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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 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 megohmcm, which are simply the reciprocal of conductivity
(µS/cm). The chart below shows the relationship between these units.
Conductivity (µS/cm)
0.056
0.1
1.0
2.5
10.0
Conductivity and Temperature
Conductivity in aqueous solutions reflects the concentration, mobility, and
charge of the ions in solution. The conductivity of a solution will increase with
increasing temperature, as many phenomena influencing conductivity such as
solution viscosity are affected by temperature.
The relationship between conductivity and temperature is predictable and
usually expressed as relative % change per degree centigrade. This temperature
coefficient (% change per degree) depends on the composition of the solution
being measured. Extremely pure water exhibits a temperature coefficient of
5.2%, and concentrated salt solutions about 1.5%.
Since temperature affects the conductivity measurement so profoundly, usual
practice is to reference the conductivity to a standard temperature - typically
25°C. Select 20°C or 25°C as the normalisation temperature in advanced setup.
Enter the temperature coefficient which best suits your sample and use an ATC probe
to automatically temperature compensate back to the chosen reference temperature.
Conductivity Theory
Resistivity (Mega ohm-cm)
18
10
1.0
0.4
0.1
9
27
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