EP0577679A1 - Acoustic flowmeter - Google Patents

Acoustic flowmeter

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Publication number
EP0577679A1
EP0577679A1 EP19920907504 EP92907504A EP0577679A1 EP 0577679 A1 EP0577679 A1 EP 0577679A1 EP 19920907504 EP19920907504 EP 19920907504 EP 92907504 A EP92907504 A EP 92907504A EP 0577679 A1 EP0577679 A1 EP 0577679A1
Authority
EP
European Patent Office
Prior art keywords
signal
signals
generate
frequency
acoustic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19920907504
Other languages
German (de)
French (fr)
Inventor
Lawrence Anthony Endress + Hauser Limited Jones
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Endress and Hauser Ltd
Original Assignee
Endress and Hauser Ltd
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Endress and Hauser Ltd filed Critical Endress and Hauser Ltd
Publication of EP0577679A1 publication Critical patent/EP0577679A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
    • G01F1/00—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow
    • G01F1/66—Measuring the volume flow or mass flow of fluid or fluent solid material wherein the fluid passes through a meter in a continuous flow by measuring frequency, phase shift or propagation time of electromagnetic or other waves, e.g. using ultrasonic flowmeters
    • G01F1/667—Arrangements of transducers for ultrasonic flowmeters; Circuits for operating ultrasonic flowmeters

