Electricity: measuring and testing – Electrolyte properties – Using a conductivity determining device
Reexamination Certificate
1998-07-10
2002-07-30
Le, N. (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
Using a conductivity determining device
C324S694000
Reexamination Certificate
active
06426629
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method of measuring an electrical parameter of a fluid. In particular the present invention concerns the measurement of the electrical conductivity of fluid.
BACKGROUND OF THE INVENTION
Electrical conductivity is a property of a fluid which may be used in the identification of fluids and in the measurement of the concentration of certain constituents of a solution or mixture of different materials, including bubbles and solids in a fluid medium. The principle of such concentration measurement is that the constituents have a different electrical conductivity as compared to that of the base material, such that the electrical conductivity of the combination varies with the concentration.
Electrical conductivity is measured by passing an electrical current through the fluid and calculating the electrical resistance from the voltage drop across the fluid divided by the current. This is done in a device called a conductivity cell, which contains at least two electrodes separated by the fluid to be measured. The electrical current enters and leaves the fluid by means of electrodes, which are normally arranged so that they are electrically insulated from one another apart from the fluid path.
The electrical conductance is calculated as the inverse of the electrical resistance. Thus, electrical conductance depends not only on the electrical conductivity, but also on the geometry of the electrical path through the fluid and any additional electrical paths (which are normally minimized). In many situations, the conductance C is related to the conductivity P by a simple constant k (which is usually termed the cell constant) as shown in equation 1.
&rgr;=
kc
1
This relationship follows from simple electrical theory, which states that the conductance equals the conductivity times the conductor cross-sectional area divided by the conductor length. In other words, the cell constant equals the conductor length divided by the conductor cross sectional area. Conventionally, the cell constant is established by calibrating the conductivity cell with fluid of known conductivity—typically potassium chloride.
To prevent polarization of the electrodes, whereby an excess (or deficit) of charge builds up at and around the point at which the electrical signal enters and leaves the fluid, alternating current is conventionally used instead of direct current.
Conductivity cells may be arranged to accept a flowing fluid or to measure a static pool of fluid according to the nature of the application.
The conductivity of a fluid depends on temperature and a change of about 2% per degree centigrade is fairly typical. Therefore, in many conductivity measuring devices, a temperature probe is used to measure the temperature, so that a correction may be made for any difference between the actual measurement temperature and the reference temperature (which is often 25° C.).
However, accurate temperature compensation is difficult for a number of reasons. The first of these is that the point and time at which the temperature is measured is not (in general) the same point and time at which the conductivity is measured. Thus, any change in the temperature between the temperature measurement and the conductivity measurement results in an error. This problem is exacerbated by the fact that both the temperature measurement and the conductivity measurement usually generate heat as a by product of the measurement technique. At low flow rates, these heating effects have a greater affect on the temperature of the fluid and, at the same time, the temperature probe must be better isolated from the environment to ensure that is neither added nor removed from the fluid by the temperature probe. The lower the flow rate, however, the greater the errors introduced by these problems such that the accuracy which can be obtained with known techniques reduces as the flow rate reduces.
These problems also affect the calibration of the cell when the temperature must also be determined very accurately. This means that the errors are encountered twice in the total measurement process, rather than just at the time of the conductivity measurement of the test fluid. In addition, if the variation of the conductivity with temperature is not accurately known for both the calibration fluid and the test fluid, further errors may be generated in the temperature correction process.
An additional problem with conductivity cells is that the cell constant may change with time and regular re-calibration is required if accurate performance is to be maintained.
PCT Application No. WO-A-9 604 401 discloses methods and apparatus for measuring the differential conductivity of decomposed and undecomposed urea solutions. The present invention is also applicable to such application, and the entire disclosure of WO-A-9 604 401 is incorporated herein by reference thereto.
SUMMARY OF THE INVENTION
In accordance with the present invention, these and other objects have now been accomplished by the discovery of a method for measuring an electrical parameter of a test fluid comprising maintaining the test fluid and a reference fluid at substantially the same temperature and in thermal proximity to each other, equalizing the temperature of the test fluid and a reference fluid prior to the measuring step, measuring the electrical parameter of the test fluid and the reference fluid at substantially the same time, and standardizing the measurement to a predetermined temperature. Preferably, measuring of the electrical parameters is carried out in the measurement cell. In a preferred embodiment, the standardizing of the measurement to the predetermined temperature is carried out on the basis of the relationship:
&rgr;
b
(&thgr;)=&rgr;
b
&rgr;
a
(&thgr;)−&rgr;
a
)
wherein &thgr; comprises a standard temperature,
&rgr;
b
(&thgr;) comprises the electrical parameter of the test fluid at the standard temperature,
&rgr;
a
(&thgr;) comprises the electrical parameter of the reference fluid at the standard temperature,
&rgr;
b
comprises the electrical parameter of the test fluid at the measured temperature, and
&rgr;
a
comprises the electrical parameter of the reference fluid at the measured temperature.
In accordance with one embodiment of the method of the present invention, the reference fluid comprises a calibration fluid.
In accordance with another embodiment of the method of the present invention, the reference fluid is provided by subjecting the test fluid to a predetermined process wherein the electrical parameter is altered. In a preferred embodiment, measuring of the electrical parameter comprises comparing the electrical parameter of a test fluid to the electrical parameter of the reference fluid.
In accordance with a preferred embodiment of the method of the present invention, standardizing of the measurement to the predetermined temperature is determined on the basis of the relationship:
&rgr;
b
(&thgr;)−&rgr;
a
(&thgr;)=&rgr;
b
−&rgr;
a
+25(&agr;
b
−&agr;
a
)−
T
(&agr;
b
−&agr;
a
)
wherein T comprises the measurement temperature,
&agr;
b
comprises the temperature coefficient of the electrical parameter of the test fluid, and
&agr;
a
comprises the temperature coefficient of the electrical parameter of the reference fluid.
In accordance with another embodiment of the method of the present invention, the electrical parameter comprises conductivity. In accordance with another embodiment, the method includes disposing the measurement cell in thermal proximity to flowing primary heat exchange fluid. In a preferred embodiment, the measurement cell is thermally symmetrical about a plane wherein the test fluid and the reference fluid are symmetrically disposed about that plane.
In accordance with another embodiment of the method of the present invention, the method includes providing the primary exchange fluid to the measurement cell and withdrawing the primary heat exchange fluid from the measurement cell in that plane. In accordance w
Edgson Raymond
Wilkinson Eric
Gambro AB
Kerveros James
Le N.
Lerner David Littenberg Krumholz & Mentlik LLP
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