GB2144550A

GB2144550A - Improved volume measuring system

Info

Publication number: GB2144550A
Application number: GB08416627A
Authority: GB
Inventors: John Brown Pond
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-07-01
Filing date: 1984-06-29
Publication date: 1985-03-06
Also published as: GB2144550B; GB8416627D0

Abstract

The method comprises applying to the gas or vapour within the confined space or container repeated compressions of predetermined waveform by means of a barrier or diaphragm; generating electrical signals representative of the resultant variation of pressure within the confined space or container; repeatedly sampling said electrical signals at regular predetermined instants, or over regular predetermined time ranges, corresponding to predetermined points or regions of said waveform; combining or integrating the sampled signals to generate thereby a time-averaged output signal; and calculating the free volume of the confined space or container from the output signal thus obtained. In one embodiment, the apparatus comprises the diaphragms of a moving coil loudspeaker 51 for applying repeated compressions to said confined space V1 with a predetermined waveform a pressure-sensitive transducer M1 for measuring pressure changes within the volume which is itself to be measured; electrical circuit means connected to said pressure- sensitive transducer and to said drive means, means 9, 11, 10, 12 for sampling the electrical output of said pressure-sensitive transducer at predetermined times related to the frequency at which the diaphragm means is driven and means 15, 16 for generating a time-averaged output signal from the signals generated by said electrical circuit means; and means 6 for determining from said time- averaged output signal a value representative of the free volume being measured. A reference volume V2 may be provided, and the state of fill of space V1 may be indicated. Portable measuring apparatus is also described (Figs. 2-3, not shown).

Description

SPECIFICATION Improved volume measurement system This invention relates to a method of, and apparatus for, measuring the free (that is gaseous or vapourous) volume within a confined space or a container. One technique for effecting such measurements is by compressing the volume and considering the resultant pressure change. An example of this technique is disclosed in U.S. Patent No.

3,237,451 dated 1st March, 1966. Many potential applications of this technique however, are prevented by the problem of noise which is too great simply to be removed by the use of filters. Noise arises from a variety of sources. There is the electrical noise associated with the conversion of the pressures undergoing monitoring into electrical signals, and with the amplification and subsequent processing of those signals (termed hereinafter "the wanted signals"). Such electrical noise appears as unwanted signals added to the wanted signals and may be produced by the measurement system itself (self-generated electrical noise) or by electrical or magnetic influence from the surroundings (pickup electrical noise).

There is also noise which, although eventually appearing as unwanted electrical signals, has its origin in pressure fluctuations or mechanical vibrations caused by the measurement system itself (self-generated acoustic noise) or by influence from the surroundings (pickup acoustic noise). In many instances, the magnitude of the noise is so great that it tends to swamp the wanted signals. Filtering techniques are inadequate to cope with such situations and can, in practice, lead to a deterioration in the eventual output signal since successive filtering stages tend to add their own noise to the original signal and are thereby self-defeating.

Noise may also be defined as locked or nonlocked. Locked noise is the acoustic or electrical noise present as a by-product of the intentional pressure changes and hence has a definite time relationship with these pressure changes. Because of this relationship, it is often not difficult to reduce a problem of locked noise. Non-locked noise is not related in this way to the intentional pressure changes and, in many potential applications of the technique under consideration, can be an insuperable problem.

Where the volume to be measured is very large so that the intentional pressure changes are of necessity very slight with the result that the required electrical signals need large amplification, unavoidable self-generated noise obscures the wanted signals. Also, in some applications, pickup noise may be much greater than the wanted signals even when these can themselves be made quite large.

Various systems have been suggested to reduce the problem of noise, for example that described in USSR Patent SU678,316 dated 29th August, 1979, but in may potential applications this problem has still remained intractable.

The present invention aims to reduce the problem of noise when measuring a volume by the technique involving compression of the volume and consideration of the resultant pressure change.

According to one aspect of the present invention, there is provided a method of measuring the free volume within a confined space or a container, which comprises applying to the gas or vapour within the confined space or container repeated compressions of predetermined waveform by means of a barrier or diaphragm: generating electrical signals representative of the resultant variation of pressure within the confined space or container; repeatedly sampling said electrical signals at regular predetermined instants, or over regular predetermined time ranges, corresponding to predetermined points or regions of said waveform; combining or integrating the sampled signals to generate thereby a time-averaged output signal; and calculating the free volume of the confined space or container from the output signal thus obtained.

