GB2472237A

GB2472237A - Ultrasonic Non-Penetration Level Measurement

Info

Publication number: GB2472237A
Application number: GB0913233A
Authority: GB
Inventors: Jingdong Zhang
Original assignee: Mobrey Ltd
Current assignee: Rosemount Measurement Ltd
Priority date: 2009-07-30
Filing date: 2009-07-30
Publication date: 2011-02-02
Anticipated expiration: 2029-07-30
Also published as: GB2472237B; GB0913233D0

Abstract

An ultrasound non-penetration level sensing method and apparatus as applied to the outer wall of a tank. A level sensing transducer 10 is mounted to the exterior of a tank, the tank wall having an outer surface 18, an inner surface 19 and a thickness `t'. The transducer 10 has a transducer body 20, one end surface 21 being the surface from which ultrasonic energy is transmitted and by which reflected energy is received. A sealed chamber 23 is defined at least between surface 21 and adjacent surface part 24 of the tank outer wall 18. The chamber 23 is filled with an acoustic coupling fluid. A further aspect of the invention resides in the manner in which operating frequencies are selected to maximise the difference between wet and dry reflected signals.

Description

IMPROVEMENTS IN OR RELA TING TO LEVEL MEASUREMENT

Field of the invention

This invention relates to level measurement and is applicable, in particular, to level measurement using ultrasound.

Background

The use of ultrasound to measure the level of contents in tanks and spiliways has been known for a considerable period of time, two examples of this type of apparatus being described and claimed in US patent 4,596,144 (Federal Industries) and UK patent 2,230,608 (Hycontrol). These rely on pulses of ultrasound being directed toward the surface of the tank contents, reflected from the surface back to the source of the pulses, and the time difference between the transmission and receipt of the pulses being converted into a measure of the distance between the ultrasound transmitter and the surface.

The above form of apparatus require direct exposure of the tank contents to the ultrasound source. For some contents, particularly highly flammable contents, this is undesirable and can be dangerous.

Level sensors are currently available in which a sensor is attached to the outer surface of the tank and measurement pulses, both transmitted and reflected, are passed through the tank wall. The size of the reflected pulse varies according to whether the tank contents are or are not present within the tank at the level at which the sensor is attached to the outer surface of the tank. A greater signal is reflected when the contents of the tank are below the level of the sensor than when the contents are at or above the level of the sensor.

An example of a sensor of this general type is described in US Patent 4,630,245. Such sensors are commonly referred to in the art as non-penetration sensors and variations are available based on radar, capacitance and ultrasonic technologies.

In general, radar-based non-penetration level measurement systems are expensive and suffer from the particular problem that they cannot be applied to metal tanks.

Capacitance-based systems are less expensive but, similarly, cannot be applied to metal tanks.

Ultrasonic-based non-penetration level measurement systems can be applied to metal tanks and are generally relatively low in cost, reliable, and simple to install. However, existing ultrasonic-based systems exhibit two significant shortcomings. Firstly, the coupling between the sensor and the tank wall is invariably unreliable. Currently available sensors use a thin layer of oil or water-based acoustic coupling gel between the ultrasound transducer, and the tank wall. With the passage of time, and changes in temperature, this thin coupling layer gradually disappears and/or looses its functionality which, in turn, leads to the sensor providing incorrect outputs. A second problem with existing systems is that they are too sensitive to their environment. Incorrect readings can be obtained if the sensor is mounted too close to the top or to the bottom of the tank.

Similarly, existing sensors are very sensitive to stresses imposed or induced by the installation, condensation on the inner wall of the tank, the geometry of the tank and the like. The net effect of all of these shortcomings is that the difference in signal between wet' and dry' conditions is too small to process effectively and reliably.

It is an object of the invention to provide an ultrasonic-based non-penetration level sensing method and apparatus which will go at least some way to addressing the aforementioned drawbacks; or which will at least provide a novel and useful choice.

Summary of the Invention

Accordingly, in one aspect, the invention provides a method of providing a non-penetration level sensing apparatus on a tank having an inner and an outer wall surface, using an ultrasound transducer having a transducer body and a signal transmitting and receiving surface, said method comprising mounting said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and providing a coupling medium between said transmitting and receiving surface, and said outer wall; said method being characterized in that it further includes defining a sealed chamber enclosing the space between said transmitting and receiving surface, and said outer wall.

