Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
2001-09-10
2004-03-09
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C702S023000, C702S050000, C702S055000, C324S644000, C342S124000
Reexamination Certificate
active
06701783
ABSTRACT:
The present invention relates to a device, as well as a process, for determining the position of the border areas between different mediums, specifically for determining the layer thickness of the uppermost of two superimposed filling materials inside a container by means of electromagnetic waves.
Devices and processes of this kind are known from WO 00/43739 and WO 00/43806, among other sources. To be sure, these documents describe only a functional dependence of the dielectric constants &egr;
r
on the reflection factor, and thus the relation of the voltage that returns on the cable to the departing voltage. Furthermore, neither of these documents discloses attenuation losses in the line.
The present invention is therefore based on the problem of elaborating the processes and devices named in the two documents, in such a way that the border areas between the two mediums can be more precisely determined.
First, however, the technological background of the present invention will be elucidated.
For some time the measurement of filling-levels in industry has employed measuring systems which precisely determine the distance between a sensor and the filling material, as based on the measured transit time of electromagnetic waves that travel from a sensor mounted on the container lid above the filling material to the surface of the filling material and back again. Thus, given the container height, conclusions can be reached about the filling level in the container. Sensors of this kind, which are known as filling-level radar sensors, are based overall on the property exhibited by electromagnetic waves of propagating at constant speed within a homogeneous, non-conductive medium and of being at least partially reflected at the border area between different mediums.
Different radar principles are known to the prior art for determining the wave transit time. The two principally applied methods are pulse radar and FMCW radar. Pulse radar makes use of the pulsed amplitude modulation of the emitted wave and determines the direct period of time between transmission and reception of the pulses. FMCW radar determines the transit time indirectly by transmitting a frequency-modulated signal and ascertaining the difference between the transmitted and the received momentary frequency.
In addition to the different radar methods, various frequency ranges can be used for the electromagnetic waves, depending on the application. For example, there are pulse radar systems with carrier frequencies in the range between 5 and 30 GHz, as well as those that operate in the base band as so-called monopulse radar systems without a carrier frequency.
Furthermore, a series of processes and devices are known which conduct the electromagnetic wave to the surface of the filling material and back again. A basic distinction is made here between a wave emitted into space and one conducted through a cable. Examples of the first type have an antenna that emits the wave with a sufficient degree of focus in the direction of the filling material and then receives it back again. This kind of sensor system is described, e.g., in DE 42 40 492 C2. Radar sensors which guide the electromagnetic wave through a cable to the reflection point and back again are often referred to as TDR (time domain reflectometry) sensors. The cable employed here can have any form customary in high frequency technology. By way of example, single-wire cables, as described in DE 44 04 745, may be mentioned, as well as waveguides, as described in DE 44 19 462.
In addition to the conventional filling-level radar measurements, which determine only the position of the border area between the filling material and the gaseous space above it (air in the usual containers), there are applications in which the goal is determine the position of the bordering layer between two different filling materials, or the layer thickness of the upper layer. Since every border layer between two mediums with different dielectric constants produces an echo, a radar sensor in this case will receive reflections from several points. In addition to the usual reflection at the border area between the gas and the uppermost filling material, an echo will arise at the border between the two filling materials. Under certain circumstances, other echoes may follow from other border areas of filling materials and also from (metallic) container floors. With the appropriate signal evaluation it is possible in any case to clearly identify the echo that results from the reflection at the gas/uppermost filling material border and the one that results from the reflection at the border leading to the next filling material. The sought after layer thickness of the uppermost filling material can be determined from the interval of time that separates the two echoes if the propagation speed of the wave-within this filling material is known. This propagation speed v depends on the dielectric constants &egr;
r
of the filling material and the permeability &mgr;
r
of the filling material. The following formula applies in a calculation based on the propagation speed V
0
in a vaccum.
v
=
v
0
·
1
ϵ
r
·
μ
r
(
equation
1
)
Since the filling materials almost never have a magnetic property the permeability is known (&mgr;
r
=1), and the dielectric constant remains the only unknown. In the past it has often been very difficult to determine this constant, since the user of filling-level sensors frequently has no knowledge of the material properties of the filling material. In addition, many containers are alternately filled with materials whose dielectric constants differ, and this necessitates continuous correction through renewed input of the value. Heretofore a further problem has resulted from the fact that the dielectric constant of many mediums is both temperature-dependent and also dependent on the frequency of the electromagnetic wave. Thus even if this material constant is known for a given temperature and a defined frequency range, for example several kilohertz, it can be assumed that for other filling material temperatures and sensor frequencies in the high and maximum frequency range the measuring result based on this predetermined value for the constant will not provide an exact outcome.
With the present invention it is possible to avoid the manual input of the dielectric constants that has heretofore been necessary in measuring separating layers with electromagnetic waves. Instead, a process is proposed, along with a device corresponding to this process, which makes it possible to determine the actual parameters that are dependent on the filling material and that are needed for ascertaining the layer thickness.
Furthermore this invention can be applied when a radar sensor works according to a process like that described in DE 42 33 324. Instead of directly determining the position of a filling level surface from a reflection that under certain circumstances may be relatively weak for filling materials with a low dielectric constant, the echo from the container floor is located, which is usually strong in this case. With a knowledge of the dielectric constants and the distance to the floor for an empty container it is easy to ascertain the filling height of the container.
Whereas the distance to the floor can be measured by a sensor without difficulty in the case of an empty container, or can be input a single time, the description just given applies to the dielectric constant. For radar determination of the filling level according to this method of floor tracking, the present invention also allows input of the dielectric constant to be advantageously replaced by a internal determination of the needed computing factor using a sensor.
With the present invention it is thus possible to determine the dielectric constant from the reflection factor at the border area of the filling material whose dielectric constant is being sought. The reflection factor, in turn, can be determined by measuring the echo amplitude, while incorporating a knowledg
Fehrenbach Josef
Griessbaum Karl
Berkowitz Marvin C.
Miller Rose M.
Nath & Associates PLLC
Vega Grieshaber KG
Williams Hezron
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