Definitions

  • the present invention relates to an acoustic flowmeter.
  • Acoustic flowmeters are used to measure the rate of flow of for example a gas within a pipe.
  • Such flowmeters use conventional techniques to launch an acoustic signal into the flow from a suitable transducer and to detect the signal after it has traversed a predetermined path through the flow.
  • the signal is detected by a transducer which is of course also sensitive to acoustic noise propagating through the flow.
  • a transducer which is of course also sensitive to acoustic noise propagating through the flow.
  • Unfortunately it is generally the case that large acoustic disturbances are generated in pipes through which fluids are flowing and this background noise can be sufficient to swamp the acoustic signal which is deliberately propagated through the flow.
  • acoustic flowmeters must be capable of operating with very low signal to noise ratios and as a result practical systems must rely upon complex signal processing techniques. These techniques can only be implemented at considerable cost and as a result acoustic flowmeters have not been widely used. It would be possible to improve the signal to noise ratio by introducing a band pass filter into the signal processing system.
  • the sort of frequencies that have been proposed for use in flowmeters are typically from a few hundred Hertz to a few thousand Hertz. The reason for this is that provided the acoustic wave length is larger than the diameter of the pipe through which the monitored flow is passing the performance of the flowmeter is independent of the flow profile.
  • the Weaver method relies upon the use of low pass filters to reject unwanted sidebands. It is an object of the present invention to apply low pass filters to the problems outlined above with regard to the signal to noise ratio encountered in acoustic flowmeters.
  • an acoustic flowmeter comprising an acoustic signal source, means for launching a signal output from the source into a flow path, means for detecting the signal after transmission through the flow path, means for measuring the time taken for the signal to be transmitted through the flow path, and means for calculating the rate of flow within the flow from the measured transmission time
  • the signal to noise ratio of the eighth signal is very much larger than the signal to noise ratio of the first signal as a result of rejection of unwanted signal components in the low pass filters.
  • low pass filters having the required filter characteristics.
  • commercial low pass elliptic switched capacitor filters are readily available.
  • the system in accordance with the invention may be regarded as a translation of the low pass filter to the frequency of the second signal together with a mirroring of the low pass filter characteristic around the second frequency.
  • the result is equivalent in performance to the provision of a band pass filter with its centre frequency at the frequency of the second signal and a band width equal to twice the cut off frequency of the low pass filters.
  • the eighth signal is applied to a phase locked oscillator to provide in effect a memory the content of which is available in the event of a temporary loss of signal.
  • Figure 1 is the acoustic spectrum of noise in a pipe through which a fluid is flowing and into which a signal at 316.25 Hz has been launched;
  • Figure 2 illustrates the signal to noise ratio achieved using a simple acoustic pick up to detect the signal represented in Figure 1;
  • Figure 3 illustrates the improvement in the signal to noise ratio which can be achieved using a narrow band-pass filter;
  • Figure 4 illustrates an embodiment of the present invention which enables characteristics such as those illustrated in Figure 3 to be achieved;
  • Figure 5 illustrates the characteristic performance of low pass filters illustrated in Figure 4 and the equivalent band-pass filter characteristics achieved by the use of low pass filters in Figure 4;
  • Figure 6 illustrates a circuit for generating the fixed frequency signals utilised in the embodiment of Figure 4; and
  • Figure 7 illustrates results obtained with an embodiment of the present invention.
  • this illustrates the acoustic spectrum of noise in a pipe together with a signal of 316.25 Hz launched into that pipe.
  • the launched signal is detected by a conventional transducer and if the output of that transducer is not further processed the signal to noise ratio is very poor due to the high level of the noise floor as illustrated in Figure 2.
  • Figure 3 illustrates how the signal to noise ratio could be greatly enhanced if it was possible to provide a narrow band pass filter centred on the wanted signal frequency of 316.25 Hz.
  • the improvement in the signal to noise ratio is a corollary of the narrow band noise floor.
  • the signals illustrated in Figures 1 and 2 are those which are obtained using conventional acoustic flowmeter techniques in a noisy environment.
  • the conditions illustrated in Figure 3 are unfortunately not achievable at sensible cost as it is not possible to produce a band pass filter with a suitably sharp narrow pass band. At the frequencies shown typically a pass band of only 6 Hertz would be required.
  • FIG 4 this illustrates an embodiment of the invention in which the wanted signal that is launched into the monitored flow is at a frequency of 1758 Hz.
  • the signal is received by a loudspeaker configured as a microphone and the output of that transducer is applied as a first signal to an input 1.
  • the first signal is the combination of the wanted signal at 1758 Hz and the accompanying acoustic noise.
  • the detector circuitry illustrated in Figure 4 comprises a local oscillator the output of which is at a frequency 1762 Hz and is applied to input 2.
  • the local oscillator signal is applied to a phase shifter 3 having outputs 4 and 5 to which signals at 1762 Hz are applied in quadrature. That is to say there is a 90° phase shift between the signals appearing on outputs 4 and 5.
  • the signal on output 4 represents a second signal which is multiplied in a mixer 6 with the first signal to generate a third signal that is applied to a low pass filter 7.
  • the signal on output 5 represents a fourth signal which is applied to a mixer 8 with the first signal so as to produce a fifth signal that is applied to a low pass filter 9.
  • the mixers 6 and 8 may be for . example LM1596/LM1496 balanced modulator - demodulators available from National Semiconductor Corporation and the filters 7 and 9 may be LTC1064-1 low noise, eighth order, clock sweepable elliptic low pass filters available from Linear Technology.
  • the low pass filters receive a clock signal applied to input 10 at 439.5 Hz which gives the filters a cut ⁇ off frequency of 4.395 Hz.
  • the outputs of the mixers 6 and 8 carry signals at the frequency of the sum and difference of the signals applied to inputs 1 and 2.
  • the difference (beat) frequencies are passed by the filters 7 and 9 and the other signals are rejected.
  • the outputs of filters 7 and 9 are at 4 Hz.
  • the output of the filter 7 is applied to a mixer 11 with the second signal appearing on the output 4 of the phase shifter 3.
  • the output of the filter 9 is applied to a mixer 12 with the fourth signal appearing at the output 5 of the phase shifter 3.
  • the outputs of the mixers 11 and 12 can be considered as sixth and seventh signals which are combined to form an eighth signal. This occurs in an amplifier 13 to produce a reconstituted version of the original signal without the noise.
  • a phase locked oscillator 14 is then locked onto this signal.
  • the output of the phase locked oscillator is at 1758 Hz, that is the frequency of the wanted signal.
  • the circuit operates to translate the low pass filters in frequency such that the low pass filter characteristics are mirrored about the frequency of the local oscillator.
  • the result is effectively a band pass filter with its centre frequency at the frequency of the local oscillator.
  • the filters are of the switched capacity type and cannot respond to DC signals the local oscillator frequency is also suppressed.
  • the good signal to noise ratio characteristics enable acoustic systems to operate on relatively low power levels.
  • An embodiment of the invention has been tested measuring a gas flow of 36 metres per second in a four inch pipe with only 62 mW of power launching the signal into the gas flow. Results obtained are illustrated in Figure 7. It will be seen that the acoustic velocity (the output of the meter embodying the invention) accurately tracks the reference velocity (the velocity of the flow determined by other means) Earlier acoustic gas flow measurement systems have used much higher power to overcome the signal to noise ratio problem, for example 70 watts.
  • the switched capacitor filters that are used avoid phase drifts since the clock signal is derived from a crystal. Furthermore, switched capacitor filters have added immunity from noise due to the use of sampling. Finally, the use of a phase locked oscillator that is locked to the transmitting frequency means that the stability of the whole circuit is only dependent on the stability of the 10.245 MHz crystal.

