Experiences with Novel Secondary Conductivity Sensors within the ...

Vol. 3 No. 2 • June 2008
Primary cells
(of the national
metrology institute)
Reference standards
(Standard cells)
Commercial measuring
instruments and devices
for electrolytic conductivity
Calibration objects
Figure 1. Metrological hierarchy of electrolytic conductivity.
The aim of the project work done in the
DKD-K-06901 was the development of a
metrological concept to measure the electrolytic
conductivity in a wide range from
2 µS/cm to 100 mS/cm. At the beginning
only one standard measuring cell was
available in the laboratory. This cell was
applicable only for a limited measuring
range from 100 µS/cm to 12 mS/cm. In
order to be able to measure in the whole
range of interest commercial devices and
sensors were used. By means of this procedure,
it was not possible to realize the
low target uncertainty the customers are
asking for. On the other hand, an obvious
limitation of the commercial instruments
was the non-linearity. The metrological
hierarchy for electrolytic conductivity is
shown in Fig. 1.
The standard cells in the middle of the
pyramid are the result of the development
project. In the level of the secondary
standards commercial measuring
instruments and devices with several
Transfer standards:
Certified reference solutions
for electrolytic conductivity
sensors are used (e.g., 2-pole and 4-pole
cells from WTW or Radiometer). 1
1.3 Measuring Principle
The measurement principle described in
the following is valid likewise for
primary cells and the standard cells. The
complex conductance, G, or its reciprocal
the complex resistance, is evaluated
from a measurement at different frequencies.
For a dc measurement, either
of these two quantities would be enough.
But in general the measurement is
carried out with alternating current by
means of an LCR-meter. The detected
complex resistance during a measurement
with ac is the impedance, Z, consisting
of a real part, resistance, R, in
ohms, and an imaginary part, X, the so
called reactance. The relationship
between these quantities is shown in
equation (1):
Z = R + j X .
(1)
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In equation (1), j is the imaginary unit.
Similarly, the conductance can be
described as the complex conductance,
Y, consisting of a real, G, and an imaginary
part, B:
Y = G + j B .
(2)
The quantity B is referred the susceptance.
In all measurements of electrolytic
conductivity with primary and standard
cells, the susceptance is part of the measured
impedance. Therefore the conductance
of the solution at infinite frequency
has to be separated from the complex
result. Because an infinite frequency is
not realizable, the conductance at different
frequencies is measured and plotted
as a function of the reciprocal frequency
(see Fig. 2).
There is a good linearity between the
conductance and the reciprocal of the frequency
over a given range. The intersection
with the ordinate corresponds with
the conductance at an infinite frequency.
This value is used for the determination
of the cell constant of the standard measuring
cells if the conductivity of the reference
solution is known (e.g., certified
reference solutions from DFM, National
Institute of Standards and Technology
(NIST) or Physikalisch-Technische Bundesanstalt
(PTB)). After the cell constant
is known, this value is used for the calculation
of the electrolytic conductivity of
solutions to be calibrated.
For a precise evaluation, the choice of
a suitable frequency range is very important.
[8] A good linearity is not given
using the whole frequency range. There
are limits in the linearity at determined
frequencies, but the limits depend on the
electrolytic conductivity of the solution.
In general it is possible to say that it is
better to use low frequencies for solutions
with low electrolytic conductivity
and high frequencies for solutions with
high electrolytic conductivity.
During the work in the development
project, different cell constants were
realized for the whole measuring range
from 2 µS/cm to > 100 mS/cm. The sig-
1 Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately describe the experimental procedure. Such
identification does not imply recommendation or endorsement by the author or NCSL International, nor does it imply that the materials or equipment
identified are the only or best available for the purpose.
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