Electricity: measuring and testing – Electrolyte properties – Using a conductivity determining device
Reexamination Certificate
1999-09-29
2002-04-09
Wong, Peter S. (Department: 2838)
Electricity: measuring and testing
Electrolyte properties
Using a conductivity determining device
C324S425000
Reexamination Certificate
active
06369579
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for determining the electrical conductivity of fluids by means of conductively operable conductivity measuring cells using analysis and processing measurement technology, wherein the conductivity of the fluid is determined from a measured resistance or impedance value and the geometric measuring cell constants.
BACKGROUND OF THE INVENTION
A conductively operable conductivity measuring cell is understood to mean one in which electrodes are disposed in immediate contact with the fluid to be measured. In order to determine the electrolytic conductivity of the fluid, the resistance or conductance of the electrode measurement path in the fluid is determined, for example, by using an alternating current measuring bridge. If the cell constant k is known, then the electrolytic conductivity ó can be determined.
In contrast to the conductive conductivity measurement, there is also the principle of inductive conductivity measurement through the use of an electrodeless conductivity measuring cell in which the fluid to be measured is used as an inductively acting coupling medium between an excitation coil and a measurement coil. The present invention, however, relates to the conductive conductivity measurement.
The physical interrelationships of current conduction in electrolyte fluids are as follows:
Depending on the dissociation behavior, the electrolytic components under consideration in the fluid are broken down to a greater or lesser extent into their ions (dissociated). These ions are responsible for the electrical current transport in the fluid. The actual degree of dissociation of an electrolyte therefore depends on the temperature and on the concentration of this material in the fluid. The carrier medium as such, for example the water in aqueous solutions, also contributes to the conductivity by means of dissociation. Over wide regions, the electrolytic conductivity increases along with the concentration of the electrolytes.
In addition to the determination of the pH value, the measurement of the electrolytic conductivity also represents a simple, high-precision means for analysis and process control, particularly with automated processes.
The electrolytic conductivity ó of a fluid is defined as the product of a geometric constant of the electrode measuring cell, the so-called cell constant k, and the reciprocal electrical, ohmic resistance of the fluid between the electrodes.
σ
=
k
·
1
R
For a measurement path with a length L and a cross sectional area A with a corresponding area A of the electrodes, the electrical resistance R that can be determined as a measurement quantity is produced as the product of the specific resistance ñ and the quotient length/area, i.e. L/A
R
=
ρ
·
L
A
Therefore the specific resistance ñ can be calculated from the electrical resistance measured since its reciprocal value is defined as the conductivity ó. Consequently the following is true:
ρ
=
R
·
A
L
σ
=
1
ρ
=
1
R
·
L
A
The geometric constant mentioned at the beginning (cell constant) of this particular electrode measurement path is therefore L/A.
The measurement range of a conductivity measuring cell is limited on the one hand to a high conductivity by the physical events of the phase transition from the solid metal electrode into the fluid. In the ideal case, this impedance of the phase transition behaves in a purely capacitive manner, in fact because of the electrochemical double layer being formed. Actually, however, the impedance of this phase transition also has a real, i.e. ohmic component. The measurement result is therefore distorted if this real part of the impedance of the phase transition is non-negligible in relation to the resistance of the fluid that is of interest. This limits the measurement range of a conductivity measuring cell toward the top since up till now, only the ohmic resistance of the fluid and not that of the phase transition from the solid metal electrode into the fluid was taken into consideration in the evaluation.
SUMMARY OF THE INVENTION
Based on this, the object of the current invention is to reduce errors in the determination of conductivity according to the process of the type mentioned at the beginning.
This object is attained by means of a process of the type mentioned at the beginning, which according to the present invention is characterized by means of the following process steps:
By using a measurement converter, the impedance of the measurement cell immersed in the fluid is determined in at least two frequency values of an alternating current; the frequency-independent parameters n, Q and the desired resistance R are determined from the determined impedance values, on the basis of an equivalent circuit diagram comprised of a parallel connection of a capacitor C representing the measuring cell capacitance, an ohmic resistance R
f1
representing the desired resistance of the fluid inside the conductivity measurement cell, and component Z that has an electric, frequency-independent parameter n, Q with a frequency-independent phase, wherein the latter R, Z are connected to each other in series.
After the desired resistance R
f1
has been determined in the above manner, the conductivity ó can be calculated based on the interrelationship described at the beginning between the resistance R
f1
and the conductivity ó through the use of cell constants k (&sgr;=1/R
f1
·k).
Since the physical events at the phase transition from the solid metal electrode to the fluid can be taken into account according to the present invention, by virtue of the fact that according to the present invention, the ohmic resistance R
f1
, which alone can be attributed to the measurement fluid that is of interest, is determined by means of calculation from a number of measurement values on the basis of the above-described equivalent circuit diagram, errors in the determination of the electrolytic conductivity in the region of the upper end of the given measurement range for a specific measurement cell device can be reduced. In other words, the conductivity measurement range of a particular measuring cell device can be extended upward. This can achieve a measurement range extension by a factor of 10.
According to a preferred embodiment of the present invention, the component which is intended to simulate the physical events in the region of the phase transition from the solid metal electrode to the fluid comprises two frequency-independent parameters and, in order to be able to calculate these parameters as well as the desired resistance, a measurement of the real part and the imaginary part of the impedance of the measurement cell immersed in the fluid is respectively determined at two frequency values.
However, if only measurement converters are available to carry out the process, in which the real part or the amount of the impedance of the measuring cell device immersed in the fluid can be determined, then impedance measurements in at least three frequency values are required.
It should also be noted at this point that the present invention turns out to be particularly advantageous for the measurement of the electrical conductivity of fluids in the measurement range of less than 200 &mgr;S/cm, in particular of less then 100 &mgr;S/cm, because by means of the measurement range extension according to the invention, calibrating solutions can use whose conductivity values previously could not lie within the measurement range of respectively considered conductivity measuring cell. To this extent, it turns out to be particularly advantageous that the conductivity measurement range, for example of a super-clean water measuring cell with a cell constant of 0.01 cm
−1
can be increased from approximately 20 &mgr;S/cm to 200 &mgr;S/cm. However, this offers the possibility of calibrated this conductivity measuring cell by using a calibrating solutions in the conductivity range from 100 to 200 &mgr;S/cm, for example by means of using calibrating solutions a
Endress +Hauser Conducta Gesellschaft fur Mess- und Regelte
Jones,Tullar & Cooper, P.C.
Luk Lawrence
Wong Peter S.
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