The method of the invention makes the wanted signals repetitive and selects the timing of signal sampling with respect to the timing of the signals themselves so that, when the signals are combined or integrated, the effect of noise is relatively diminished.

The container or confined space whose volume is to be measured will generally be filled with a gas or vapour, for example with air or with a mixture of air and petroleum vapour.

The frequency of the repeated compressions will generally be constant while measurements are being taken, but it is possible and may be advantageous to perform a sweep through a frequency range prior to taking measurements in order to select a frequency with a view to optimising noise reduction. The selected frequency will be a function of the size of the volume being measured, large volumes using lower frequencies and vice versa. The frequency must be sufficiently low to prevent the occurrence of standing waves, and low enough to avoid problems associated with Helmholtz resonance. In general, the frequency should be sufficiently low for the pressure response to tend towards equilibrium within the volume concerned. Given this condition, higher frequencies are more advantageous than lower ones because of the greater ease of eliminating noise from the wanted signals.Typically, a volume of about 1 cubic metre can be measured accurately with a frequency of about 30 Hz, while a volume of about 100 cubic metres can be measured accurately with a frequency of around 3 Hz.

The source of repeated compressions can conveniently be in the form of an electricallydriven diaphragm. In order to allow equilibration of the static pressure across the diaphragm, there is advantageously provided means, e.g. a small by-pass duct or a hole in the diaphragm, which facilitates such equilibration while maintaining acoustic (or subacoustic) separation between the gases or vapours on opposite sides of the diaphragm.

The presently preferred waveform of the repeated compressions is a square wave profile.

The resultant variation of pressure within the confined space or container can be measured by a suitable pressure-sensitive device, for example a microphone sensitive to low frequencies. This can be used to give an electrical output which is directly proportional to pressure variation within the volume undergoing measurement.

The repeated sampling of signals related to the pressure variations in the unknown volume can be effected using circuitry incorporating one or more triggers which respond to the waveform of applied compressions and which actuate one or more switches which act to pass the signals to further processing circuitry.

Where a square wave drive is applied to the diaphragm, the signal sampling advantageously commences a short time after the diaphragm has reached the limit of its movement, whether in the forward or in the rearward directions; this may be accomplished by appropriate delay circuitry, and has the advantage that any initial pressure fluctuations, caused for example by Helmholtz resonance, are ignored by the signal sampling arrangement.

In one embodiment of the invention, the diaphragm is positioned so that one face thereof applies repeated compressions to the volume undergoing measurement, while the opposite face of the diaphragm applies repeated compression (of opposite phase) to a reference volume, e.g. a small chamber of known volume. This reference volume is preferably small, e.g. of the order of from 10 to 100 cubic centimetres. Pressure variations in both the unknown and reference volumes are used to generate electrical signals, e.g. by use of pickup transducers such as microphones, positioned so as to respond to those pressure changes, and the signals are processed as described above.Since noise is unlikely to be a serious problem with respect to the signals related to the reference volume, these signals may be rectified instead of being subject to the sampling and time-averaging technique which is used for the signals related to the unknown volume.

According to another aspect of the present invention, there is provided apparatus for measuring the free volume within a confined space or a container, which apparatus comprises (1) diaphragm means, for applying repeated compressions to said confined space or container; (2) drive means for moving said diaphragm means in accordinace with a predetermined waveform; (3) a pressure-sensitive transducer for measuring pressure changes within the volume which is itself to be measured; (4) electrical circuit means connected to said pressure-sensitive transducer and to said drive means, for sampling the electrical output of said pressure-sensitive transducer at predetermined times related to the frequency at which the diaphragm means is driven and for generating a time-averaged output signal from the signals generated by said electrical circuit means; and (5) means for determining from said time-averaged output signal a value representative of the free volume being measured.

