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Conductivity is a
measurement of the ability of a solution to conduct an electric
current. An instrument measures conductivity by placing
two plates of conductive material with know area and distance apart
in a sample.
Then a voltage potential is applied and the resulting current is
measured.
Using Ohms Law , V= iR and knowing
conductivity G= 1/R
then G can be determined as
G= 1/R = i/V
The number of ions that are conductive, metals, salts,
etc, provides the conductive path between two electrodes
of the conductivity cell. Higher ionic concentration yields higher
conductivity. Typically an AC signal is used to
prevent ionization of the electrodes.
Temperature effects and
compensation:
Increase temperatures can make the ions in the
water move faster
Conductivity levels falsely increases
approximately 2% per °C --- more for resistive waters (up to 4 or 5%
per °C )
Terminology
The Terminology used to express a unit of electrical
conductance is a microSiemen (Formerly a micromho). High
conductivity values can be expressed as milliSiemens.
Below 1 microSiemen, we express units of measure
as ohms of resistance rather than fractions or decimals of
conductance.
1 micromho = 1 microSiemen
1,000,000 ohms = 1 megaohm.
1/1,000,000 ohms is = microSiemen
1000 micromhos = 1000 microSiemens = 1 milliSiemen.
Many years ago, the water treatment industry adopted a
nomenclature of PPM. Correlating PPM to microSiemens can be
difficult, as water can be make up of different salt concentrations
and dissolved metals, which can alter the conversion factor. It is
preferable to use microSiemens as a unit of measure, however if you
need to convert to PPM, you can use the following formula:
1 ppm = 1.5 microSiemen.
1 ppm (sodium chloride) ˜ 2 micro siemens (<30,000
uS).
1ppm (mixed salts) ˜ 1.5 micro siemens (<1,000 uS).
A more exact conversion factor is:
ppm = 0.64 x conductivity
Cell Constant (K) Values
Cell constants define the volume between the electrodes. Cell
constant k is directly proportional to the distance separating the 2
conductive plates and inversely proportional to their surface area.
K = L/a, where a(area) = A x B.
Materials of Construction
The basic conductivity probe is comprised of two conductive
surfaces separated by a given distance in a body. The body material
can be anything from PVC, CPVC, PVDF, TEFLON, PEEK or even stainless
steel. The measuring surfaces (usually pin configuration) are
typically constructed of graphite, stainless steel, titanium or
platinum. The basic criteria for determining which is best are based
on cost and performance requirements.
Cleaning and Maintenance
Some care should be taken when cleaning conductivity probes.
Scratches and abrasions on the surface of the pins increases the
surface area which alters the cell constant and provides a retention
area for old samples which can cause calibration and measurement
difficulties. Graphite being a soft material is most susceptible.
Cleaning should be done with chemicals and soft non-abrasive
cloths. Sanding is not recommended. HCL
is an excellent material to dissolve many coatings.
Alternative Technologies
The basic 2-pin conductivity cell is all we have discussed to
this point. There is 4-pin technology that tries to better control
the field surrounding the conductivity sensor to improve stability.
These are known as contacting type conductivity cells.
Another type of technology is the non-contacting (Toroidal) cell,
which uses a magnetic field to sense conductivity. A transmitting
coil generates a magnetic alternating field that induces an electric
voltage in a liquid. The ions present in the liquid enable a current
flow that increases with increasing ion concentration. The ionic
concentration is then proportional to the conductivity. The current
in the liquid generates a magnetic alternating field in the
receiving coil. The resulting current induced in the receiving coil
is measured and used to determine the conductivity value of the
solution. Advantages to this type of cell are:
- No polarization
- Reduced maintenance and resistance to chemical attack
- Complete galvanic separation of measurement from medium
(eliminates ground
loss)
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