Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2001-11-07
2004-05-25
Deb, Anjan K. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S674000, C324S685000, C324S689000, C324S453000
Reexamination Certificate
active
06741083
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention refers to an oscillating device to determine the purity of single or multi-component liquids, on the basis of their dielectric permittivity, in a continuous form under strict temperature control and through frequency changes in the static permittivity region and to the associated measurement procedure.
More precisely it refers to an oscillator to determine in a continuous fashion the degree of coincidence of the global composition of a liquid, either single or as a mixture of two or more components with a previously established standard, by means of a high sensitivity oscillating circuit containing a temperature controlled capacitance cell.
For this purpose it is based on dielectric permittivity measurements (previously called dielectric constant, until this denomination was changed by the International Union of Single and Applied Chemistry, as stated in the 2
nd
Edition of Quantities, Units and Symbols in Physical Chemistry, published in 1993) taking into account that it is one of the physical properties that is most sensitive to the composition of liquids.
2. Brief Description of the Prior Art
The verification and continuous following of the global composition of a liquid, either single or multi-component, during production processes, at the end of it and before its use in the final destination, is one of the most important technologic problems. Especially because, very frequently, a detailed knowledge of the composition is not needed and it is only important whether or not it is within the requirements of a previously, in general physical, established standard.
Under these conditions devices that require the taking of a sample and its later analysis in a place, near or far; from the production line, are not adequate because they imply either the interruption of the process until the answer is available or the risk that an out of specification product continues in the process or is used in its final application.
This is the typical case of solvents, liquid fuels, liquid lubricants, transformer liquids and isolating liquids in general.
As is known, the permittivity of any given substance is obtained from measurements in capacitors comparing their capacitance values in vacuum and containing the substance to be examined through the following equality:
∈=
C
f
/C
e
wherein ∈ is the permittivity, C
f
, C
e
respectively the capacitance of the filled and empty capacitor. Actually the latter value corresponds to the air filled capacitor since for all practical purposes there is no substantial difference between the vacuum permittivity, taken as unity and that of atmospheric air, which under normal conditions of temperature and pressure is ∈=1, 00059.
To determine these capacitances RLC, LC or RC oscillating circuits are used, among other devices, having oscillating frequencies that run from fractions of Hertz to Gigaherts, that are clearly in the microwave region.
Depending on the frequency measuring range used for permittivity determination, the latter can be divided into two well differentiated classes: static and complex permittivities. Static permittivity is determined at frequencies never exceeding 5 MHz, although in general operations are carried out below 1 MHz. These values depend on the size of the molecules of the substance being examined because this is what determines their ability to “follow” the oscillations of the applied electric field. In other words, plotting permittivity as a function of frequency, the first region (0 Hz to 5 MHz) will be represented by a straight line parallel to the abscissa (frequency) axis. Once this limit is surpassed the permittivity becomes complex being the result of the sum of two components, one real and the other imaginary, because the molecules oscillation frequency suffers a progressive phase shift with regards to the oscillation frequency of the electric field. This is the so called relaxation phenomenon that is in turn the fundamental characteristic of this region of the frequency spectrum and its amplitude changes with the size of the molecules involved.
Both the theory and the oscillators used in the determination of both types of permittivities are completely different. In general the former (static permittivity) are far more accessible and simpler than the latter (complex permittivity). In any case both have very important fields of application that vary depending on the nature of the problems to be solved.
The most important advantage of the static permittivity determinations are: their evident independence from the oscillation frequency with the consequent ease to reproduce measured values, the low influence of residual capacitances (resulting from cables, connections and eventual empty spaces in the capacitor) and the precision and high sensitivity that can be obtained with the modem circuits based on integrated electronic components. This is not the case of the complex permittivity because determinations of similar quality have not been achieved.
This latter situation is the result of the well known fact that while with complex permittivity and relaxation, determinations in general do not provide results with better than the second decimal, in the case of static permittivity the fourth decimal can be reached without difficulties.
Therefore static permittivity becomes an ideal parameter to establish macroscopic characteristics of substances (especially fluids) and compare them with preestablished appropriate standards.
Presently known procedures, that provide these results, are based on electronic circuits of the RLC type wherein the capacitance (C) is formed by a number of capacitors in parallel: a measuring cell, a standard capacitor and a micrometric capacitor. This is quite adequate for discontinuous measurements and although the sensitivity and precision are acceptable, the standard and micrometric capacitor, as mechanical devices, are a source of important difficulties, such as: mechanical backlash, sensitivity to ambient temperature, complexity in the connections that require a rigidity not easy to achieve (any movement during the measuring procedure causes appreciable changes in the results), the impossibility to develop any type of automatic processing or continuous recording, preventing the use of computers and the need to make several capacitance measurements before the desired permittivity can be calculated.
Regarding this latter mentioned fact it is necessary to take into account and review briefly the manner in which the measurements are carried out, in what can be called the traditional way, that is using capacitors through the so called substitution method.
As stated above an oscillator is used the capacity of which is formed by three capacitors in parallel:
a measuring cell
a standard capacitor, that can be any one available in the market as for instance of the General Radio brand (although any other one of similar characteristics is equally acceptable). These capacitors have a range that covers from 150 to 1000 picoFarads (pF) and a sensitivity of 0.5 pF.
a micrometric capacitor, formed by a micrometric screw, of the caliper type, with a 3 to 4 cm stem that can be introduced in a bronze cylinder the inside diameter of which is machined so that with the outer diameter of the stem a 2 pF capacitor is obtained for a length of 20 divisions of the screw.
In general the substitution method comprises the steps of:
determining the capacitance of the circuit without the three mentioned capacitors,
determining the capacitance of the empty cell, subtracting from it the value of the so called residual capacitance, that corresponds to that part of the cell that is not in contact with the liquid to be examined and all the connections that lead to the oscillator,
determining the capacity of the cell once the dielectric has been introduced.
The instruments used comprise two oscillators. One (fixed) is based on a crystal of oscillating frequency chosen between 100 and 500 kHz, depending on what is available regarding st
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