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Olly,
>1) What are you currently paying for analysers, and replacement sensors?
People pay about 300 or so bucks for the Miniox type from most scuba places.
They sometimes don't bother shelling out the extra $100 or so for a flow
regulator which is a MUST if you want any accuracy or repeatability from the
device.
>2) Are the sensors user-replaceable?
Yes but...
>3)What accuracy do the manufacturers claim? What accuracy is actually required?
The electrochemical sensor head that is used in the Miniox claims 1% accuracy
if calibrated properly. What most people ignore, is the calibrated properly.
It is not linear through its range, so the proper procedure to get the stated
accuracy is to calibrate within 5% of the desired final reading. Since most
divers do not carry around a calibration quality gas that is within 4% of
their desired mix, the mini-ox can be off by as much as 4-10% depending on how
you actually calibrated it.
Now, how accurate does it need to be?
Suppose we want NOAAI:
32% MOD at 1.4 is 4.38 ata, or 111.54 fsw.
MOD at 1.6 is 5.00 ata, or 132.00 fsw. <<-- BTW this is MOD for NOAA
with good accuracy +/- 1% we get
31% MOD at 1.4 is 4.38 ata, or 111.54 fsw.
MOD at 1.6 is 5.00 ata, or 132.54 fsw.
33% MOD at 1.4 is 4.24 ata, or 106.92 fsw.
MOD at 1.6 is 4.85 ata, or 127.05 fsw.
So the actual MOD will range within 5fsw with 1% accuracy. With ppo2 up to
1.65, or 1.45 if you're aiming for the lower limit.
Probably acceptable.
Now, suppose were off by +/- 5%, and we still want NOAAI above:
27% MOD at 1.4 is 5.19 ata, or 138.11 feet. <<-- yeah! we can go deeper!
MOD at 1.6 is 5.93 ata, or 162.56 feet. <<-- way deeper!
38% MOD at 1.4 is 3.68 ata, or 88.58 feet. <<-- Oh, oh...
MOD at 1.6 is 4.21 ata, or 105.95 feet. <<-- hmmm...
So, the mod will range almost 60 feet with +/- 5% accuracy. And +/- 5% is
what most people get. If you miss by the higher percentage, you will be
diving an ppO2 of 1.9. That is, you assume you can go to 5atm, but you
actually have a 38% mix.
In other words, miniox's are NOT adequate with the calibration procedures
used by most divers. The biggest improvement you could do for an O2 analizer
would be to include circuitry to make the sensor linear. Of course, the
sensors are not designed to have the exact same curve, so you'd have to
calibrate each instrument each time you change the sensors.
> 4) How rugged are the analysers you currently use?
Pretty tough. But they don't have to be.
> 5) What features do you want to see on an analyser?
It should regulate its own flow. It should have a linear sensor head, so that
you can calibrate with air or pure o2.
You can move to paramagnetic sensors instead of electrochemical ones. But they
are much more expensive. You can buy electrochemical sensors for about $12
a pice in qty from medical supply houses. Paramagnetic sensors cost about
$700 a pice in similiar qtys, but you never have to replace them, they are
already linear in response, are accurate to .1%, and have good repeatability.
You pays you money, you takes you choice.
--
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/* William E. Sadler | email: sadler_b@bs*.bs*.bl*.co* */
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