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William Sadler sez:
>They are all bad for use as dive meters.

>I recently obtained the specs for the electrochemical sensors that they
>use, and they are not linear throughout their range.  They are only
>correct within about 4% of their calibration point.  If you calibrate at
>21% (air) you will be off (reading a low O2 value) at 32-36%.  If you
>calibrate at 100% (pure o2) you will be off by as much as 10% at 32-36%.
>If you use 2 points, and move the curve about until it crosses at 21% and
>100%, then you will be off by about 4%.

>The ONLY way to use those meters properly is to use a calibration gas, say
>a known 34% mixture.  Then the sensor will be nearly linear from 32%-36%.

>This assumes, of course, that you are trying to calibrate for NOAAI or II.
>If you are trying to calibrate for another mix, you must use a known
>calibration gas within 4% of the value you are trying to reach or your
>results will be off.

>Also, these sensors have BAD hysterisis (sp?) meaning the values are not the
>same going up as coming down, instead of drawing the same line, it draws a
>crescent moon type shape.  The effects of this are reduced with proper
>calibration procedures too.

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.

>The only meter that I've seen that works are the paramagnetic type.
>They measure the O2 via its magnetic properties.  Yes, Oxygen has magnetic
>properties, as it is a metal.  Which I guess I could have deduced, but
>didn't.

>Anyway, paramagnetic sensors cost about $1500 just for the sensor, at
>wholesale (OEM) prices.  Complete instruments cost about 4k.  But they are
>linear over the range 0-100% O2, there is no hysterisis, and very good
>repeatability. --

Have you run any head to head comparisons between the two types? - Tony

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