Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2000-04-21
2002-12-17
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S667000, C324S675000, C324S682000, C324S765010, C324S678000, C073S30400R
Reexamination Certificate
active
06496020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus for determining the capacitance of a dielectric medium.
2. State of the Art
The on-line and real-time measurement of the biomass content of fermentations is still an active area of research. New sensors are continuously being developed and the already established technologies are constantly being improved. The reason for this need for innovation is that the fermentation industry works with a very broad range of cell types using a very wide range of culturing conditions and under an ever-growing blanket of regulatory requirements.
To fit easily into an already-established fermentation installation, a biomass monitoring system must be generic and capable of working via the currently available probe ports, must not pose a significant contamination risk, be capable of withstanding in-situ the high temperatures and pressures or caustic nature of sterilisation processes, and the probe materials must be inert. The equipment must be supplied in a form that can easily be incorporated into (and survive in) a fermentation hall environment. This is true before any consideration can be given to its ability to work with the cell system being monitored.
The fermentation broth itself is a particularly hostile environment for any sensor to work in. The growth of the cells and the feeding and control regimes used ensure that it is an ever changing environment. The presence of vigorous and fluctuating aeration, and time-dependent temperature profiles, are a particular problem. The medium used can be highly viscous and often contains a wide range of non-biomass solids and immiscible liquids, particularly at the start of the fermentation. The medium's constitution can change markedly as the cells grow and consume its components.
An ideal measuring system should be capable of working with a wide variety of cell types, ranging from bacterial cells and yeasts, through filamentous fungi and bacteria to animal and plant cells, in both free and immobilised forms. The ability to measure a wide range of biomass concentrations is important, as is the inclusion of an in-situ cleaning system that can remove cellular growth on the sensing probe.
A widening body of literature has demonstrated that the measurement of biomass by use of a capacitive probe provides the best and most generic method in practical situations. Sensor methods based on two-pin electrodes and electromagnetic coupling are being developed but as yet have not been refined enough to work in anything other than laboratory model systems. The most highly developed technology that has found practical utility in industry is the Biomass Monitor (BM). This instrument fulfils the majority of the criteria outlined above and published work has shown it to work well for bacteria, yeast, filamentous fungi and bacteria, animal and plant cells, immobilised cells, for solid substrate fermentations of filamentous fungi, and in assessing cytotoxicity. Its major applications in industry so far have been for controlling the pitching of yeast slurries in brewing and for monitoring microbial fermentations in the pharmaceutical industry.
Details of biological dielectrics and the theory behind capacitive (dielectric) biomass measurements are well known to those skilled in the art. For the purposes of this specification a simplified heuristic model will be described. Cells in a suspension can be regarded as having a three-component structure. Outside and inside the cells is a conducting aqueous ionic medium, the former being the suspension medium the latter the cell cytoplasm. Surrounding the conducting cell core is the thin essentially non-conducting plasma membrane. This means that a cell suspension can be regarded, from an electrical point of view, as a suspension of spherical capacitors containing a conducting matrix surrounded by a conducting suspension medium. To make measurements on this system, an electric field is applied via a set of electrodes. The resulting electrical current paths have two routes through the suspension, either around the cells via the external conductance or through them via the membrane capacitance and internal and external conductances. At low radio-frequencies and below (<0.1 MHz), the cell membrane has a very low admittance, most of the current flows around the cells and, as the membrane capacitance is nearly fully charged, the capacitance of the suspension is very high. The more cells that are present per unit volume, the more spherical capacitors are charged and so the higher is the capacitance of the suspension. The low-frequency capacitance gives a measure of cellular volume fraction. As non-biomass material (including necromass) lacks an intact plasma-membrane, it does not give a significant capacitance contribution. At frequencies above 10 MHz, the membrane capacitance is shorted out, the induced charge held by the membranes is very low and so the capacitance of the suspension approaches that of the water in the suspending medium.
From these arguments, one expects the capacitance of a cell suspension to go from a high low-frequency plateau to a low high-frequency one. This fall in capacitance is called the &bgr;-dispersion (FIG.
1
). The high-frequency residual capacitance due mainly to water dipoles is called C
∞
. The height of the low frequency plateau above C
∞
is called the capacitance increment &Dgr;C
&bgr;
and its magnitude is proportional to the biomass content of the suspension. The frequency when the fall from &Dgr;C
&bgr;
+C
∞
to C
∞
is half completed is called the critical frequency f
c
. The steepness with which capacitance falls as frequency increases is described by the Cole-Cole &agr; value. This has values in the range 0=<&agr;<1 and is supposed to reflect the distribution of relaxation times in the suspension due to heterogeneity. Shown on
FIG. 1
are the curves for &agr; equals 0 (no distribution of relaxation times) and &agr; equals 0.2. Increasing &agr; from 0 does not change &Dgr;C
62
, f
c
or C
∞
, its major effect on the a equals 0.2 plot shown on the figure is that in the frequency window shown the low-frequency plateau is not achieved.
From these arguments, one can see that to estimate the biomass in a fermentation broth, all that is necessary is to measure the &Dgr;C
&bgr;
of the suspension. On the BM this is done in either of two ways.
FIG. 2
line (a) shows a &bgr;-dispersion with a spot measuring frequency typically 0.4 MHz) marked by an arrow and the capacitance at that frequency marked by a dot on the curve. As can be seen, the capacitance at the measuring frequency is a good approximation of the &Dgr;C
&bgr;
and hence biomass concentration. In reality, what is done for these single-frequency biomass measurements, and what has been done on this Figure, is to back off the capacitance due to the suspending medium to zero at the spot measuring frequency prior to inoculation. Then any change in the capacitance at that frequency reflect changes in &Dgr;C
&bgr;
and hence biomass concentration. The second capacitive biomass method uses two frequencies, the spot measuring frequency as before and also a high frequency reference (10 MHz). From
FIG. 2
it will be seen that the difference in the capacitance between the spot measuring frequency (0.4 MHz) and that at 10 MHz also gives a good estimate of &Dgr;C
&bgr;
.
For both methods to work reliably the spot biomass measuring frequency should be well into the low-frequency plateau of the &bgr;-dispersion, since the f
c
of the dispersion can move with changes in the medium conductance. If the measuring frequency were on the falling part of the dispersion, then movements in the f
c
could cause significant changes in the capacitance measured at the spot frequency and result in corresponding errors in the biomass determination.
To be well onto the plateau means that for most &bgr;-dispersions it is necessary to use a measuring frequency below 0.5 MHz. However, this forces the use of a frequenc
Davey Christopher Lyndon
Kell Douglas Bruce
Gallagher Thomas A.
Gordon David P.
Hamdan Wasseem H.
Jacobson David S.
Le N.
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