Thermo Fisher Scientific CyberScan PC 5000 - Manual (Page 170)

Instruction Manual CyberScan PC5500/ 5000
165
Thus comes the term specific conductance or conductivity.
The relationship between conductance and specific conductivity is:
Specific Conductivity, S.C.= (Conductance) ( cell constant, k) =
siemens * cm/ cm2 = siemens/ cm
C is the Conductance (siemens)
K is the cell constant, length/ area or cm/ cm2
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backwards- however, this term has been replaced by the term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
(uS/ cm) or milli-Siemens per cm (mS/ cm).
This meter automatically converts conductivity readings from micro or
milli Siemens to other derived units that are widely used. These are
ppt salinity, ppm TDS (total dissolved solids), and Resistivity.
The salinity scale 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 approximates 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 is simply the
reciprocal of conductivity (uS/ cm). The chart below shows the
relationship between these units.
Conductivity, µS/cm Resistivity, megohm cm
0.056
0.1
1.0
2.5
10.0
18
10
1.0
0.4
0.1