Preferably said method further includes sealingly fixing a mounting cavity to said outer wall surface; and at least partly locating said transducer body within said mounting cavity.

Preferably said mounting cavity is defined by a tubular member, said method comprising fixing said tubular member to said outer wall surface. The method of fixing will depend on the materials of the tubular member and of the outer wall surface. If both are of metal, fixing may be effected by welding.

Preferably said method is applied to an ultrasound transducer having driving electronics, said method further including causing said driving electronics to drive said transducer in pulse mode with a carrying frequency so as to substantially satisfy the expression: = nX/2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.

Preferably said method includes causing said driving electronics to drive said transducer at frequencies in the range 50kHz to 20MHz.

Preferably said method further includes measuring the amplitude of a reflected signal in a selected time interval after the signal has been transmitted from said ultrasound transducer; and determining the presence of a liquid in the tank, at the level of said transducer, by applying a threshold to the amplitude in said time interval. The threshold may be applied to a single reflected pulse or to a profile of reflected pulses.

In a second aspect, the invention provides a non-penetration level sensing apparatus for a tank having an inner and an outer wall surface, said apparatus including an ultrasound transducer having a transducer body and a signal transmitting and receiving surface; a mounting facility to, in use, mount said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and a coupling medium between said transmitting and receiving surface, and said outer wall; said apparatus being characterized in that said mounting facility defines a sealed chamber enclosing the space between said transmitting and receiving surface and said outer wall.

In a third aspect, the invention provides a method of providing a non-penetration level sensor on a tank having an inner and an outer wall surface separated by a wall thickness, using an ultrasound transducer having a transducer body and a signal transmitting and receiving surface, said method comprising mounting said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and providing a coupling medium between said transmitting and receiving surface, and said outer wall surface; said method being characterized in that it includes driving said transducer with a carrying frequency so as to substantially satisfy the expression: t = nX/2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and ?. is the wavelength of the ultrasound wave in the material defining the tank wall.

In a fourth aspect the invention provides a non-penetration ultrasonic level sensor for application to the exterior of a tank having a wall thickness, said sensor being characterized in that it is configured to emit ultrasound in pulse mode whose carrying frequency can be adjusted to satisfy the following expression: -nAJ2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.

Many variations in the way the invention may be performed will present themselves to those skilled in the art, upon reading the following description. The description should not be regarded as limiting but rather as an illustration, only, of one manner of performing the invention. Where appropriate any element or component should be taken as including any or all equivalents thereof whether or not specifically mentioned.

Brief Description of the Drawings

One preferred method of, and apparatus for, reducing the present invention to practice will now be described with reference to the accompanying drawings in which: Figure 1: shows a schematic diagram of a non-penetration level sensor according to the invention; Figure 2: shows an elevational, part-sectional view of one means of mounting the ultrasound transducer in Figure 1; Figure 3: shows a theoretical plot of reflection coefficient against frequency to illustrate another aspect of the invention; Figure 4: shows an experimentally derived echo signal array from a level switch according to the invention working in a dry condition; Figure 5: shows the same as in Figure 4 but with the switch sensing a wet condition; Figure 6: shows the 3rd echo of a received signal with its FFT spectrum, in a dry condition; and Figure 7: shows the same as in Figure 6 but in a wet condition.

Detailed Description of Working Embodiment

Referring firstly to Figure 1, the invention provides a non-penetration based method and apparatus for establishing a wet or dry state within a tank. In the known manner, ultrasound pulses are transmitted from an ultrasound transducer through the tank wall and, depending on the reflected signal, an assessment is made as to whether fluid in present in the tank at the level of the transducer. The transducer may be incorporated into a switching circuit to start or stop a pump, or to otherwise initiate some other process control function connected with the particular level.

The present invention advances the known art by providing a method and apparatus for mounting the ultrasound transducer so as to ensure the reliable ultrasound coupling of the transducer to the outer wall of the tank. In another aspect the invention provides a method of establishing the frequency of the transmitted ultrasound energy so as to highlight the difference in reflection losses between a dry and a wet condition.

