Instruction Manual CON 6/TDS 6
42
areas and separation distances, it is possible to standardize or specify
conductance measurements.
Thus derives the term specific conductance or conductivity.
The relationship between conductance and specific conductivity is:
Specific Conductivity, S.C. = (Conductance) (cell constant, k)
= siemens * cm/cm
= siemens/cm
where C is the conductance (siemens)
k is the cell constant, length/area or cm/cm
2
2
Since the basic unit of electrical resistance is the ohm, and conductance is the
reciprocal of resistance, the basic unit of conductance was originally designated
a “mho” – ohm spelled backwards – however, this term has been replace by the
term “siemen”. Conductivity measurements are reported as Siemens/cm, since
the value is measured between opposite faces of a cell of a known cubic
configuration. With most aqueous solutions, conductivity quantities are most
frequently measured in microSiemens per cm (µS/cm) or milliSiemens per cm
(mS/cm).
The salinity value which ranges from 2 to 42 is a measure of all salts, not just
sodium chloride. This scale was originally devised for seawater, and is based
on seawater at 15 °C having a conductivity equivalent to that of a potassium
chloride solution of a known concentration. This solution (0.44 molal) is defined
as having a salinity of 35 ppt.
The total dissolved solids scale approximate the ppm TDS in surface waters by
multiplying the conductivity of a sample by a factor, 0.66.
