EIJKELKAMP 18.52.01 Manuel D'instructions page 135

concentration of ionic species present, as well as their charge and mobility. It is intuitive that higher
concentrations of ions in a liquid will conduct more current.
Conductance derives from Ohms law, E = IR, and is defined as the reciprocal of the electrical
resistance of a solution.
One can combine Ohms law with the definition of conductance, and the resulting relationship is:
In practice, conductivity measurements involve determining the current through a small portion of
solution between two parallel electrode plates when an ac voltage is applied. Conductivity values are
related to the conductance (and thus the resistance) of a solution by the physical dimensions - area
and length - or the cell constant of the measuring electrode. If the dimensions of the electrodes are
such that the area of the parallel plates is very large, it is reasonable that more ions can reside
between the plates, and more current can be measured. The physical distance between the plates is
also critical, as it affects the strength of the electric field between the plates. If the plates are close and
the electric field is strong, ions will reach the plates more quickly than if the plates are far apart and the
electric field is weak. By using cells with defined plate 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
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 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
micro Siemens per cm (µS/cm) or mill Siemens per cm (mS/cm). The 185001 & 185201 meters not
only measures conductivity readings from micro or milli Siemens but also reads resistivity (Ohms,
kOhms and MOhms), TDS (ppm and ppt), and salinity (ppt).
Note: ppm = parts per million, ppt = parts per thousand, 1000 ppm = 1 ppt. The total dissolved solids
scale complies with the TDS ISO/EN/DIN/NEN27778.
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 (µS/cm). The chart below shows the relationship between these units.
Conductivity, µS/cm
0.056
0.1
1.0
2.5
10.0
CONDUCITVITY 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. However, for most medium range salt
concentrations in water, 2% per degree works well. Extremely pure water exhibits a temperature
coefficient of 5.2%, and concentrated salt solutions about 1.5%.
This meter permit you to enter the temperature coefficient which best suits your sample and use an
ATC probe to automatically temperature compensate back to the chosen reference temperature.
It is important to choose an electrode with an appropriate cell constant. The following table lists the
optimum conductivity ranges for electrodes with cell constants of 0.1, 1, and 10.
C = 1/R C is conductance (siemens) R is resistance (ohms)
C = I/E I is current (amps) E is potential (volts)
Resistivity, megohm. cm
18.0
10.0
1.0
0.4
0.1
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