Abstract

Débitmètre acoustique comportant une source de signaux acoustiques servant à lancer un signal de sortie dans une voie d'écoulement et à détecter le signal après son passage dans celle-ci. La mesure de la durée du passage du signal dans la voie d'écoulement permet de calculer le débit de l'écoulement à partir de la durée d'émission du signal ainsi mesurée. Afin d'améliorer le rapport signal/bruit du débitmètre, les signaux acoustiques émis dans la voie d'écoulement génèrent un premier signal qui se mélange à un deuxième signal ayant une fréquence différente afin de générer un troisième signal. Le premier signal se mélange également à un quatrième signal afin de générer un cinquième signal, les deuxième et quatrième signaux ayant la même fréquence et étant en quadrature. Les troisième et cinquième signaux s'appliquent à une paire de filtres passe-bas (79) dont on sélectionne la fréquence de coupure de telle sorte que soient transmis les signaux dont la fréquence est sensiblement égale ou inférieure à la différence de fréquence entre les premier et deuxième signaux. Le troisième signal filtré se mélange au deuxième pour générer un sixième signal, et le cinquième signal filtré se mélange au quatrième signal pour générer un septième signal. Les sixième et septième signaux se mélangent l'un à l'autre pour générer un huitième signal représentant le signal émis dans ladite voie d'écoulement. Le rapport signal/bruit du huitième signal est considérablement plus grand que celui du premier signal en raison du rejet dans les filtres passe-bas des composantes indésirables de signal.Acoustic flow meter comprising an acoustic signal source used to launch an output signal in a flow path and to detect the signal after it has passed through it. Measuring the duration of the signal passing through the flow path makes it possible to calculate the flow rate from the duration of transmission of the signal thus measured. In order to improve the signal-to-noise ratio of the flow meter, the acoustic signals emitted in the flow path generate a first signal which mixes with a second signal having a different frequency in order to generate a third signal. The first signal also mixes with a fourth signal to generate a fifth signal, the second and fourth signals having the same frequency and being in quadrature. The third and fifth signals apply to a pair of low-pass filters (79), the cut-off frequency of which is selected so that the signals whose frequency is substantially equal to or less than the difference in frequency between the first are transmitted. and second signals. The third filtered signal mixes with the second to generate a sixth signal, and the fifth filtered signal mixes with the fourth signal to generate a seventh signal. The sixth and seventh signals mix with each other to generate an eighth signal representing the signal transmitted in said flow path. The signal-to-noise ratio of the eighth signal is considerably greater than that of the first signal due to the rejection in unwanted filters of unwanted signal components.