Advantageously, the electrical circuit means comprises a first averaging element which is connected to said transducer only during times (or parts thereof) of maximum diaphragm excursion in one direction, and second averaging element which is connected to said transducer only during times (or parts thereof) of maximum diaphragm excursion in the opposite direction, together with means for determining the difference between the outputs of said first and said second averaging elements, said outputs representing, respectively, first and second averaged signals derived from the response of said free volume to said repeated compressions.

According to a further aspect of the invention, there is provided apparatus for measuring the free volume within a confined space or a container, which apparatus comprises (1 ) diaphragm means, for applying repeated compressions to said confined space or container; (2) a chamber of known volume supporting said diaphragm means; (3) drive means for moving said diaphragm means in accordance with a predetermined waveform; (4) means for allowing equilibration of the static pressures across said diaphragm means while maintaining acoustic separation across the diaphragm means; (5) a first, pressure-sensitive transducer for measuring pressure changes within the volume which is itself to be measured; (6) a second, pressure-sensitive transducer for measuring pressure changes within said chamber of known volume; (7) electrical circuit means connected to said first pressuresensitive transducer and to said drive means, for sampling the electrical output of said first, pressure-sensitive transducer at predetermined times related to the frequency at which the diaphragm means is driven and for generating a time-averaged output signal from the signals generated by said electrical circuit means; (8) means for generating a second output signal from the second, pressure-sensitive trans ducer; and (9) comparator means for comparing said time-averaged output signal derived from the first, pressure-sensitive transducer with a second output signal derived from said second, pressure-sensitive transducer.

The comparator means preferably comprises a logic element which can be programmed to perform calculations and comparisons necessary to give an output directly indicative of the value of the unknown free volume, and/or to calculate an associated parameter (such as the state of fill of the container), given an appropriate input of data (such as the free volume of the container when it is empty).

The apparatus advantageously further includes display means responsive to the output of said comparator means. Such display means is preferably an alphanumeric display and can be arranged to display warnings and/or instructions to the user.

The diaphragm means can be a moving coil loudspeaker mounted in a duct forming part of, and communicating with the main volume of, said chamber of known volume.

The means for allowing static pressure equilibration across the diaphragm means can be, for example, a small hole in the diaphragm itself or a fine-bore duct by-passing the diaphragm.

The presently preferred electrical drive means for the diaphragm is a square wave generator.

In one embodiment, the apparatus of this invention is in portable form and comprises a duct which is intended to couple onto the volume which is to measured; the duct forms an extension from the chamber of known volume and can terminate, for example, with an O-ring seal. Within the duct are located the diaphragm which imparts repeated compressions to the unknown volume and to the reference volume, and the pressure-sensitive transducer for generating an electrical signal related to the unknown volumes. The reference chamber may include within it the second transducer for generating electrical signals related to the reference volume.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 illustrates schematically one embodiment of apparatus in accordance with the invention; Figure 1A illustrates the variation of pressure with time within a container whose free volume is being measured using the apparatus of Fig. 1; Figure 2 illustrates schematically how apparatus of the invention may be arranged as a portable unit; Figure 2A shows another embodiment arrangement of the apparatus as a portable unit; and Figure 3 illustrates a variation in part of the apparatus shown in Fig. 2.

Referring to Fig. 1, the volume to be measured, V1, is given periodic compressions by the movements of S which is the diaphragm of a moving coil loudspeaker mounted between V1 and a reference volume V2 and whose driving coil (indicated schematically as S1) is fed from a square wave generator 4.

Thus S also gives related compressions to V2.

A first, pressure-sensitive transducer in the form of a microphone M1 is located within the volume V1 and a second, pressure-sensitive transducer in the form of a microphone M2 is located within the chamber V2 of known volume. The output of microphone M1 is amplified by an amplifier 1, the output of which is fed to elements 11 and 1 2 which are components of two arms, in parallel, of a signal rectifying/averaging circuit. Square wave generator 4 also feeds into this circuit, which includes timing/pulsing elements 9 and 10 and an inverter 8 connected between generator 4 and element 9. Elements 11 and 1 2 are connected, respectively, to elements 10 and 9 and function as variable conductance elements, e.g. switches (where the conductance is effectively varied from zero to infinity).In one arrangement, each of elements 9 and 10 is a differentiating element, e.g. a Schmidt trigger device which, in response to the square waves generated by 4, generates pulses which alternately energise or de-energise the switch (11 or 1 2) to which it is connected. In this way, switch 11 is only ON at times when (or part thereof) when diaphragm S is at the limit of its movement in one direction, and switch 1 2 is ON only at times (or part thereof) when diaphragm S is at the limit of its movement in the opposite direction. Switch 1 2 is OFF whenever 11 is ON, and switch 11 is OFF whenever 1 2 is ON. Elements 13 and 14 are resistances, and 1 5 and 1 6 are capacitances.Elements 8, 9, 10, 11, 12, 13, 14, 1 5 and 1 6 perform a signal rectification function as well as a signal averaging function. Elements 13, 14, 15 and 1 6 can be realised electronically, as can the other circuit elements, and can be adjustable.

