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Analyzing tanks (Fwd)
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Date: Fri, 16 Oct 1998 01:35:54 -0700
From: "Sami Laine" <sami@kelpdiver.com>
Subject: Analyzing tanks
To: ba_diving@lists.best.com
I first didn't want to answer on the list because this thread is bordering
pollution already :-) However, your post reminded me of couple of small
things to keep in mind about analyzing. Although everything I'm writing
here is covered in a basic nitrox class, and hanging on the Net isn't a
substitute for a class, out there in the real world there's a lot of
confusion. Here's some more...
>For about $200+ you could get an O2 sensor to check every tank.
The analyzer alone isn't doing you much good in a situation where you don't
know with certainty as to what is in the tank. Why? Oxygen analyzers
commonly used in the field (MiniOX, Ceramatec) are based on
electrochemical sensors that tend to be unstable and need to be calibrated
before every use with a known gas. If you're not sure what gas you have in
the cylinder, and don't have a reference gas available, the analyzer won't
help much.
Another thing to remember is that these electrochemical sensors do not
measure the fraction of oxygen in the tank, they measure the partial
pressure of oxygen in contact with the sensor. Therefore the flowrate into
the sensor is critical to get correct results, not only while analyzing the
gas but also when calibrating the sensor. I see people on the boats all the
time "calibrating" a MiniOX while it's just sitting in the box, then
hooking it to a nitrox tank at 4 liters per minute. The reading on the
nitrox can be a whole percentage point off or more, luckily in this case
indicating a higher 02 percentage than actual (luckily since IMO getting
bent beats toxing).
Speaking of cheap analyzers I had the "pleasure" of checking out a new
everyman analyzer, which was called a Nitrox Stick or something to that
effect. All in one, plastic encased stick, just pluck it against a valve
opening. Well, depending on how accurately I cracked the valve, guessed the
flow and how I held it, I got readings between 19% and 24%. On an air tank.
Sigh.
>If there are O2 bottles around you can observe the fill process
>to be sure they don't get connected to "regular air" tanks.
Yes, if they are doing partial pressure fills. Now, how do you know what's
in your tank if they have mix in the whips, doing continuous bleding? ;-)
What if the same whips may be pushing air or mix, depending on how valves
are set etc? This is not hypothetical, BTW, this is real life.
To take this thinking further, how do you know the reference "air" in the
tank you just used to calibrate your sensor had air in it? 8-)
Sami The Death Gas Instroketor
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