Making an electrode for a conductivity meter

Thu Dec 30, 2004 1:02 am   

Wayne wrote:


In the U.K. we got platinum and similar stuff from Goodfellow Metals

http://www.goodfellow.com/csp/active/gfHome.csp

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

Thu Dec 30, 2004 1:17 am   

Rich Grise wrote:

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

Thu Dec 30, 2004 1:25 am   

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

Thu Dec 30, 2004 1:28 am   

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

Thu Dec 30, 2004 1:28 am   

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

Thu Dec 30, 2004 8:48 am   

I read in sci.electronics.design that bill.sloman_at_ieee.org wrote (in
<1104366355.241964.316220_at_z14g2000cwz.googlegroups.com>) 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

Thu Dec 30, 2004 9:39 am   

Hello,

Try Goodfellow. They should have this.

Best regards.


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Thu Dec 30, 2004 12:08 pm   

"John Woodgate" <jmw_at_jmwa.demon.contraspam.yuk> schreef in bericht
news:BZuFQ0A$K70BFwuy_at_jmwa.demon.co.uk...

If you wind your wire around the toroid in the obvious way - starting at 0
degrees and adding turns until you get around to 360 degrees - you've made a
loop around the toroid.

Rayner and Kibble's "Coaxial AC Bridges" (ISBN 0-85274-389-0 and available
from NPL
http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html)
discusses this at length in section 4.2.1.

They list a couple of ways of making the winding non-progressive or
"astatic", starting with taking the end of the coil back around the toroid
to the start, which roughly cancels the loop, proceeding through the
Ayrton-Perry or "bootlace" technique where you put on half the turns going
clock-wise around the toroid, and the remainder back-tracking anti-clockwise
over the top of the first lot, to a more complicated variant where you wind
the first 25% of the turns over 180 degrees of the toroid, going around the
toroid clockwise, then wind the next 50% over the full 360 degrees, going
back around the toroid anti-clock wise, then reverse again to wind the last
25% over 180 degrees, going around the toroid clockwise back to your
starting point.

The last variant gives you a coil with less self-capacitance.

I posted most of this on s.e.d. on Mar 12 2003, at 1:32 am, in the thread
"Conductivity meter probe questions".

--------
Bill Sloman, Nijmegen

Thu Dec 30, 2004 1:28 pm   

I read in sci.electronics.design that Bill Sloman <bill.sloman_at_ieee.org>
wrote (in <cr0npf$8ta$1_at_reader10.wxs.nl>) about 'Making an electrode for
a conductivity meter', on Thu, 30 Dec 2004:


Thank you for the information.
--
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