Description

ACOUSTIC FLOWMETER The present invention relates to an acoustic flowmeter. Acoustic flowmeters are used to measure the rate of flow of for example a gas within a pipe. Such flowmeters use conventional techniques to launch an acoustic signal into the flow from a suitable transducer and to detect the signal after it has traversed a predetermined path through the flow. The signal is detected by a transducer which is of course also sensitive to acoustic noise propagating through the flow. Unfortunately it is generally the case that large acoustic disturbances are generated in pipes through which fluids are flowing and this background noise can be sufficient to swamp the acoustic signal which is deliberately propagated through the flow.
Thus acoustic flowmeters must be capable of operating with very low signal to noise ratios and as a result practical systems must rely upon complex signal processing techniques. These techniques can only be implemented at considerable cost and as a result acoustic flowmeters have not been widely used. It would be possible to improve the signal to noise ratio by introducing a band pass filter into the signal processing system. The sort of frequencies that have been proposed for use in flowmeters are typically from a few hundred Hertz to a few thousand Hertz. The reason for this is that provided the acoustic wave length is larger than the diameter of the pipe through which the monitored flow is passing the performance of the flowmeter is independent of the flow profile. Given such frequencies a significant improvement in the signal to noise ratio can only be achieved by using a band pass filter having a very narrow pass band, typically a pass band only a few Hertz wide. Unfortunately it is not a practical proposition to manufacture such a filter using discrete components. The tolerances required would be two rigid and such a filter would drift off frequency when exposed to temperature variations.
It has been known for many years in radio frequency systems to adopt the "Weaver" method to generate single side band signals. The Weaver method relies upon the use of low pass filters to reject unwanted sidebands. It is an object of the present invention to apply low pass filters to the problems outlined above with regard to the signal to noise ratio encountered in acoustic flowmeters.
According to the present invention, there is provided an acoustic flowmeter comprising an acoustic signal source, means for launching a signal output from the source into a flow path, means for detecting the signal after transmission through the flow path, means for measuring the time taken for the signal to be transmitted through the flow path, and means for calculating the rate of flow within the flow from the measured transmission time, wherein the detecting means comprises a transducer which is responsive to acoustic signals transmitted through the flow path to generate a first signal, means for mixing the signal with a second signal to generate a third signal, the first and second signals being of different frequency, means for mixing the first signal with a fourth signal to generate a fifth signal, the second and fourth signals being at the same frequency and in quadrature, a pair of low pass filters to each of which a respective one of the third and fifth signals is applied, each low pass filter having a cut-off frequency selected such that signals at a frequency substantially equal to or less than the difference in frequency between the first and second signals are passed, means for mixing the filtered third signal with the second to generate a sixth signal, means for mixing the filtered fifth signal with the fourth signal to generate a seventh signal, and means for combining the sixth and seventh to generate an eighth signal which is representative of the signal transmitted through the flow path.
The signal to noise ratio of the eighth signal is very much larger than the signal to noise ratio of the first signal as a result of rejection of unwanted signal components in the low pass filters.
It is a relatively easy matter to produce low pass filters having the required filter characteristics. In particular, commercial low pass elliptic switched capacitor filters are readily available. The system in accordance with the invention may be regarded as a translation of the low pass filter to the frequency of the second signal together with a mirroring of the low pass filter characteristic around the second frequency. The result is equivalent in performance to the provision of a band pass filter with its centre frequency at the frequency of the second signal and a band width equal to twice the cut off frequency of the low pass filters. Preferably the eighth signal is applied to a phase locked oscillator to provide in effect a memory the content of which is available in the event of a temporary loss of signal.
An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Figure 1 is the acoustic spectrum of noise in a pipe through which a fluid is flowing and into which a signal at 316.25 Hz has been launched;
Figure 2 illustrates the signal to noise ratio achieved using a simple acoustic pick up to detect the signal represented in Figure 1; Figure 3 illustrates the improvement in the signal to noise ratio which can be achieved using a narrow band-pass filter;
Figure 4 illustrates an embodiment of the present invention which enables characteristics such as those illustrated in Figure 3 to be achieved;
Figure 5 illustrates the characteristic performance of low pass filters illustrated in Figure 4 and the equivalent band-pass filter characteristics achieved by the use of low pass filters in Figure 4; Figure 6 illustrates a circuit for generating the fixed frequency signals utilised in the embodiment of Figure 4; and Figure 7 illustrates results obtained with an embodiment of the present invention.