The basic components of a non-penetration sensor according to the invention are shown in Figure 1 and comprise an ultrasound transducer 10 acoustically coupled to the outer wall of a tank 11 in a manner which will be described in detail below. An adjustable frequency driving circuit 12 under the control of microprocessor 13 determines the frequency of the ultrasound energy transmitted by the transducer 10. The transducer 10 also acts as a receiver of echo signals reflected back from the tank. Reflected signals are separated out from the transmit signals in splitter 14, and are passed to a wide band receiver 15. A threshold may then be applied at 16 to give an output representing a wet or dry state, within the tank 11, at the vertical level of the transducer 10.

Referring now to Figure 2, and as stated above, the manner in which the transducer 10 is mounted to the exterior of the tank 11 is a characterising feature of the invention. Figure 2 shows a connection between the transducer 10 and a section of the tank wall, the tank wall having an outer surface 18, an inner surface 19, and a thickness t'. The transducer has a transducer body 20, one end-surface 21 of the body 20 being the surface from which ultrasonic energy is transmitted and by which reflected energy is received. As can be seen, the surface 21 is aligned substantially parallel to the adjacent surface of the tank 11. The transducer 10 is operatively connected to a housing 22 which contains the components and circuitry shown schematically as 12, 13, 14, 15 and 16 in Figure 1.

Broadly, one aspect of the invention comprises defining a sealed chamber 23 at least between the surface 21 and adjacent surface part 24 of the tank outer wall 18. In the particular embodiment shown, for simplification of the mounting and sealing, the chamber 23 also extends to surrounding part of the longitudinal wall section 25 of the transducer body 20.

To define chamber 23, a section 26 of tube is sealingly fixed to the tank outer surface 18.

If the tank and tube are both formed from metal, fixing can conveniently be effected by welding along weld line 27. Other forms of fixing will present themselves to those skilled in the art, given the nature of the materials employed, and include adhesives and mechanical fastenings.

As can be seen the section 26 of tube has an annular flange 28 about the outer end thereof. A further tubular section 29, which serves to locate and retain the transducer body 20, is also provided with an annular flange 30 about the front edge. Upon assembly the flanges 28 and 30 are bolted together with a sealing gasket 31 there-between. A seal is also created between the outer surface of the transducer body an the inner surface of the tubular section 29, to complete the chamber 23. This latter seal is conveniently effected using 0-rings 32.

In the form shown, the housing 22 is conveniently mounted on the rear of the tubular section 29.

The chamber 23 is filled with an acoustic coupling fluid such as oil or grease. The coupling fluid is conveniently inserted through hole 34, and the hole sealed with a bung 35, after all the described components have been assembled and sealed together. The coupling fluid should be free of air bubbles and be selected to retain its acoustic properties in the working temperature range of the sensor. A soft foam sleeve 33 with a sealed surface may be provided within chamber 23 to accommodate variations in the volume of coupling fluid with temperature.

By way of example, the mounting section 26 may have an outer diameter of 50 mm and a projecting length of about 80 -100 mm. The transducer body 20 may have an outer diameter of about 3 0-40 mm and be mounted within section 26 so that the spacing between the surface 21 of the transducer, and the tank outer surface 18 is in the order of I -50mm. The precise spacing depends on the working frequency of the transducer.

The transducer can be any type of acoustic transducer which has a wide frequency band.

A piezoelectric transducer is preferred because of simplicity and cost. The transducer is preferably operated at a frequency above 50kHz to minimise the effects from the top and bottom of the tank, and the tank geometry, on the detection function. Further, different transducers with different working frequencies can be chosen to suit tanks of different wall thickness, it being appreciated that the selection of transducer involves a trade-off between band width and signal strength.

As is well known, these types of apparatus measure the reflected signal amplitude in a selected time interval after the signal has been transmitted. The signal amplitude in the selected time interval is compared with a pre-determined threshold to determine whether gas (normally air) or liquid is present within the tank, at the level of the transducer 10.

The determination of wet' or dry' may be made by applying a threshold -either to a particular reflected pulse or to a profile of reflected pulses.

To experimentally test the arrangement shown in Figure 2, a 5MHz Panametric transducer was mounted over a steel plate of 6 mm thickness. Acoustic coupling to the plate was effected using Soundsafe', a water-based couplant gel supplied by Diagnostic Sonar Limited (see www.diagnosticsonar.com). The emitting surface of the transducer was spaced about 10 mm from the surface of the plate and the metal tube in which the transducer was mounted was butted to the plate. A Panametrics 5072PR pulsed gate was used to generate and pass narrow pulses into the transducer, and to receive the reflected signals.

