Hi
I am trying to make a conductivity meter and would like to know if any body can advise on how to make the electrodes? Do I make them from paladium or can I use something like alluminum? Also, would I get an improved conductivity value using an AC signal?
Cheers
Wayne
of them
Very true.
This is a "standard" conductivity cell - you can make the plates bigger and closer together if you want to see a lower resistance when measuring the conductivity of a particular solution. Making them smaller and further apart lets you see a higher resistance, but you end up averaging the conductivity over a range of paths through the liquid, and the maths can get complicated.
that
You really have to measure conductivity with an AC signal - you get additional voltage drops at the electrode surfaces in addition to the resistive losses through the bulk of the liquid, and you have to minimise these additional losses if your measurement is going to say anything useful about the conductivity of the solution. Even with an AC signal, you've always got a capacitative impedance at the electrodes in series with the resistance of the solution - a higher frequencies this can be just the "double layer capacitance" but at lower frequencies you can also see the Warburg impedance which looks like a frequency dependent capacitance in series with a frequency dependent resistance - see
it has
to the
Platinum is used in standard cells because it doesn't corrode, and because it is easy to electroplate a layer of "platinum black" onto a platinum surface, which vastly increases the effective surface area and reduces the voltage drops at the electrode surfaces in the same proportion. Carbon is often used in big cells where cost is a problem, but it is much more difficult to work with.
Stainless steel is just as bad as aluminium - it is "stainless" because the surface is protected by a thin layer of Cr2O3 and I've had conductivity measurements wrecked by the additional resistance of this layer (which changes with the chemical nature of the solution being measured). My bosses were not pleased with having to pay for platinum rather than stainless steel, but should have been more upset about ever having wasted time on stainless steel electrodes in the first place.
------- Bill Sloman, Nijmegen
Hello,
1- It exist conductivity meters in the market. You can just buy one of them : you'll save your time and money. 2- If you really want to make one by yourself for pratice, then :Best regards.
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Thanks Bill Do you know where I could buy a few of these 1cm^2 platinum plates. My research is taking on a great deal of equipment and I will be modifying and customising the equipment that is why I can not just buy one.
Cheers
Wayne
Wouldn't a potted torroidal inductor make an interesting conductivity sensor? No contacts to crud up.
John
That's bizarre. Why not a single torroid?
John
OK, that makes sense for low-conductivity liquids.
John
Well, some experimental calibration would be in order. But the loss associated with a resistive shorted turn should pretty well scale to conductivity, after subtracting core loss.
I'd think that the single torroid would work pretty well for reasonably conductive stuff. I'll try it maybe.
John
It certainly would. And they are a commercial product. They contain two toroidal cores, axially aligned. The winding on one produces an EMF in the liquid, driving a current that passes through the hole. The winding on the second one acts as a current transformer, measuring that current.
-- John Popelish
I think the two core approach makes it easier to separate the small current transformer currents from the large excitation current.
-- John Popelish
platinum
will be
buy one.
In the U.K. we got platinum and similar stuff from Goodfellow Metals
but I think you can also get it from jewellery suppliers. For platinum black you need hexachloroplatinic acid (H2PtCl6) which we got from our regular chemical supply house (it's poisonous and corrosive ...).
You ought to be able to get platinum plated onto printed circuit board type materials - which include Teflon/PTFE and alumina - if you chase around a bit. My bosses weren't prepared to mess around with the mechanical design of our conductivity sensor to take advantage of this, so I've not tried this out in practice.
The better solution to measuring solution conductivity is to skip the electrodes completely, and use the inductive technique (which we've discussed on s.e.d. within the last few years). You need a rather bigger volume of solution than you can get away with in a 1cc conductivity cell, and it doesn't seem to be too good below the milliSiemen/cm level, but you lose all the voltage drops at the elelctrode surfaces.
-------- Bill Sloman, Nijmegen
platinized
electrodes.
22-carat gold ought not to tarnish, but it contains enough copper and silver for a tarnish layer to form if there is a lot of sulphide around. 24-carat - 0.9999 pure gold - won't tarnish, but it is very soft, and probably as hard to get hold of as platinum.------- Bill Sloman, Nijmegen
Only if you do it that way - if you put just enough current through the second coil to precisely cancel the induced voltage, the inductor impedance would fall out. Nulling techniques are always nice.
And don't forget that the windings on both inductor cores have got to be non-progressive, otherwise you get an effective single turn around each of the toroidal cores, which rather messes up the concept.
---------- Bill Sloman, Nijmegen
In fact the commercial inductive sensors don't seem to be too good below about 1mS/cm so they wouldn't be too good with tap water. which isn't my idea of a low-conductivity liquid.
----------- Bill Sloman, Nijmegen
In fact the commercial inductive sensors don't seem to be too good below about 1mS/cm so they wouldn't be too good with tap water. which isn't my idea of a low-conductivity liquid.
----------- Bill Sloman, Nijmegen
I have always used pure Gold plating for conductivity meter electrodes. Do you have any information as to why Platinum would be prefered?
-- Guy Macon
I read in sci.electronics.design that John Larkin wrote (in ) about 'Making an electrode for a conductivity meter', on Wed,
29 Dec 2004:That was my initial reaction. But calculating the conductivity from the resistive component of the inductor impedance is not at all simple.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
The overvoltage of hydrogen to platinum in water is zero volts.
Rene
-- Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
Could you give a bit more detail about the practical results of this? I am a lot more familiar with electronics than I am with chemistry, and you are using the term "overvoltage" in a way that I am unfamiliar with.
-- Guy Macon
I read in sci.electronics.design that snipped-for-privacy@ieee.org wrote (in ) about 'Making an electrode for a conductivity meter', on Wed, 29 Dec 2004:
I don't understand 'progressive' in that context. Could you please enlarge?
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
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