YSI 3100 - Manual (Page 23)

19
7. Principles of Operation
7.1 3100 Principles
The YSI 3100 obtains a conductance value by varying the amplitude of a square-wave current
forced through the cell so that the center-sampled magnitude of the cell voltage for each half-
cycle is constant and is equal to a reference voltage. In this condition, the current and
conductance are directly proportional. To convert this conductance value to conductivity, it is
multiplied by the cell constant which has units of reciprocal cm (cm
-1
). For most applications, the
cell constant is automatically determined (or confirmed) with each deployment of the system
when the calibration procedure is followed. Solutions with conductivities of 1.00, 10.0, 50.0, and
100.0 mS/cm, which have been prepared in accordance with recommendation 56-1981 of the
Organization International De Metrologie Legale (OIML) are available from YSI. The instrument
output is in
µ
S/cm or mS/cm for both conductivity and temperature compensated conductivity.
The multiplication of cell constant times conductance is carried out automatically by the
software.
7.2 Conductivity Principles
Introduction
Conductivity measurements are used in waste water treatment, industry, pharmaceutical, and
military etc. as a measurement of the purity or the condition of a process. Conductivity is used as
a measurement of a solution’s ability to conduct electric current. The ability of a solution to
conduct electric current depends upon ions: their concentration, size, mobility, viscosity, valence
and the temperature of the solution. Inorganic solutions are relatively good conductors. Organic
solutions are poor conductors.
Conductivity Fundamentals
Electrical conductance (k) is defined as the ratio of the current (I) in a conductor to the
difference in the electrical potential (V) between its ends (k=I/V), measured in mhos or siemens
(S). Conductance, therefore, is not a specific measurement. Its value is dependent upon the length
of the conductor. Conductivity (
ℵ ℵ ℵ ℵ
), or specific conductance, is the conductance per unit of
conductor length. For our purposes, conductivity is defined as the conductance in mhos or
siemens measured across the sides of a one centimeter cube of liquid at a specified temperature.
Looking at our electrodes as sides of a cube, it becomes apparent that
the conductance changes as the geometry of the cube changes. If the
cube lengthens with respect to the area of the sides, then the
conductance will decrease. If the area of the sides increases with
respect to the distance between them, then the conductance will
increase. The conductivity, however, will remain the same, regardless
of the geometry, provided that the temperature and composition of the
measured solution remain constant. A factor called the cell constant
(K) relates conductivity to conductance. The cell constant is defined
as the ratio of the distance between the electrodes (d) to the area