jpsmith123
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Current Density Uniformity For Simple Electrolytic Cell Geometry
I wanted to measure the conductivity of distilled water from various sources using a simple arrangement of a 600 ml beaker and two 0.25" diameter
cylindrical electrodes spaced 1.5" apart on center, so I attempted to model it using a 3D EM simulator.
Anyway, I was also curious as to how the current density at the electrodes varies with position.
For this simple case, as it turns out, the current density at the minimum point is very nearly 50% of that at the maximum point. (A picture is
attached).
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Twospoons
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I love FEA. What software did you use? I use FEMM4.0 a lot but it is only 2 d.
Helicopter: "helico" -> spiral, "pter" -> with wings
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jpsmith123
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That's CST EM Studio. They have a product out now called "Studio Suite", which includes all their various programs.
The geometry editor (modeler) built into their S/W is very easy to use...so easy that now I'm spoiled.
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microcosmicus
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Since each rod is essentially a line source, the electric potential due to that
electrode is going to fall off as log r, the field as 1/r. The total field is the
sum of the two fields --- if you want to be careful, you can make corrections
for the boundary effects of the beaker and the finite length of the rods but,
even without such correction, treating the problem as two line sources is
a reasonable approximation. as your plot shows.
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bio2
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To measure a solutions or waters conductivity it is necessary to use an AC voltage controlled source read through a voltage divider. The current is
calculated in the electrolyte (water) as one leg of the voltage divider. This is then plugged into the standard conductivity formula using the
recommended electrode spacing and area for the expected range of micro Siemens
Using DC will not work as the reading continuously changes
due to the properties of the electrolyte (water) at the
electrodes surfaces.
[Edited on by bio2]
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jpsmith123
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You're right that AC is generally used in practice. Since I'm not looking for high accuracy but more or less a ball park idea, I'm hoping to get by
with DC. (Although if need be, I think I can simulate it with AC, too).
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bio2
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I have found that using DC to test water conductivity will give a momentary
reading at the very beginning that is somewhere close.
Within seconds the reading then quickly drops (resistance drops) as the
electrodes polarize and reactions occur. Electrodes about 10mm apart.
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jpsmith123
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That is my observation as well. BTW, did you make your own test cell or buy it?
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dann2
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Hello,
I just realized the other day that I used to work in a place that used a 'DLC' coater. (Diamond Loaded Coating). It put a coating of DLC onto Silicon
as an interface coat for another hydrophobic coating (top coat). I actually have some samples from the machine. The coating are very colourful when
not done properly. (Different thinknesses I guess).
Will see if they are conductive. The silicon that they were deposited on is non conducting as it is undoped so I don't think I have a ready make
anode.
The machine was a complicated contraption. I did not have access to it.
Dann2
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bio2
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........BTW, did you make your own test cell or buy it?.......
Made it with a 6 volt transformer and two reverse parallel LM317 regulators for 12VAC.
Calibrated a couple precision resistors with the 3rd resistor of the divider being
the liquid being tested.
Used a Fluke 87 multimeter to read the voltages then calculate the solution (water)
resistance and converting to micro siemens then to PPM.
You can find the circuit on National Semiconductors website in the main LM317
application note entitled "AC Voltage Regulator".
I compared my results to known solutions and Class 1 water and got within a percent or so. The probe was made from two test lead tips 10mm apart and
the constant calculated from the measurements.
This is run at line frequency and was quite stable and repeatable although commercial
conductivity meters run at 1KHz or so.
My homemade DI water is less than 1/4PPM my RO water 60PPM and the tap water is
280PPM (very hard).
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