Experiences with Novel Secondary Conductivity Sensors within the ...

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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. MEASURE | 63
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 procedure 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- 64 | MEASURE www.ncsli.org
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