The inputs to a differential amplifier 3 are connected through the resistances 1 3 and 1 4 to elements 11 and 1 2, respectively.

The time profile of the pressure in V1 is somewhat as shown in Fig. 1A where for clarity a constant pressure (the static pressure), usually much greater than the pressure variation, has been subtracted. For clarity, acoustic noise is shown as a small disturbance on the roots (minima) and crests (maxima) of the wave; in many instances, the amplitude of the noise will in fact greatly exceed that of the applied compressions, being of such a magnitude that it cannot be handled by the mere use of filters.

After conversion of the pressure signals into electrical signals by M1 and their amplification by 1 there is obtained an electrical signal similar in principle in time profile to that shown in Fig. 1A (given that in Fig. 1A the amplitude of the noise is reduced by a large factor for clarity of presentation) but with added electrical noise. Invertor 8 and timing/pulsing elements 9 and 10 alter the electrical conductances of elements 11 and 1 2 so that essentially resistance 1 3 is connected to the signals from 1 during definite parts (shown as A of the crests and 14 is connected during parts (shown as B) of the roots. While A and B can occupy all of the crests and roots respectively in simpler versions of the apparatus, decreasing A and B can help to reduce locked noise.

Thus the output of amplifier 3 is a voltage related more and more closely to the difference of averages of root and crest pressure as the number of samples A and B taken increases. The waiting time for an acceptable average (which may be shown by a relatively steady voltage from 3) may be reduced by altering the electric charges in 1 5 and 1 6 by known means (not shown).

The voltage from 3 is led to a comparator/ logic element 6 to be compared with a voltage which may be derived in an analogous manner from the pressure changes in V2. In the illustrated version of the apparatus which is adequate where noise in the signals from V2 is not a problem, the signals from V2 are rectified by element 5 without averaging. It will be appreciated that rectifying element 5 may be replaced by a circuit analogous to that comprising elements 3, 8, 9, 10, 11, 12, 13, 14, 15 and 16.

The comparison of signals to lead to the measurement of volume may be done by analogue or digital means. In the version shown here comparator 6 contains analogue to digital convertors and calculator logic so that the voltages from V1 and V2 may be made into a ratio, multiplied by desired constants and subtracted from desired constants.

7 is an alphanumeric display so that information about the state of fill of a container may be displayed.

Fig. 2 illustrates a portable version of the apparatus. The electronic are not shown. In the prior art, volume measurement systems employing pressure changes are not considered as portable and versatile instruments which can be coupled to different containers as and when required. Fig. 2 shows an arrangement which can be so used. A duct D couples at C on to an orifice on the container.

The exact nature of the coupling is not important as long as it is reasonably gas tight; the orifice can even be an adventitious hole. M1 and S both couple into the duct rather than into V1 itself. A volume V1 may also be measured with the portable apparatus taken right inside it.

Fig. 2A shows an alternative portable arrangement, intended for use in measuring relatively large volumes, and in which duct D is dimensioned as shown and is relatively squat. Diaphragm S occupies all or substantially all of the width of duct D.

Fig. 3 illustrates a modification of the apparatus of Fig. 2 in which provision is made for the flushing out of the interior of reference chamber V2 with gases or vapours from the interior of the volume which is to be measured. The principle is that a pump (shown here as a piston P together with non-return valves 1 and 2) can operate to expel gas/vapour from V2 and draw in fresh gas/vapour from V1. Some by-pass to S is required and this is shown diagrammatically as B. Some means of preventing by-pass during the induction of pressure changes is required. This can be achieved for example by strong springs on 1 and 2. The reason for flushing and replacement is to equalise the nature and hence the compressibilities of the gas/vapour contents of V1 and V2. This is important in some applications and is particularly so with a portable instrument.

