Electricity: measuring and testing – Electrolyte properties – Using a conductivity determining device
Patent
1989-12-22
1991-03-26
Eisenzopf, Reinhard J.
Electricity: measuring and testing
Electrolyte properties
Using a conductivity determining device
324445, 324450, G01N 2702
Patent
active
050032674
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus for measuring the impedance of a low conductivity sample. In an important example, the apparatus can be used to monitor low level microbial growth where it is typically necessary to measure electrical conductivities in the range 0.001 to 30 mS/cm.sup.-1.
2. Description of Related Art
It is well established that measurement of the electrical impedance of bacteria suspended in a growth medium can provide useful information concerning bacterial growth. This has been done in some cases by inserting electrodes in the sample and making a direct impedance measurement. Advantages have been seen, however, in employing non-invasive methods and a number of oscillometric techniques have been suggested which can readily be operated in a non-invasive manner, that is to say with no physical contact between the sample and any electrode of the measuring system. In these oscillometric techniques, a sample of the medium is disposed in relation to an electrical resonant circuit in such a manner that the frequency of reasonance is indicative of the sample impedance.
The known techniques often fall into two categories: inductive techniques where the sample is placed within a measurement coil, and capacitive techniques in which the test sample is placed between electrodes to form a capacitor. Inductive oscillometers perform best with highly condcutive samples and are therefore used only rarely in this and related apllications. A number of different arangements of capacitive oscillometers have been proposed but difficulties have been found with existing arrangements, particularly in their resolution. It is believed that for microbial suspension studies, a resolution is desirable of 1 in 10.sup.-8 or better.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved apparatus for measuring the electrical impedance of a low conductivity sample and in particular to provide apparatus having high resolution.
Accordingly, the present invention consists in apparatus for measuring the electrical impedance of a low conductivity sample, comprising a sample container having an electrically insulating wall; coil means positioned adjacent the container; electrode means provided on the external wall surface; feedback means serving to apply to the coil means a current related to the voltage sensed by the electrode means; and monitoring means for deriving an output indicative of the electrical impedance of the sample.
Advantageously, said monitoring means serves to determine the natural frequency of oscillation.
Preferably, electrical screening means are provided between said coil means and said electrode means.
In an important form of the invention, there is provided variable gain amplifier means between said electrode means and said coil means, the gain of said amplifier means being adapted to reduce with increasing frequency.
Advantageously, the gain of said amplifier is adapted to reduce with increasing amplitude of sensed voltage.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example with reference to the accompaning drawings in which:
FIG. 1 is a somewhat diagrammatic view of apparatus according to this invention;
FIG. 2 is an electrical circuit diagram of the apparatus shown in FIG. 1, including an equivalent circuit for the sample.
FIG. 3 is a graph illustrating the gain characteristics of an amplifier forming part of the circuit shown in FIG. 1;
FIG. 4 is a calibration curve for the apparatus of FIG. 1.
FIG. 5 is a diagram similar to FIG. 1, illustrating a modification; and
FIG. 6 is a part view of apparatus according to this invention illustrating a further modification.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a double toroid coil 10 is disposed about one end of an electricallly insulating sample tube 12. Each toroid comprises a single winding of 0.25mm insulated copper upon a ferrite core. The sample tube is formed of glass having in this example an
REFERENCES:
patent: 3417329 (1968-12-01), Landis et al.
patent: 3735247 (1973-05-01), Harker
patent: 4376026 (1983-03-01), Hoffman et al.
patent: 4876504 (1989-10-01), Blake et al.
Eisenzopf Reinhard J.
Public Health Laboratory Health Service Board
Regan Maura K.
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