GB2199948A - Measurement of liquid levels by resonating gas or vapour above the liquid - Google Patents

Abstract

A method or apparatus for measuring a liquid level, particularly in an enclosed container (1) which operates to vibrate a column of gas or vapour (4) above the liquid (2) at a resonant frequency thereof, to detect such frequency and to calculate the liquid level and optionally volume there from. The resonant frequency is preferably detected by a microphone (6) and a microprocessor (8) stores the information required to perform the necessary calculation. The column of gas or vapour is vibrated in organ-pipe manner either with an aperture and a lip or by a reed. The apparatus may be used to control inlet and outlet valves in transport containers. <IMAGE>

Description

Measurement of liquid levels This invention relates to liquid level measuring apparatus and methods and relates particularly but not exclusively to an apparatus and method suitable for measuring the liquid level in an enclosed container such as one forming part of or carried by a road, rail or sea tanker or the like.

Traditionally liquid level measurement in e.g. a road tanker has been carried out manually by means of a dip-stick which has obvious practical disadvantages.

Whilst various means have been proposed for automatically measuring or monitoring liquid levels in the form of e.g. ultrasonic level guages and the like, many of these are relatively complex mechanically and/or electronically, and some earlier proposals also suffer from lack of adapility e.g. for use with different types of liquid. For example, one earlier system involves vibrating a rod which is partiality immersed in the liquid and then detecting the natural resonance frequency of the rod; such frequency is dependent upon the length of rod above and below the liquid. However, such an arrangement cannot readily adapt to different liquids since changes in liquid density will affect the calculation which has to be applied by processing means to deduce the liquid level from the measured frequency.

Viewed from one aspect the invention provides a method of measuring a liquid level which comprises vibrating a column of gas or vapour above the liquid at a resonant frequency thereof, detecting such frequency, and calculating there from the liquid level.

Viewed from a second aspect the invention provides apparatus for measuring a liquid level which comprises means for vibrating a column of gas or vapour above the liquid at a resonant frequency thereof, means for detecting such frequency, and processing means for calculating therefrom the liquid level.

It will be appreciated that the frequency at which resonance of a gas or vapour column occurs depends on its length and thus in accordance with the invention on the level of the upper surface of the liquid which defines the lower edge of the gas or vapour column. Since the length of the gas or vapour column depends on the level of the upper surface of the liquid the system is unaffected by changes in liquid density. It will be appreciated that a gas or vapour column has a fundamental resonant frequency and also a number of harmonics of higher frequency. The method and apparatus in accordance with the invention preferably operates to vibrate the column at its fundamental resonant frequency, which will provide the greatest amplitude response, although alternatively the gas or vapour column could be vibrated only at one or more of its harmonics to determine its length.

The invention is applicable to enclosed containers, in which case the gas or vapour column which is vibrated may be defined by an upper part of the container itself, above the liquid therein. Alternatively, a conduit may be provided which extends into the liquid and which defines a gas or vapour column above the liquid. In the latter arrangement, the invention may be used to measure liquid levels in either closed or open containers or indeed in other environments, since there is no need for a container holding the liquid to define a volume there above in which gas or vapour is vibrated.

The means for vibrating the gas or vapour column at its resonant frequency may take any convenient form. In the presently preferred arrangement, such means operates on the "organ pipe" principle, i.e. a stream of gas is passed through a recess in an aperture formed in one end of a conduit defining the gas or vapour column, such stream impinging on a lip so as to set up vortex motion which in turn causes resonance vibration of the gas or vapour column. Alternatively, a vibrating reed could be arranged periodically to interrupt an inlet gas stream, which likewise sets up resonance vibrations of the column. It is also envisaged that a scanning tone generator or the like could be used to establish resonance vibrations of the column.

The means whereby the resonant frequency is detected likewise may take any convenient form, and the presently preferred system uses a microphone located in the vicinity of the gas or vapour column.

It will be appreciated that various factors need to be considered in calculating liquid level from the resonant frequency of the gas or vapour column thereabove, including temperature (which affects the speed of sound in the gas or vapour and thus affects the relationship between frequency and wavelength of the resonance vibration which is used to calculate column length), information concerning the relationship between the gas or vapour column length and liquid level height for e.g. a container to which the invention is applied, and information as to whether the column is open ended or closed since this affects the relationship between the wavelength of the resonance vibrations and the gas or vapour column length.Other factors which might need to be taken into account include pressure variations, and, in a preferred system which is further adapted to provide an indication of the total volume in a container, information relating to the relationship between liquid level and volume for the size and shape of the container concerned. In a preferred embodiment such information is prestored in processing means in the form of a microprocessor programmed to calculate liquid level, and optionally the corresponding volume, from the detected resonant frequency.

