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From: owner-techdiver (INET.OWNERTE7) at DIAMOND
Date: 1/26/96 1:21PM
To: DAVID B DRAKE at SNFLD$61
*To: *techdiver
Subject: Re: More (long) than you ever wanted to know, was O2 Sensor
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G-man says:
Bingo - do it each time, this covers you for output, temp, humidity, etc.
Don;t worry about how, as you are already beyond the accuracy of the sensor
anyway. -G
<end>
Others have posted about the nonlinear behavior of the sensors. If
you don't see a linear behavior, your problem can be the line you're
drawing, not the sensor. The miniox and other low-end meters use an
incorrect line model (every time). When the sensor behavior doesn't
fit the bad model, it leads you to think the sensor is at fault, i.e.
behaving nonlinear (which it still may be), when in fact the meter is
creating a bad linear model to begin with. It's not comforting to me
to think that I'm diving with gas measured by a meter based on a bad
algorithm. This is the whole reason why this
diver/captain/instructor/physicist decided to make his own sensors
that I've been talking about.
I got the information on the sensor I mentioned in my last sensor
post. The accuracy and therefore linearity is pretty good, at least
for the 0-50% range. The zero offset explains a lot of the error we
see in typical meters. BTW, you can't expect to calibrate a miniox or
the like with two different gases like air and 100%. The model the
meter uses for the linear model uses the zero volts= zero O2 as one
point of the line which is incorrect. The other point is your
calibration point. As your fixed point is OFF, the line is OFF. The
real line only crosses the meter's bad line at one point, your
calibration point. Therefore, if your sensor behaves linearly, the
only place the meter will give you an accurate reading is at the
calibration point and that's *if* the meter's repeatability is good.
If you measure any other percentage other than calibration, it will be
off from reality. For instance you should get a high reading when
checking 100% if you calibrate with air first on the miniox. The
difference isn't as bad if you calibrate with 100% first and then
check air for reasons I stated in my last post *if* the sensor IS
behaving linearly.
The way to check the zero reading of your sensor according to these
fellows is listed below with some other notes they sent with the unit:
1...The zero adjustment is made in the presence of an inert gas such
as helium or nitrogen. Helium is easier because the cell can be
placed in an inverted container while pure helium is fed to the
sensor.
2. A good flow for reading is about 3 liters per minute. Faster
flow doesn't seem to have much of an effect.
3. The cell is internally thermally compensated so in general it
has an excellent rise time and is very stable. If you encounter
problems with your cell, please return it for replacement under the
warranty program.
4. This cell is quoted as being good for 900,000% hours, that is
900,000 hours in 21% air. This is about 5 years...
5. The instrument is tested against a calibrated mix of 40.26% as
measured by a HOLOX mass spectrograph... With careful calibration
and several cycles, readings within 0.1% are common. Random tests
are within 0.5%. Most other manufactures units will be 1% or more
high for readings in this range unless the zero offset has been
removed. These units will provide accuracies of less than 0.5%
between 0 and 50%.
I would be interested in what the minioxes come up with doing the
above helium test. I suspect they will still read some small % O2
with a reasonable gas flow of around 3 liter/min. Will someone try
this and let me know what the results are?
David Drake EDS/SATURN Infrastructure 8-320-4190 on
GMnet Spring Hill, TN USA Internet: saturn.ddrake05@gm*.co*
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