Referring to Figure 1, this illustrates the acoustic spectrum of noise in a pipe together with a signal of 316.25 Hz launched into that pipe. The launched signal is detected by a conventional transducer and if the output of that transducer is not further processed the signal to noise ratio is very poor due to the high level of the noise floor as illustrated in Figure 2. Figure 3 illustrates how the signal to noise ratio could be greatly enhanced if it was possible to provide a narrow band pass filter centred on the wanted signal frequency of 316.25 Hz. The improvement in the signal to noise ratio is a corollary of the narrow band noise floor.
The signals illustrated in Figures 1 and 2 are those which are obtained using conventional acoustic flowmeter techniques in a noisy environment. The conditions illustrated in Figure 3 are unfortunately not achievable at sensible cost as it is not possible to produce a band pass filter with a suitably sharp narrow pass band. At the frequencies shown typically a pass band of only 6 Hertz would be required.
Referring now to Figure 4, this illustrates an embodiment of the invention in which the wanted signal that is launched into the monitored flow is at a frequency of 1758 Hz. The signal is received by a loudspeaker configured as a microphone and the output of that transducer is applied as a first signal to an input 1. The first signal is the combination of the wanted signal at 1758 Hz and the accompanying acoustic noise. The detector circuitry illustrated in Figure 4 comprises a local oscillator the output of which is at a frequency 1762 Hz and is applied to input 2. The local oscillator signal is applied to a phase shifter 3 having outputs 4 and 5 to which signals at 1762 Hz are applied in quadrature. That is to say there is a 90° phase shift between the signals appearing on outputs 4 and 5.
The signal on output 4 represents a second signal which is multiplied in a mixer 6 with the first signal to generate a third signal that is applied to a low pass filter 7. The signal on output 5 represents a fourth signal which is applied to a mixer 8 with the first signal so as to produce a fifth signal that is applied to a low pass filter 9. The mixers 6 and 8 may be for . example LM1596/LM1496 balanced modulator - demodulators available from National Semiconductor Corporation and the filters 7 and 9 may be LTC1064-1 low noise, eighth order, clock sweepable elliptic low pass filters available from Linear Technology. The low pass filters receive a clock signal applied to input 10 at 439.5 Hz which gives the filters a cut¬ off frequency of 4.395 Hz. The outputs of the mixers 6 and 8 carry signals at the frequency of the sum and difference of the signals applied to inputs 1 and 2. The difference (beat) frequencies are passed by the filters 7 and 9 and the other signals are rejected. Thus the outputs of filters 7 and 9 are at 4 Hz.
The output of the filter 7 is applied to a mixer 11 with the second signal appearing on the output 4 of the phase shifter 3. The output of the filter 9 is applied to a mixer 12 with the fourth signal appearing at the output 5 of the phase shifter 3. The outputs of the mixers 11 and 12 can be considered as sixth and seventh signals which are combined to form an eighth signal. This occurs in an amplifier 13 to produce a reconstituted version of the original signal without the noise. A phase locked oscillator 14 is then locked onto this signal. The output of the phase locked oscillator is at 1758 Hz, that is the frequency of the wanted signal.
In effect, the circuit operates to translate the low pass filters in frequency such that the low pass filter characteristics are mirrored about the frequency of the local oscillator. The result is effectively a band pass filter with its centre frequency at the frequency of the local oscillator. As the filters are of the switched capacity type and cannot respond to DC signals the local oscillator frequency is also suppressed.
It will be appreciated that the two sides of the circuit need to be balanced and that in order to maintain good frequency stability all signals should be derived from a common source. Such an arrangement is illustrated in Figure 6 in which a stable 10.245 MHz crystal source 15 supplies a first divider 16 which outputs a frequency at 1762 Hz and a second divider 17 which outputs a frequency of 1758 Hz. A third divider 18 generates an output at 439.5 Hz.
By way of further explanation of the circuit of Figure 4, the above mentioned first to eight signals can be represented as follows:
First Signal cos Wct
Second Signal cos Wmt
Third Signal cos (Wct - Wmt) - cos (Wct + mt)
Fourth Signal cos (Wmt + 90)
Fifth Signal cos (Wct - [Wmt + 90] - cos (Wct + [Wmt + 90])
Sixth Signal cos Wct - cos (2Wmt - Wct)
Seventh Signal cos Wct + cos (2Wmt - Wct)
Eighth Signal 2 cos Wct
To summarise the advantages of the invention, the good signal to noise ratio characteristics enable acoustic systems to operate on relatively low power levels. An embodiment of the invention has been tested measuring a gas flow of 36 metres per second in a four inch pipe with only 62 mW of power launching the signal into the gas flow. Results obtained are illustrated in Figure 7. It will be seen that the acoustic velocity (the output of the meter embodying the invention) accurately tracks the reference velocity (the velocity of the flow determined by other means) Earlier acoustic gas flow measurement systems have used much higher power to overcome the signal to noise ratio problem, for example 70 watts.
In the illustrated embodiment of the invention the switched capacitor filters that are used avoid phase drifts since the clock signal is derived from a crystal. Furthermore, switched capacitor filters have added immunity from noise due to the use of sampling. Finally, the use of a phase locked oscillator that is locked to the transmitting frequency means that the stability of the whole circuit is only dependent on the stability of the 10.245 MHz crystal.

