LaMotte CDS 5000 Conductivity Meter - Manual (Page 4)

CONDUCTIVITY
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. This
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 sea water has a large number of ions, and a high
conductivity.
Although a conductivity reading provides an overall measurement of the
ionic content 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 which 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; 0.7 is a
commonly used factor.
The conductivity measurement can also be used to estimate the salinity of
water, or the total amount of all salts dissolved in the water. Typically, the
conductivity reading is converted to salinity using charts, such as the one
found on page 10. These charts are based on water containing the same
amount and proportion of ions as standard seawater, so this form of
measurement is most effective for low concentrations and dilutions of
seawater.
Conductivity is measured in microsiemens per centimeter
(msiemens/cm).
Siemens are also called mhos. In waters of higher conductivity,
msiemens/cm may be mul million. Therefore, using the information
discussed above:
msiemens/cm x 0.7 = ppm TDS
Salinity is usually measured in parts per thousand (ppt). The chart on page
10can be used to convert conductivity readings to salinity.
Because it is a quick, reliable, and inexpensive way of monitoring the
ionic content of a solution, conductivity measurements are widely used in
many areas of water testing, from environmental monitoring to municipal
water supplies to many industrial applications.
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