In the presently preferred embodiment discussed above which operates on the "organ pipe" principle, it is necessary for the processing means to calculate the detected frequency e.g. by counting the detected cycles against time. In an alternative arrangement where resonance vibrations are established by means of a scanning tone generator, it is envisaged that the processing means might be adapted to detect a maximum in the magnitude of the vibrations of the column indicative that the tone generator frequency is equal to a resonant frequency of the column.

The resonant frequency may thus be deduced from the drive frequency of the tone generator rather than from an analysis of the detected response.

The method and apparatus in accordance with this invention is applicable to containers forming part of road, rail and sea tankers and the like for measuring the volume of the contents thereof, in which case the processing means can be adapted to display information to the operator and may also be used to control other operations such as automatic opening and closing of inlet and outlet valves when certain liquid levels are reached.

The invention is however applicable to any other contexts where it is desired to measure or monitor liquid levels or volumes.

Viewed from a further aspect the invention provides a liquid transport container of a road, rail or sea tanker or the like incorporating liquid level measuring apparatus as aforesaid.

Viewed from a broader aspect the invention encompasses a method or apparatus which operates to detect a liquid level by vibrating gas or vapour above the liquid at a resonant frequency thereof.

An embodiment of the invention will be described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a schematic cross-sectional view through a liquid container e.g. forming part of a road tanker which incorporates apparatus in accordance with an embodiment of the invention; Fig. 2 is a schematic diagram of the processing means; Fig. 3 is a schematic diagram illustrating the calculation applied by the processing means.

Referring firstly to Fig. 1, an enclosed container 1 contains a liquid 2. Mounted in an upper part of the container and extending downwardly into the liquid therein there is a tubular conduit 3 the upper part of which defines therein a column of air 4. It will be appreciated that the lower end of the air column 4 is defined by the upper surface of the liquid 2, and therefore the column length depends on the liquid level. The upper end of the conduit 3 receives a stream of air 5 which is forced past a lip and aperture which, as is well known in the field of organ pipes and other musical instruments, establishes vibrations of the air column 4 at one or more resonant frequences thereof.

The sound produced by the resonance vibrations of the air column 4 is detected by a microphone 6 connected via a screened lead 7 to processing means in the form of microprocessor 8.

The operation of the processing means is illustrated schematically in Figs. 2 and 3. Thus, the output from the microphone 6, together with the output from a temperature sensor (not shown in Fig. 1) is fed to a conditioning unit 9 which converts the detected signals to ones acceptable to a CPU 10. The CPU calculates the detected frequency and, from the measured temperature, the speed of sound in the gas or vapour column 4. From the detected frequency and calculated sound velocity, the wavelength of the resonance vibrations can be deduced in accordance with the following equation: v = where v is the speed of sound, f is the frequency and X is the wavelength of the vibrations.Once the wavelength has been calculated, then the length of the gas column can be calculated in accordance with the following equation in the case of a pipe having an open upper end 1 = n 2 where 1 is the length of the column, n is an integer corresponding to the number of the harmonic (= 1 for the fundamental vibrations).

The factors in the equations will vary if a different method is used for producing the resonance.

Once the length of the vibrating air column has been calculated, the liquid level and thus the volume contained in the container can be calculated from prestored information concerning the shape and size of the container and the position of the conduit 4 therein.

Modifications may be apparent to persons of relevant skill to the apparatus and method described herein, and the disclosure hereof is intended to encompass such modifications even if certain of the features presently described and/or illustrated are omitted.

Claims (33)

CLAIMS:

1. A method of measuring a liquid level which comprises vibrating a column of gas or vapour above the liquid at a resonant frequency thereof, detecting such frequency, and calculating there from the liquid level.

2. A method as claimed in claim 1 which operates to vibrate the column at its fundamental resonant frequency.

3. A method as claimed in claim 1 or 2 wherein the gas or vapour column is vibrated at one or more of its harmonics.

4. A method as claimed in any of claims 1,2 or 3 applied to an enclosed container, wherein the gas or vapour column which is vibrated is defined by an upper part of the container, above the liquid therein.

