Experiences with Novel Secondary Conductivity Sensors within the ...

TECHNICAL PAPERS
Experiences with Novel Secondary
Conductivity Sensors within the
German Calibration Service (DKD)
Ulrich Breuel, Barbara Werner, Petra Spitzer and Hans D. Jensen
Abstract: International efforts concentrate on the traceability of electrolytic conductivity at the field level having small
associated measurement uncertainties. Although the measurement of conductivity at the primary level has been widely
developed during the last decade, the dissemination of the small measurement uncertainty to the field level is lagging.
There is a lack of easy to handle and reliable secondary calibration methods and transfer standards.This paper describes
a procedure for determination of the electrolytic conductivity on the secondary level appropriate for calibration laboratories.
The procedure was developed within a joint project of the German calibration laboratory (ZMK ANALYTIK
GmbH, DKD-K-06901) together with the Physikalisch-Technische Bundesanstalt and the Danish Metrology Institute.
Altogether a chain of five measuring cells has been used so that it is possible to measure the conductivity over a wide
range from 2 µS/cm up to 100 mS/cm.Experiences and first results using the five secondary cells are given including
evaluation of comparisons with metrological institutes and partners from industry.The results obtained by the ZMK
are in good agreement with the results obtained by the metrological institutes.
1. Introduction
1.1 Measuring Devices for Electrolytic Conductivity
In general to determine the electrolytic conductivity at a certain
level of the metrological hierarchy (national and international)
the followed measuring devices are used:
1. Primary measuring cells.
2. Special designed standard measuring cells.
3. Commercial conductivity measuring instruments (with 2pole
and 4-pole cells).
Ulrich Breuel
Barbara Werner
Zentrum für Messen und Kalibrieren -ANALYTIK- GmbH
D-06766 Wolfen, Germay
Email: info@zmk-wolfen.de
Petra Spitzer
Physikalisch-Technische Bundesanstalt (PTB)
D-38116 Braunschweig, Germany
Hans D. Jensen
Danish Institute of Fundamental Metrology (DFM)
DK-2800 Kgs. Lyngby, Denmark
1.2 Metrological Hierarchy
The metrological hierarchy is the basis for the metrological
infrastructure that is valid nationally and internationally.
1.2.1 Primary Measuring Cells
Primary measuring cells are a part of national standards at the
highest level of a calibration hierarchy in a country. For the
development of a calibration procedure in the DKD-K-06901,
these cells were important because there are commonalities
in the measurement with primary cells and standard cells.
(e.g., the determination of the conductance with impedance
measurement).
The measurement of electrolytic conductivity with primary
cells is traceable to the International System of Units (SI). There
is a differential measurement principle. Changes of cells constants
can be measured very exactly by dimensional measurements.
[1–4] Fringe effects can be canceled out. The cell
constant can be determined by geometrical measurements only.
1.2.2 Secondary Measuring Cells
The measurement of electrolytic conductivity by means of secondary
cells is also carried out in a large scale in national metrological
institutes. From the Danish Institute of Fundamental
Metrology (DFM) several standard measuring cells were developed.
[5, 6, 7]
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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)
TECHNICAL PAPERS
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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TECHNICAL PAPERS
G, µS
838.00
836.00
834.00
832.00
830.00
828.00
826.00
nificant frequencies were determined.
1.4 Temperature Measurement
Constant temperature of the measuring
cells is an important precondition for the
determination of reproducible measurement
results with low uncertainties.
The homogeneity of the thermostatic
bath must be assured. Usually thermometers
with platinum elements of
100 Ω (Pt100) or 25 Ω (Pt25)resistance
1/f, Hz –1
y = 970.1x + 837.7
R 2 = 0.9995
0.0000 0.0020 0.0040 0.0060 0.0080 0.0100 0.0120
Figure 2. Conductance as a function of the reciprocal value of frequency.
G, µS
28.67
28.66
28.65
28.64
28.63
28.62
28.61
28.60
28.59
28.58
28.57
0.0000 0.0100 0.0200 0.0300 0.0400 0.0500 0.0600
Figure 3. Determination of the cell constant.
1/f, Hz –1
y = 1.7862x + 28.673
R 2 = 0.9976
are used as temperature sensors. The
indication of the results is carried out with
resistance bridges (e.g., IsoTech Model
TTI-2). It is important that the measurement
uncertainty of the bath temperature
is not higher than U = ± 5 mK (k = 2).
