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Sorry to waste bandwidth for those who know or know they don't care.
Tim Taylor writes:
A friend of mine just came back from DEMA and said that he was told
that the proper method was to keep the sensor in a humid environment
(a dive bag for example). Don't bother sealing the sensor away.
Supposedly the moisture keeps the sensor head from drying out and
oxidizing. This same person told my friend that his sensors typically
lasted 3 years with this treatment.
<end>
I talked in depth with the developers/manufactures of a new 5 yr. cell
at TEK. They said that moisture was bad and water is death to their
sensor. It is also a galvanic cell. Keeping it dry, cold (but not
frozen), and away from it's fuel, O2, is the best thing you can do for
cells in general. Read that as keep it in a sealed container full of
inert gas in a refrigerator. Regarding the O2 gas difusing into a
plastic bag, for those long Michigan winters, try pulling a vacuum on
the bag as you seal it and put that in a masor jar purged with inert
gas as best you can and seal that up before putting it in the frig.
I'd hope the double seal would at least slow down the diffusion
process.
They also mentioned why their meter/sensor combination was worth $380
(negotiable)... According to them, the sensors are going to give a
positive voltage out even in a 0% O2 environment. This is due to the
O2 caught in the materials surrounding the reaction site diffusing out
into the reaction area. The surrounding material is in effect,
decompressing and off-gassing O2 due to the low partial pressure of
O2.
The reason this is relevant is the typical meter is calibrated
assuming a straight line behavior (linear). The two points that
determine this line are by default 0% O2 and whatever you calibrate
at. Most meters are calibrated at no voltage = 0% O2 which is wrong
according to these guys (two PhDs in physics and engineering between
them). This zero reading is handy because people expect to see that
when there is no sensor plugged into the meter or the sensor is dead
and it also removes a difficult calibration step required for
accuracy.
The troublesome step required for accuracy is correctly determining the
voltage that should be interpreted for 0% O2 for each individual
sensor. For those of you that know your linear algebra this is the y
intercept, b, in the y = mX + b formula. Y being voltage, X the % O2,
and m the slope or ratio of voltage to % O2. Note that the % O2 is
based on the partial pressure of oxygen. Crank up the gas flow and
pressure, and you'll see a higher O2 than is really there.
The Miniox and others simply ignore the b factor in this equation.
How can you tell, well if the cell gives off positive voltage even in
a zero O2 environment, you must subtract this voltage out for your
meter to read zero with a positive voltage. If you disconnect your
cell, then the meter will see the drop in voltage and read a negative
percentage. These fellows typically see around 2-5 mV of background
voltage in their sensors when in a 0 % O2 atmosphere.
Why does this mean anything? Well take an x-y chart with 10
centimeters on each side, each centimeter on the x axis is 10% O2.
Plot (x,y) = (0,0.5) for the 0% O2 and a 5 mV background voltage.
Now plot (2.09, 2.0) for an air calibration. Draw a line through
those two points to the far right of the graph. This is now your
chart that the new meter uses to figure the %O2. Y= mX + 0.5. Label
the line "alpha". The m is not from empirical data, just convenient
for discussion. I didn't bother to calculate it.
To see what the miniox would have given you for 50 % O2, draw a line
between the lower left corner (0,0) and the air calibration point
(2.09,2.0) and extend it to the edge of the graph where X=10cm or 100%
O2. Label it beta for the miniox. The minox line modeling your gas is
y=mX + 0. Go to 5.0cm (50 %) on the x-axis and then straight up till
you hit the 1st line you drew for the new meter (alpha). Mark that
point. From that point draw a line horizontally, parallel to the
x-axis, till you intersect the miniox beta line. Drop straight down to
the x axis and read your value, multiply by 10 for the % O2. The
difference in the x values at a given Y (voltage from the cell) is the
automatic error built into your readings with the miniox due to a poor
mathematical modeling. It can easily approach 2% w/o you doing
anything wrong. Notice as the lines travel to the right, higher O2 %,
the worse the error. This is due to the poor model and extrapolation
(guessing what the sensor's behavior will be outside of the
calibration zone). In this case the assumption is the same cell used
for both meters, is calibrated correctly for the alpha meter and it's
voltage for like mixes is the same. Yes you can switch cells, but
with some considerations regarding behavior.
The surest way to reduce error is to calibrate with a gas as close to
the mix you are measuring as possible. For instance, if you are
mixing air (20.9%)from N2 and O2, your best calibration gas is air.
You shouldn't see a difference between the meters when this is done
correctly. The worst thing you can do is extrapolate, that would be
using air to calibrate and the checking a higher O2 percentage like
80% (the higher the more the likelyhood for error).
These guys' new meter takes the background voltage into account in
it's linear model of the sensor's behavior. Other meters do not. The
more you're off from your calibration %, the more the difference
between meters. Also, there meter is not finicky WRT flow rates. As
long as you have some gas moving it'll read the same (I witnessed
this) as long as you don't over pressurize it. The differences
between the sensors and the meters and the significance of it is up to
you to decide.
The way to "do it right"tm, is to have each meter and sensor paired. If
you switch sensors, then the meter must be recalibrated to the new
sensor. I am waiting for information on how to do this correctly. You
can see with the limited life of miniox O2 sensors, why the
sensor/meter calibration step is not commercially attractive. For
those who'll pay a bit more, it seems you can get longer (5yrs) and
more accurate life. Note the "can" in that statement. I do not vouch
for the co. or there product, just passing the info along as it's been
a big subject before that has been oversimplified and misunderstood.
If people want the manufacturer's name & # post a request.
To use the charts directly yourself with an accurate voltmeter, simply
set up your chart and axis values, plot the calibration values (the
more the merrier if you use a best fit linear model), draw your line.
Note, if you don't have the 0% O2 reading for the cell, your line can
not be drawn unless you use two calibration gases to establish a line.
Measure the voltage out of your cell for your subject mix, go up the y
axis to that value and then go out to the right parallel to the x axis
until you cross your line. At that point, you drop straight down to
the x axis for your % O2. At least this was how it was explained to
me...don't use this for your diving purposes, you might kill yourself
and your family will sue me. To know if you have an accurate meter,
go to a good electronics place and get it tested.
What, no algebra or math skills?, then don't dive gas.
What can you do to the miniox to make it read more accurate? Well
you simply can't change it's modeling method (hard wired), but you
can do a few things to help it out. First, follow all the procedures
you're supposed to for measuring mix. Use your miniox for calibration
and target mix in the high O2 percentages. The intercept plays a
decreasing role in the differences between the two models as the O2
calibration and target mix values get higher. Most importantly,
calibrate as close to the unknown gas you will be analyzing as you
can. For instance, if you have 80% targeted for your final mix, then
get a bottle of 80% (near as you can) certified from a gas supply co.
and use it for calibration. Do the same for other common O2
percentages. This closer-to-target calibration gas will improve all
analyzers performances if they repeat the same readings for the same
gas and work reasonably well to begin with.
Got all that? Let me know politely if something doesn't check here.
David Drake EDS/SATURN Infrastructure 8-320-4190 on GMnet
Spring Hill, TN USA Internet: saturn.ddrake05@gm*.co*
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