5. A method as claimed in any of claims 1, 2 or 3 wherein a conduit is provided which extends into the liquid and which defines a gas or vapour column above the liquid.

6. A method as claimed in any preceding claim wherein a stream of gas is passed through a recess in an aperture formed in one end of a conduit or container defining the gas or vapour column, such stream impinging on a lip so as to set up vortex motion which in turn causes resonance vibration of the gas or vapour column.

7. A method as claimed in any of claims 1 to 5 wherein a vibrating reed is arranged periodically to interrupt an inlet gas stream, setting up resonance vibrations of the column.

8. A method as claimed in any of claims 1 to 5 wherein a scanning tone generator is used to establish resonance vibrations of the column.

9. A method as claimed in any preceding claim wherein the means whereby the resonant frequency is detected comprises a microphone located in the vicinity of the gas or vapour column.

10. A method as claimed in any preceding claim wherein information is prestored in processing means in the.form of a microprocessor programmed to calculate the liquid level, and optionally the corresponding volume, from the detected resonant frequency.

11. A method as claimed in claim 10 wherein the detected frequency is calculated by the processing means.

12. A method as claimed in claim 10 wherein the processing means is adapted to detect a maximum in the magnitude of the vibrations of the column.

13. A method as claimed in claim 8 wherein the resonant frequency is deduced from the drive frequency of the tone generator.

14. A method as claimed in any of claims 10 to 13 wherein the processing means is adapted to display information to the operator or to control other operations.

15. A method which operates to detect a liquid level by vibrating gas or vapour above the liquid at a resonant frequency thereof.

16. Apparatus for measuring a liquid level which comprises means for vibrating a column of gas or vapour above the liquid at a resonant frequency thereof, means for detecting such frequency, and processing means for calculating there from the liquid level.

17. Apparatus as claimed in claim 16 which operates to vibrate the column at its fundamental resonant frequency.

18. Apparatus as claimed in claim 16 or 17 wherein the gas or vapour column is vibrated at one or more of its harmonics.

19. Apparatus as claimed in any of claims 16 to 18 applied to an enclosed container wherein the gas or vapour column which is vibrated is defined by an upper part of the container above the liquid therein.

20. Apparatus as claimed in any of claims 16 to 18 wherein a conduit is provided which extends into the liquid and which defines a gas or vapour column above the liquid.

21. Apparatus as claimed in any of claims 16 to 20 wherein a stream of gas is passed through a recess in an aperture formed in one end of a conduit or container defining the gas or vapour column, such stream impinging on a lip so as to set up vortex motion which in turn causes resonance vibration of the gas or vapour column.

22. Apparatus as claimed in any of claims 16 to 20 wherein a vibrating reed is arranged periodically to interrupt an inlet gas stream, setting up resonance vibrations of the column.

23. Apparatus as claimed in any of claims 16 to 20 wherein a scanning tone generator is used to establish resonance vibrations of the column.

24. Apparatus as claimed in any of claims 16 to 23 wherein the means whereby the resonant frequency is detected comprises a microphone located in the vicinity of the gas or vapour column.

25. Apparatus as claimed in any of claims 16 to 24 wherein information is prestored in processing means in the form of a microprocessor programmed to calculate the liquid level, and optionally the corresponding volume, from the detected resonant frequency.

26. Apparatus as claimed in claim 25 wherein the detected frequency is calculated by the processing means.

27. Apparatus as claimed in claim 25 wherein the processing means is adapted to detect a maximum in the magnitude of the vibrations of the column.

28. Apparatus as claimed in claim 23 wherein the resonant frequency is deduced from the drive frequency of the tone generator.

29. Apparatus as claimed in any of claims 25 to 28 wherein the processing means is adapted to display information to the operator or to control other operations.

30. Apparatus which operates to detect a liquid level by vibrating gas or vapour above the liquid at a resonant frequency thereof.

31. Apparatus for measuring a liquid level substantially as herein described with reference to the accompanying drawings.

32. A liquid transport container incorporating liquid level measuring apparatus as claimed in any of claims 16 to 31.

33. A method of measuring a liquid level substantially as herein described with reference to the accompanying drawings.