2. Description of the Calibration
Procedure
As a result of the development project in
the DKD-K-06901 a calibration proce-
dure was developed that is based on the
use of standard measuring cells. Now it is
possible to calibrate reference solutions
for electrolytic conductivity over a range
from 2 µS/cm to > 100 mS/cm.
2.1 First Measurements with Standard
Measuring Cells
At the beginning, there was one standard
measuring cell available in the calibration
laboratory, DKD-K-06901. Because
of its cell constant (K = 1.67 cm -1 ), this
measuring cell was suitable for the measurement
in the range above 100 µS/cm.
The question arose as to whether it is
possible to also measure lower electrolytic
conductivities with this cell.
With decreasing electrolytic conductivity
increasing deviations from a strong linearity
were found for the conductance as
a function of the reciprocal value of the
frequency. Because of these strong deviations
from a good linearity this standard
measuring cell was not suitable for low
conductivities in the pure water range.
The development of a second cell with
a lower cell constant was necessary.
The cell constant of this second cell
(K = 0.1745 cm -1 ) was determined with
a certified reference solution (see Fig. 3).
During the development project, it was
apparent that the two standard measuring
cells are not enough to carry out a
measurement over such a wide range of
electrolytic conductivity from 2 µS/cm to
> 100 mS/cm. Altogether five standard
measuring cells with different cell constants
were used for the measurement of
electrolytic conductivity.
2.2 Standard Measuring Cells
The standard measuring cells are special
products (Sensortechnik Meinsberg
GmbH) manufactured on the basis of a
metrological conception. For a clear differentiation,
each cell got a letter from A
to E and a measuring range.
All five standard measuring cells
consist of a glass tube with two circular
measuring electrodes (platinum, diameter
20 mm). The distance between the
measuring electrodes is different from
cell to cell. The measuring cells are fixed
in a retaining device. A cover plate is
made of metal and has grips to set the
cell into the thermostatic bath. Connec-
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Figure 4. Standard measuring cell D (example) for the determination
of electrolytic conductivity.
2 connections
(for LCR-meter)
tion to the LCR meter is realized with four connector sockets.
Another connector socket is used for an earth ground cable (see
Figs. 4 and 5).
The realization of different cell constants was reached by
changing the distances and a treatment of the surface (platinising)
of the measuring electrodes. With it measurements in different
ranges of electrolytic conductivity are possible.
Vol. 3 No. 2 • June 2008
Openings for filling and emptying
Measuring
electrodes
(Pt plate)
Pt plate
Solder connection
(gold)
2 connections
(for LCR-meter)
Figure 5. Schematic image of a standard measuring cell in the
DKD-K-06901.
TECHNICAL PAPERS
The ranges where a measurement of electrolytic conductivity
is possible for each cell are overlapping. For a clear assignment,
the used ranges are smaller. Figure 6 gives an overview over the
standard measuring cells.
The measuring device consists of:
1. Five standard measuring cells.
2. LCR-meter (Agilent Model 4284A).
3. Precision thermostatic bath (oil bath, Lauda Proline Model
PV 36).
4. Temperature measuring device (temperature sensor Pt25
and temperature indication instrument IsoTech TTI-2).
2.3 Impedance Measurements with the LCR-meter
Impedance measurements in the calibration laboratory DKD-K-
06901 are carried out with the LCR-meter connected to the
standard measuring cells in 4-wire circuit. The cables used have
a better electromagnetic shielding in comparison with the original
cables from the manufacturer. With the LCR-meter, frequencies
in the range from 20 Hz to 1 MHz are realizable, but,
for our measurements, only a part of this range, from 20 Hz to
5 kHz, was used.
2.4 Stabilized Standard Measuring Cells and Temperature
Measurement
During the measurement the standard measuring cells are in the
precision thermostatic bath. It is important that the time of tempering
before the start of measurement is long enough. The temperature
sensor (SPRT Pt25) used for the temperature
measurement was calibrated at temperature fixed points. It is
used together with a temperature indication instrument TTI-2.
For the realization of low measuring uncertainties it was necessary
use a thermostatic bath with a low temperature inhomogeneity
for the tempering.
The spatial inhomogeneity of the bath was determined with
platinum resistance thermometers. One of these thermometers
is the sensor Pt25 also used for the temperature control during
the measurement. The sensor was fixed on a reference position
in the centre between two measuring electrodes. Another resistance
thermometer was positioned on two other measuring posi-
4 mm 6 mm 20 mm 60 mm 60 mm
Cell A Cell B Cell E
Operating range
Real measuring ranges are overlapping
Figure 6. Operating ranges of the standard measuring cells in the DKD-K-06901.
