LaMotte 5 Series TDS Meter - Manual (Page 35)

CONDUCTIVITY THEORY
Conductivity is defined as the ability of a solution to conduct an electrical
current, or the reciprocal of the solution’s ability to resist the current. The
current is conducted by electrically charged particles called ions, which are
present in almost all solutions. Different solutions have different kinds and
amounts of ions. Distilled water has very few ions, and therefore a low
conductivity, while seawater has a large number of ions, and a high
conductivity.
Although a conductivity reading provides an overall measurement of the ionic
strength of a solution, it is not possible to distinguish the specific amounts of
individual ions. For this reason, conductivity is often used to measure the total
dissolved solids (TDS) of a solution. TDS is defined as the amount of solids
that will pass through a 45 micron filter. Rather than filtering a solution, the
TDS can be estimated by multiplying the conductivity measurement by a
predetermined factor. This factor, which is determined gravimetrically, will fall
between 0.55 and 0.9. A commonly used factor is 0.7.
Conductivity is measured using a probe which has two parallel plates separated
by a fixed distance. When a voltage from the meter is applied across the
electrodes, the ions in the solution conduct a current that flows between the
two electrodes. The greater the concentration of ions in the solution, the
larger the current generated and the higher the conductivity. Likewise, the
smaller the concentration of ions, the lower the conductivity. The meter
converts the current measured to a conductivity reading. Conductivity values
are related to the conductance of a solution by the physical dimensions - area
and length — or the cell constant of the measuring electrode. The physical
distance between the plates is also critical, as it effects the strength of the
electric field between the plates. By using cells with defined plate areas and
separation distances, it is possible to standardize conductance measurements.
The relationship between conductance and specific conductivity is:
Specific Conductivity S.C. = (Conductance) (cell constant, k)
= siemens x cm/cm 2
= siemens/cm
where C is the conductance (siemens)
k is the cell constant, length/area or cm/cm 2
Conductivity is measured in microsiemens per centimeter (
mS/cm). In waters
of higher conductivity,
mS/cm may be multiplied by 1000, giving results as
millisiemens per centimeter (mS/cm). Total dissolved solids are measured in
parts per million (ppm). Therefore:
µS/cm X 0.7 = ppm TDS
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