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What is Conductivity?
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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