47
SECTION 4,
continued
The S.I. unit for conductivity is Siemens (S) (1 Siemen = 1 mho).
Other units are: 1/ohm = 1 mho = 1000 mS = 1,000,000 µS.
Since the cell’s physical configuration significantly affects the
conductivity measurement, it must be multiplied by the
conductance to obtain the actual conductivity reading. For
example, if the conductance reading is 350 µS using a cell with
K = 0.1 cm
-1
, the conductivity value is 350
x
0.1 = 35.0 µS/cm.
Simply stated, the cell constant is defined as the ratio of the
distance between the electrodes (d) to the electrode area (A).
However, this neglects the existence of a fringe-field effect,
which affects the electrode area by the amount AR. Therefore:
K = d/(A + AR)
Normally it is not possible to measure the fringe-field effect and
the amount of AR to calculate the cell constant. For most uses,
the actual cell constant (K) of a specific cell is determined by
comparing the measurement of a standard solution of known
specific conductivity (e.g., 0.01 M KCl) to the measured
conductance.
The conductivity of a solution at a specific electrolyte
concentration will change if the temperature changes. For
accuracy, measured values should be adjusted for the solution
temperature. The temperature-compensated conductivity of a
solution is the conductivity that the solution exhibits at the
reference temperature. This temperature is either 25 °C or 20 °C.
A measurement made at reference temperature does not
need compensation.
The sens
ion
156 meter automatically compensates for
temperature during conductivity measurements using the sample
temperature. Temperature compensation is different for different
types of samples. Some examples are shown in Table 6 on
page 36. The closer the sample is to the reference temperature,
the smaller the error will be if the meter temperature coefficient is
not correct.
