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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 MEASURE | 65
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. 66 | MEASURE www.ncsli.org W D
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