Accumet XL 20, Руководство пользователя

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182
Appendix:
Conductivity Theory
XL20, 30, 50 and 60 meters
Conductance is a quantity associated with the ability of primarily aqueous solutions to carry an
electrical current, I, between two metallic electrodes when a voltage E is connected to them.
Though water itself is a rather poor conductor of electricity, the presence of ions in the water
increases its conductance considerably, the current being carried by the migration of the
dissolved ions. This is a clear distinction from the conduction of current through metal, which
results from electron transport. The conductance of a solution is proportional to and a good,
though nonspecific indicator of the 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.
C = 1/R C is conductance (siemens)
R is resistance (ohms)
One can combine Ohms law with the definition of conductance, and the resulting relationship is:
C = I/E I is current (amps)
E is potential (volts)
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 effects 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 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/cm
2
=
siemens/cm
C is the Conductance (siemens)
k is the cell constant, length/area or cm/cm
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 accumet XL20,30,50 and 60 meter 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).