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Tony,
> Then why do they work when I make up a mix using real gas laws as my basis
> for partial pressure mixing calculations? All I do is perform the
> calculations, mix accordingly, calibrate my sensor with air, and presto! On
> the first pass my mix is usually within 1 to 2 percent of the desired
> percentage. Oh sure, its not perfect, but it seems to work pretty good even
> when making mixes with high FO2's.
Thats because you don't actually know what your mix is when you finish. I'll
elaborate.
Step 1 - figure your mix, get whatever percentage O2 for your dive.
Step 2 - (for simplicity sake) drain air out of your tank. (I KNOW you
don't have too)
Step 3 - Put x psi of O2 into tank.
Step 4 - Top off with air until you hit x psi.
Unless steps 3 and 4 are done very slowly, and the tanks are allowed to cool
to the SAME temperature at each pressure measurement, you'll be off. Most
dive shops I've seen don't do that.
So, at a dive shop, your computed psi's won't mean as much since they usually
don't take into account much in the way of temp adjustments to pressure, and
they often fill at different rates from different banks, so the temp will be
different for the air top off and the O2.
Now, you measure the O2. The gases may not be mixed well. I've seen a dive
shop take a tank sit it on the floor, and roll it back and forth a few times
and announce the mix is homogeneous. Well, the CGA standards call for rolling
at a specified (and not remembered) rpm for a period of time which I believe
is an hour. A couple of kicks won't do it.
(I know that there is argument about if this is necessary - don't bust my
chops on this, thats what the book says is needed for a mixture to become
homogeneous)
So, now you calibrate your O2 sensor at 21%. You do your mix. What do you
know?
Not much.
You know that your fg's are dependenant on the accuracy of your PSI
measurement, which you know is off. But you don't know by how much.
You know that your O2 sensor will be off somewhere from 2-8% w/ a calibration
of 21%. (which should be 20.9%).
Does the PSI off cancel the O2 off?
Are the errors additive?
Did they cancel?
Point is, you don't know. In order to know, you *HAVE* to calibrate the
sensor with a gas that is within 4% of the target value, or you *DON'T*
know. You're guessing.
> Have you run any head to head comparisons between the two types? - Tony
Not personally. But I have spec sheets from both sensor types, and the
electrochemical ones are not ones that I'd choose for a life critical
application.
They don't even have repeatability within 1%, its +/- 2% for the sensors that
come with the mini-ox units. The electrochemical sensors with repeatability
of +/- 0.25% cost almost $400 a unit. The paramagnetic units are repeatable
to +/1 0.1%, accurate to within 0.25% full scale, and linear from 0-100% O2,
have neglegible histeresis (sp?), and never have to be replaced. (Calibrated
once every 5 years or so).
--
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/* William E. Sadler | email: sadler_b@bs*.bs*.bl*.co* */
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