Figures 4 and 5 show plots of amplitude versus time. Comparing the amplitudes of the 6th reflections, indicated at 40, it will be noted that there is an amplitude difference of 0.3V of the wet over the dry, a reduction of one third of the amplitude of the original dry signal.

Turning now to Figures 6 and 7, these show time-related plots 41, together with FFT (Fast Fourier Transforms) 42 of those plots. Comparing the dry situation (Figure 6) with the wet situation (Figure 7), at the 3' reflection 43, it can be seen that the maximum change of signal strength at 6.95 MHz (iY1) is 9.3 dB. This provides an adequate difference for available processing equipment to detect and initiate a switching function.

In order to maximize the difference between wet and dry signals, we have found that the greatest distinction between dry and wet is achieved if the ultrasound transducer is driven with a carrying frequency so as to satisfy the expression: t = nXJ2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.

This is illustrated in Figure 3. In Figure 3 a theoretical plot is derived where a plane wave with a sweeping frequency transmits on to a steel plate of 8mm in thickness. For this exercise, castor oil is used as the acoustic couplant while water and air are used as the media on the opposite side of the plate to represent wet and dry states respectively.

Figure 3 indicates that the lowest reflection coefficients are obtained in a narrow band about defined frequencies, these having an integral number of half-wavelengths which are equal to the plate thickness.

Claims

Claims A method of providing a non-penetration level sensing apparatus on a tank having an inner and an outer wall surface, using an ultrasound transducer having a transducer body and a signal transmitting and receiving surface, said method comprising mounting said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and providing a coupling medium between said transmitting and receiving surfaces, and said outer wall; said method being characterized in that it further includes defining a sealed chamber enclosing the space between said transmitting and receiving surface, and said outer wall.
2. A method as claimed in claim 1 further including sealingly fixing a mounting cavity to said outer wall surface; and at least partly locating said transducer body within said mounting cavity.
3. A method as claimed in claim 2 wherein said mounting cavity is defined by a tubular member, said method comprising fixing said tubular member to said outer wall surface.
4. A method as claimed in any one of claims ito 3 when applied to an ultrasound transducer having driving electronics, said method further including causing said driving electronics to drive said transducer in pulse mode with a carrying frequency so as to substantially satisfy the expression: t = nJ2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.
5. A method as claimed in claim 4 including causing said driving electronics to drive said transducer at frequencies in the range 50kHz to 20MHz.
6 A method as claimed in any one of the preceding claims further including measuring the amplitude of a reflected signal in a selected time interval after the signal has been transmitted from said ultrasound transducer; and determining the presence of a liquid in the tank, at the level of said transducer, by applying a threshold to the amplitude in said time interval.
7. A method as claimed in claim 6 wherein said threshold may be applied to a single reflected pulse or to a profile of reflected pulses.
8. A non-penetration level sensing apparatus for a tank having an inner and an outer wall surface, said apparatus including an ultrasound transducer having a transducer body and a signal transmitting and receiving surface; a mounting facility to, in use, mount said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and a coupling medium between said transmitting and receiving surface, and said outer wall; said apparatus being characterized in that said mounting facility defines a sealed chamber enclosing the space between said transmitting and receiving surface and said outer wall.
9. A method of providing a non-penetration level sensor on a tank having an inner and an outer wall surface separated by a wall thickness, using an ultrasound transducer having a transducer body and a signal transmitting and receiving surface, said method comprising mounting said transducer body adjacent to said outer wall surface such that said signal transmitting and receiving surface is substantially parallel to said outer wall surface; and providing a coupling medium between said transmitting and receiving surfaces, and said outer wall; said method being characterized in that it includes driving said transducer with a carrying frequency so as to substantially satisfy the expression: nIJ2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.
10. A non-penetration ultrasonic level sensor for application to the exterior of a tank having a wall thickness, said switch being characterized in that it is configured to emit ultrasound in pulse mode whose carrying frequency can be adjusted to satisfy the expression: -n?i2 where: t is the wall thickness of the tank (spacing between said inner and outer surfaces); n is an integer; and X is the wavelength of the ultrasound wave in the material defining the tank wall.