Conductivity meter

Linda Roundhill <artsconservation [at] attbi__com> writes

>Jeremy Wells <jeremy [at] wells__net> writes
>
>>Chris McAfee <mcafeeck [at] ldschurch__org> writes
>>
>>>I'm looking for a small, simple-to-use conductivity meter for
>>>testing my deionized water. Any suggestions?
>>
>>I've often wondered if a simple conductivity/ohm meter from Radio
>>Shack would suffice? ...
>...
>I wonder about this too.  I have used inexpensive noncalibrated
>meters when necessity demanded and found that a progressive plot of
>the numbers during desalination can be more valuable than the actual
>numbers, depending on the material being desalinated.

To measure conductivity of liquids we need to pass a current through
them which is electrolysis. The current flow is proportionate to the
degree free ions in the liquid migrate, (H2O, dissolved salts,
acids, bases, etc.) which is determined by the potential difference
(voltage) applied, the surface area of the electrodes and their
spacing.

Portable ohm meters use an internal battery as the voltage source
which provides direct current. The contacts of the test leads are
polarized, one negative, the other positive. Passing DC through a
water solution will result in the precipitation of the free negative
ions on the positive electrode (anode, anodizing), and vice-versa.
The continued passage of a direct current can also result in a
reactive corrosion of the electrodes themselves. (making sacrificial
zincs necessary on the metallic parts of boats that are below the
water-line). This build-up of precipitates and corrosion of
electrodes makes results obtained by the use of DC current
unpredictable and of little practical value.

However, an inexpensive multi-meter can be used by the addition of a
handful of readily available, inexpensive components;

Fabricate a pair of electrodes by drilling two holes in a small
piece of acrylic or other insulating material to accept the desired
conductive electrodes. These may be long machine screws with nuts
clamping them to each side of the acrylic, or if you want something
more stable and less susceptible to corrosion, use graphite pencil
leads from a mechanical pencil, or the carbon rods from the center
of a spent D cell battery. Either way, the distance of the electrodes
will be constant, determined by the spacing of the drilled holes and
you can control the surface area exposed to the liquid by the depth
of immersion. (This also will control the amount of current flowing)

Next build a circuit by using a surplus AC wall transformer (voltage
unimportant, between 3V-12V is OK) perhaps from an old princess
phone (I'm showing my age here!)or buy one at Radio Shack. Make sure
it is an AC transformer and and NOT DC adaptor!

Complete the the project by connecting the transformer, the
electrodes and a 5 Watt 100 ohm resistor (value not critical) in
series. No current will flow until you immerse the electrodes in the
liquid.

Conductivity of the liquid can be measured by measuring the AC
voltage drop across the resistor. A higher voltage reading indicates
higher current  flowing through the circuit and implies higher
conductivity in the liquid. Calibration may be achieved and
correlated to known ion densities by constant methods and
procedures. I'll gladly assist anyone in completing this project off
list.

George Schwartz
ConservArt - Master Frame Makers and Art Conservators
8177 Glades Road #16
Boca Raton, FL 33434 U.S.A.
561-482-7292
Fax: 561-482-6787


                                  ***
                  Conservation DistList Instance 17:8
                   Distributed: Tuesday, July 1, 2003
                        Message Id: cdl-17-8-001
                                  ***