Plantinization
Cell C Cell D
2 µS/cm 15 µS/cm 100 µS/cm 1 mS/cm 20 mS/cm 100 mS/cm
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TECHNICAL PAPERS
H
Reference position (Pt 25)
during the determination of
spatial inhomogenity
tions one after another. A spatial inhomogeneity
of 2 mK was determined. Furthermore
a time stability of the
thermostatic bath of 3 mK was found.
An additional temperature control is
given by two calibrated liquid-in-glass
thermometers with a resolution of
0.01 K. The thermometers are positioned
in direct near of the standard measuring
cells (see Fig. 7).
2.5 Metrological Traceability/
Validation of the Calibration
Procedure
The standard measuring cells are traceable
to the primary measuring cells of the
National Institute of Standards and Technology
(NIST), the Physikalisch-Technische
Bundesanstalt (PTB) or the Danish
Institute of Fundamental Metrology
(DFM) by certified reference solutions as
transfer standards. In the frame of validation,
extensive national and international
comparisons were carried out. The
recognition in national and international
comparisons is the highest level of validation
of the developed metrological procedure
and is an important condition for
an international offering of reference
solutions of electrolytic conductivity as
high-quality products.
3. Conclusions
Finally it is possible to say that the conception/creation
of different standard
measuring cells with accommodation to
W
1 2 3 1 2 3
D
Figure 7. Positions of the temperature sensors for the determination of spatial inhomogeneity (H, W, D are dimensions height, width and
depth of the used volume).
H
defined measuring ranges is a new
variant of the metrological trace-ability
of electrolytic conductivity. The aim of
this work is to supply reference solutions
to users in the range from 2 µS/cm to >
100 mS/cm, thereby providing users
metrological traceability to the national
level.” The standard measuring cells represent
the level of reference standard in
the metrological hierarchy of the procedure.
With it the calibration of reference
solutions of electrolytic conductivity is
possible without a break in the given
measuring range. With this development
for the industry/laboratories, there is a
new possibility to purchase reference
solutions for the monitoring of the
processes and to save the accuracy of the
measuring results.
4. Acknowledgments
We thank the Landesförderinstitut
Sachsen-Anhalt (Germany) for the financial
support of the development project.
The authors would like to acknowledge
Dr. Reinhard Lange and Mr. Frank Seifert
from Sensortechnik Meinsberg GmbH
for their suggestions for the development
of the standard measuring cells.
5. References
[1] P. Spitzer and U. Sudmeier, “Electrolytic
conductivity – a new subject field at
PTB,” PTB-report PTB-ThEx-15, PTB,
Braunschweig, pp. 37–47, 2000.
[2] K.W. Pratt, W.F. Koch, Y.C. Wu, and
Position of the electrodes
(cell C and D)
during a measurement
P.A. Berezansky, “Molality-based
[3]
primary standards of electrolytic conductivity,”
Pure App. Chem., vol. 73, no.
11, pp. 1783–1793, 2001.
R.H. Shreiner and K.W. Pratt, “Standard
reference materials: Primary standards
and standard reference materials for
electrolytic conductivity,” NIST Special
Publication 260–142, 2004.
[4] Y.C. Wu, W.F. Koch, and K.W. Pratt,
“Proposed new electrolytic conductivity
primary standards for KCl solutions,” J.
Res. Natl. Stand. Technol., vol 96, pp.
191–201, 1991.
[5] H.D. Jensen and J. Sørensen, “Electrolytic
conductivity at DFM – results
and experiences,” PTB-Bericht PTB-
ThEx-15, Braunschweig, pp. 3–8, 2000.
[6] H.D. Jensen and C. Verdier, “Towards an
improved primary standard for electrolytic
conductivity,” presentation of the
Danish Institute of Fundamental Metrology
at the NCSLI Workshop and Symposium,
2001.
[7] H.D. Jensen and N.-E. Dam, “DFM
measurement capability: Electrolytic
conductivity,” DFM-report DFM-04-
[8]
R81, Lyngby, pp. 1–7, January 2005.
K. Rommel, “Leitfähigkeitsmessungen in
Elektrolyten – Die Wahl der richtigen Frequenz,”
off print from the Fachzeitschrift
für Labortechnik, vol. 12, 1981.
[9] “Guide to the Expression of Uncertainty
in Measurement (GUM),” ISO/IEC
Guide 98, International Organization for
Standardization, Geneva, 1995.
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D