YSI 3200, Operation Manual

Page: 46 / 86

Principles of Operation Section 8
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 normal to the current flow (A):
42
Therefore, conductivity equals conductance multiplied by the cell constant.
Example: For an observed conductance of 100 micro mhos (100
microsiemens) and a cell constant of 0.1/cm
= k K
= 100 mho 0.1 / cm
= 10 mho / cm
ℵ ×
×
µ
µ
In SI units, the cell constant K=0.1/cm would become K=10/m, and the same
conductivity would be expressed:
CELL CONSTANT
The cell constant (K) is used to determine the
resistivity or conductivity of a solution. It is
defined as the ratio of the
distance between
electrode (d) to the area normal to the current
flow (A). Cells with constants of 1.0/cm or
greater normally have small, widely-spaced
electrodes, while cells with constants or 0.1/cm
Cell Constant = K =
d
A
ℵ ×
×
= k K
= 100 S 10 / m
= 1 S / m
µ
m
ℵ
= k K
×
YSI Incorporated Model 3200