email me michelle@analox.net or visit:

www.analox.net
www.amoxtec.com