Instruction Manual COND 6+, TDS 6+, SALT 6+
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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 “Siemens”.
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.
Some users prefer the use of resistivity units to describe their water, particularly where
high purity water is involved. The unit most often used to describe resistivity is megohm-
cm, which are simply the reciprocal of conductivity (µS/cm). The chart below shows the
relationship between these units.
Conductivity
(µS/cm)
Resistivity
(megOohm-cm)
0.056 18
0.1 10
1.0 1.0
2.5 0.4
10.0 0.1
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