Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1998-04-27
2001-07-24
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
C073S861270, C340S620000, C340S621000, C367S908000
Reexamination Certificate
active
06263731
ABSTRACT:
The invention relates to a method for monitoring a predetermined level of a material in a container with the aid of two ultrasonic transducers mounted on the container in line with the level to be monitored such that an interspace exists between the ultrasonic transducers into which the material enters on attaining the level to be monitored, whereby the one ultrasonic transducer is an emitter transducer energized at predetermined points in time to emit ultrasonic pulses into the interspace whilst the other ultrasonic transducer is a detector transducer which converts the detected ultrasonic pulses into electrical detection signals, the analysis of which indicates whether material is in the interspace between the ultrasonic transducers. The invention relates further to an assembly for implementing the method.
In a related method known from U.S. Pat. No. 5,269,188 it is sensed whether the detector transducer outputs a detection signal after the point in time of emission corresponding to the transit time for ultrasonic pulses to pass through a liquid contained in the interspace between the sensors. For this purpose a main time window is formed in which the detection signal is required to appear for each speed of sound materializing in the liquid. In addition a test time window is formed ahead of the main time window in which a detection signal appears when the detector transducer detects an ultrasonic pulse which has been transmitted through the support of the two transducers from the emitter transducer to the detector transducer. This test is based on the speed of sound in the solid material comprising the support being greater than the speed of sound in the liquid. The existence or lack of a detection signal in the main time window with a detection signal simultaneously present in the test time window is used as a criterion of whether or not liquid exists in the interspace. If instead of liquid, air is in the interspace then it is assumed that no ultrasonic pulse has passed through the interspace to the detector transducer; this also being true at the high ultrasonic frequencies typically employed.
In the case of the related methods known from other publications (WO 82/04122; GB Patent 1,578,031; U.S. Pat. No. 4,630,245) the criterion of the interspace between the ultrasonic transducers being filled with a liquid or not is taken to be whether ultrasonic pulses arrive at the detector transducer at a time following each point in time of emission corresponding to the transit time for the ultrasonic pulses through the liquid.
Application of these known methods is thus restricted to monitoring the level of liquids having corresponding ultrasonic transmission properties, i.e. difficulties may arise, for example, when the liquid to be monitored is heavily aerated. Furthermore, no materials can be monitored by these known methods which fail to transmit ultrasonic waves, this applying more particularly to bulk (powdery or fine-grain) material such as meal, sand or the like.
The object of the invention is to define a method of the aforementioned kind which permits monitoring the level irrespective of the ultrasonic transmission properties of the material concerned.
In accordance with the invention this object is achieved in that the emitter transducer emitting the ultrasonic pulses is activated at a frequency which is so low that the ultrasonic pulses are transmitted through the interspace to the detector transducer even when the interspace is filled with air and that it is determined whether the detector transducer outputs after each point in time of emission a detection signal in an interval corresponding to the transit time of the ultrasonic pulses from the emitter transducer to the detector transducer in air.
In the method in accordance with the invention the arrival of ultrasonic pulses transmitted through the air in the interspace is taken as a criterion as to whether the interspace is filled with air or the material being monitored. This is possible when the frequency of the ultrasonic waves is sufficiently low, preferably smaller than approximately 300 kHz since ultrasonic waves of such a low frequency are still transmitted with sufficient amplitude even through air. The ultrasonic pulses transmitted through air can be explicitly distinguished from the ultrasonic pulses transmitted through a liquid due to their longer transit time, they permitting monitoring the level even then when the material to be monitored does not transmit ultrasonic waves, i.e. in the case of powdery or fine-grain solids, for example.
When the material is a liquid an advantageous further aspect of the method in accordance with the invention enables it to be additionally determined whether the detector transducer outputs after a point in time of emission a detection signal in a time interval corresponding to the transit time of the ultrasonic pulses from the emitter transducer to the detector transducer in the liquid, as a result of which high accuracy is attained because a detection signal indicating the existing condition needs to exist in each emission cycle in both the empty and filled condition of the interspace.
When, by contrast, the material has such a consistency that it does not transmit ultrasonic waves the accuracy can be enhanced by additionally establishing whether the detector transducer outputs a detection signal after each point in time of emission in a time interval corresponding to the transit time of the ultrasonic pulses from the emitter transducer to the detector transducer through the support of the two transducers.
An assembly for implementing the method in accordance with the invention comprising two ultrasonic transducers mounted on the container in line with the level to be monitored, such that an interspace exists between the ultrasonic transducers into which the material enters on attaining the level to be monitored, whereby the one ultrasonic transducer is an emitter transducer configured and arranged so that on being energized by an electrical alternating voltage pulse it emits ultrasonic pulses into the interspace whilst the other ultrasonic transducer is a detector transducer configured and arranged so that it converts the detected ultrasonic pulses into electrical detection signals, including an energizing circuit to energize the emitter transducer at predetermined points in time of emission to emit ultrasonic pulses, and an analyzer circuit for analyzing the electrical detection signals furnished by the detector transducer to determine whether the interspace is filled with the material to be monitored or not is characterized in accordance with the invention by the energizing circuit energizing the emitter transducer for emitting ultrasonic pulses at a frequency which is so low that they are transmitted through the interspace to the detector transducer even when the interspace is filled with air and by the analyzer circuit containing a means to distinguish whether the detector transducer outputs after each point in time of emission a detection signal in a time interval corresponding to the transit time of the ultrasonic pulses from the emitter transducer to the detector transducer in air.
In one advantageous aspect of this assembly the two ultrasonic transducers are piezoelectric transducers made of a porous piezoelectric ceramic having a connectivity of
3
—
3
. Such relatively compact ultrasonic transducers are able to operate in the necessary low-frequency ultrasonic frequency range of approximately 100 to 300 kHz without requiring furthermore an adapter film.
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p
Getman Igor
Lopatin Sergej
Bose McKinney & Evans LLP
Cygan Michael
Endress + Hauser GmbH + Co.
Williams Hezron
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