Measuring and testing – Liquid level or depth gauge
Reexamination Certificate
1998-07-03
2001-03-20
Williams, Hezron (Department: 2856)
Measuring and testing
Liquid level or depth gauge
Reexamination Certificate
active
06202484
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a method for determining a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity. The invention also relates to a device for carrying out the method, having a computer unit which extrapolates from reference sound velocities to the mean sound velocity and is used to calculate the liquid level.
A particularly exact determination of the liquid level is required, for example, in the case of an underground dispensing system for fuel or generally in the case of a container for dangerous substances. In that case stringent environmental conditions require virtually any leakage to be detected and removed. In the case of a large tank with a cross-sectional area of 1 to 10 m
2
, even an erroneous measurement of the liquid level of only 1 mm leads to an erroneous determination of a liquid volume of 1 to 10 l.
A contactless ultrasonic measuring system which is known for the purpose of monitoring a fuel tank determines the liquid level with the aid of an echo time measurement of ultrasonic pulses, reflected at a liquid surface, in accordance with the echo sounding principle. It is known, moreover, to make use of a piezoceramic ultrasonic transducer which is suitable as a single component both for transmitting and for receiving ultrasonic pulses, for the purpose of measuring the echo time.
In order to calculate the filled level from the measured echo time, it is necessary to know the velocity of propagation of the ultrasonic pulses in the liquid. However, it is precisely in a large container such as a fuel dispensing system, that the density and the temperature in the liquid are not constant over the entire container volume. Rather, a density and temperature gradient forms in the vertical direction.
Since the sound velocity is a function of density and temperature, it is necessary to perform a correction of the sound velocity in order to determine the liquid level with a tolerance in the mm range. Again, no general specification of the sound velocity is possible for inhomogeneous liquids having a composition which fluctuates.
A solution which is known from U.S. Pat No. 4,748,846 is to measure the sound velocity with the aid of reference paths of known length directly in the liquid. For that purpose, additional reflectors for ultrasound are disposed at prescribed spacings in the vertical direction along the actual measuring path, along which an ultrasonic transducer emits. In order to calculate the liquid level, use is made of a mean sound velocity of liquid determined with the aid of the reflector respectively located closest to the liquid surface. That is an average sound velocity, since averaging is performed in the vertical direction over the height.
U.S. Pat. No. 3,394,589 also discloses the determination of a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity.
The accuracy achievable with the method becomes greater, with a smaller spacing between the successive reflectors.
However, as a disadvantage it is necessary to distinguish between reflections from the liquid surface and reflections from fixed reflectors, in a complicated signal detection method with the assistance of electronic masking times.
Limits to the achievable accuracy are therefore set. That holds even when reflectors are disposed separately next to the actual measuring path (U.S. Pat. No. 5,095,748), since even then the reflectors cannot be disposed at any desired density because of multiple reflections.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a method and a device for determining a liquid level with the aid of ultrasonic pulses, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, in which the method determines a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity and in particular achieves a measuring accuracy of less than one millimeter independently of the liquid level, and in which the device carries out the method with the aid of particularly expedient and simple measures.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for determining a liquid level in a container from an echo time measurement of ultrasonic pulses and a mean sound velocity, which comprises measuring a reference sound velocity for each of at least two reference paths located in a liquid; determining a characteristic of a mean sound velocity in the vertical direction over a height by extrapolation from the at least two reference sound velocities; and using the characteristic to reach a conclusion beyond actual measuring points concerning the mean sound velocity averaged up to a liquid level, and determining the liquid level therefrom by an echo time of ultrasonic pulses reflected at a liquid surface.
In this case the invention proceeds from the finding that temperature and density are varying but continuous functions in the liquid, in particular in the vertical direction. As a result, a continuous variation in the sound velocity in the vertical direction is also to be expected in the liquid. The sound velocity in the liquid can therefore be described as a function of the level above the container floor. This function can be determined and the variation of the sound velocity can thereby be extrapolated beyond the measuring points by measuring at least two reference sound velocities at different levels in the container. In particular, it thereby becomes possible to specify very exactly the mean sound velocity averaged in the vertical direction from the container floor up to the liquid level. It is likewise also possible to determine the characteristic of the mean sound velocity over height by measuring at least two reference sound velocities from the vertically aligned reference paths of different length with the same base point, and to extrapolate from this characteristic to the mean sound velocity averaged up to the liquid level.
In accordance with another mode of the invention, the determination of the liquid level takes place recursively by determining from an estimated value for the liquid level an approximated mean sound velocity which is used for recursive improvement of the estimated value for the liquid level, and the improved estimated value is equated to the liquid level. As a result of this method, the estimated value for the liquid height approaches the actual value asymptotically. The method can be terminated once the desired accuracy for the liquid level has been reached. As a rule, the estimated value of the liquid level is already corrected in the submillimeter range in a second path, with the result that it is possible to achieve a measuring accuracy of 1 mm for the liquid level after termination of the method after the first path.
In accordance with a further mode of the invention, it is particularly elegant and less computation-intensive when the extrapolation of the mean sound velocity includes a linear approximation. Since, as a rule, the characteristic of the sound velocity is not linear over height, a skillful selection of the measuring points is preferably made. In this case, two measuring points are sufficient, in particular, for reference sound velocities if one measuring point is located in the vicinity of the container floor and one measuring point is located in the vicinity of the liquid surface. The average sound velocity is yielded in this case by integrating the approximated linear characteristic of the sound velocity over height. However, it is also possible to determine the characteristic of the mean sound velocity directly with the aid of vertical reference paths. It is skillful in this case to use the shortest and the longest reference path which still dips in as a measuring range.
In accordance with an added mode of the invention, due to temperature fluctuations it is possible, precisely in long-life liquid containers,
Eisentraudt Michael
Willner Siegfried
Greenber Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
Williams Hezron
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