Claims

CLAIMS:
1. An acoustic flowmeter comprising an acoustic signal source, means for launching a signal output from the source into a flow path, means for detecting the signal after transmission through the flow path, means for measuring the time taken for the signal to be transmitted through the flow path, and means for calculating the rate of flow within the flow from the measured transmission time, wherein the detecting means comprises a transducer which is responsive to acoustic signals transmitted through the flow path to generate a first signal, means for mixing the first signal with a second signal to generate a third signal, the first and second signals being of different frequency, means for mixing the first signal with a fourth signal to generate a fifth signal, the second and fourth signals being at the same frequency and in quadrature, a pair of low pass filters to each of which a respective one of the third and fifth signals is applied, each low pass filter having a cut-off frequency selected such that signals at a frequency substantially equal to or less than the difference in frequency between the first and second signals are passed, means for mixing the filtered third signal with the second signal to generate a sixth signal, means for mixing the filtered fifth signal with the fourth signal to generate a seventh signal, and means for combining the sixth and seventh signal to generate an eighth signal which is representative of the signal transmitted through the flow path.
2. An acoustic flowmeter according to claim 1, wherein the low pass filters comprise elliptic switched capacitor filters.
3. An acoustic flow meter according to claim 1 or 2, wherein the eighth signal is applied to a phase locked oscillator.
EP19920907504 1991-03-26 1992-03-25 Acoustic flowmeter Withdrawn EP0577679A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB9106465 1991-03-26
GB919106465A GB9106465D0 (en) 1991-03-26 1991-03-26 Acoustic flowmeter

Publications (1)

Publication Number Publication Date
EP0577679A1 true EP0577679A1 (en) 1994-01-12

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EP19920907504 Withdrawn EP0577679A1 (en) 1991-03-26 1992-03-25 Acoustic flowmeter

Country Status (3)

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EP (1) EP0577679A1 (en)
GB (2) GB9106465D0 (en)
WO (1) WO1992017753A1 (en)

Families Citing this family (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP4556253B2 (en) * 1999-06-24 2010-10-06 パナソニック株式会社 Flowmeter
EP2642256B1 (en) 2012-03-23 2018-07-18 ams AG Measurement arrangement and method

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Publication number Priority date Publication date Assignee Title
US3307408A (en) * 1966-08-10 1967-03-07 Int Research & Dev Co Ltd Synchronous filter apparatus in which pass-band automatically tracks signal, useful for vibration analysis
US3949605A (en) * 1974-07-02 1976-04-13 The United States Of America As Represented By The Secretary Of The Navy Acoustic current/flow measuring system
EP0250660B1 (en) * 1986-06-05 1991-03-27 Moore Products Co. Fluid velocity measuring method and apparatus
GB2205645A (en) * 1987-06-12 1988-12-14 Milan Herman Miessler Fluid flow measurement

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
See references of WO9217753A1 *

Also Published As

Publication number Publication date
GB9106465D0 (en) 1991-05-15
GB2267568A (en) 1993-12-08
GB2267568B (en) 1994-08-03
WO1992017753A1 (en) 1992-10-15
GB9314722D0 (en) 1993-09-29

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