Claims

1. A method of measuring the free volume within a confined space or a container, which comprises applying to the gas or vapour within the confined space or container repeated compressions or predetermined waveform by means of a barrier or diaphragm; generating electrical signals representative of the resultant variation of pressure within the confined space or container; repeatedly sampling said electrical signals at regular predetermined instants, or over regular predetermined time ranges, corresponding to predetermined points or regions of said waveform; combining or integrating the sampled signals to generate thereby a time-averaged output signal; and calculating the free volume of the confined space or container from the output signal thus obtained.

2. A method according to Claim 1, in which said repeated compressions are applied to the volume by means of an electrically driven diaphragm.

3. A method according to Claim 2, wherein one face of said diaphragm is coupled to the volume and the other face of said diaphragm is coupled to a second, known volume.

4. A method according to Claim 2 or 3, wherein a moving coil loudspeaker is used to apply said repeated compressions.

5. A method according to Claim 1, 2, 3 or 4, wherein the electrical signals representative of pressure variations within the volume being measured are generated by a pickup transducer sensitive to low frequencies.

6. A method according to claim 5, wherein said pickup transducer is a micro phone.

7. Apparatus for measuring the free volume within a confined space or a container, which apparatus comprises (1) diaphragm means, for applying repeated compressions to said confined space or container; (2) drive means for moving said diaphragm means in accordance with a predetermined waveform; (3) a pressure-sensitive transducer for measuring pressure changes within the volume which is itself to be measured; (4) electrical circuit means connected to said pressure-sensitive transducer and to said drive means, for sampling the electrical output of said pressuresensitive transducer at predetermined times related to the frequency at which the diaphragm means is driven and for generating a time-averaged output signal from the signals generated by said electrical circuit means; and (5) means for determining from said timeaveraged output signal a value representative of the free volume being measured.

8. Apparatus as claimed in claim 7, wherein said electrical circuit means comprises a first averaging element which is connected to said transducer only during times (or parts thereof) of maximum diaphragm excursion in one direction, and a second averaging element which is connected to said transducer only during times (or parts thereof) of maximum diaphragm excursion in the opposite direction, together with means for determining the difference between the outputs of said first and said second averaging elements, said outputs representing, respectively, first and second averaged signals derived from the response of said free volume to said repeated compressions.

9. Apparatus as claimed in claim 8, wherein said electrical circuit means further comprises a logic element which is arranged to calculate said free volume, or a parameter associated with said free volume, from the difference between said outputs and from reference signal provided as an additional input to the logic element.

10. Apparatus for measuring the free volume within a confined space or a container, which apparatus comprises (1) diaphragm means, for applying repeated compressions to said confined space or container; (2) a chamber of known volume supporting said diaphragm means; (3) drive means for moving said diaphragm means in accordance with a predetermined waveform; (4) means for allowing equilibration of static pressures across said diaphragm means while maintaining acoustic separation across the diaphragm means; (5) a first, pressure-sensitive transducer for measuring pressure changes within the volume which is itself to be measured; (6) a second, pressure-sensitive transducer for measuring pressure changes within said chamber of known volume; (7) electrical circuit means connected to said first pressure-sensitive transducer and to said drive means, for sampling the electri cai output of said first, pressure-sensitive transducer at predetermined times related to the frequency at which the diaphragm means is driven and for generating a time-averaged output signal from the signals generated by said electrical circuit means; (8) means for generating a second output signal from the second, pressure-sensitive transducer; and (9) comparator means for comparing said timeaveraged output signal derived from the first, pressure-sensitive transducer with the second output signal derived from said second, pressure-sensitive transducer.

11. Apparatus as claimed in Claim 7, 8, 9 or 10, which further comprises display means responsive to the output of said comparator means.

1 2. Apparatus as claimed in claim 7 or 10, which further comprises a logic element and means for input of data to the logic element, the logic element being arranged to calculate from the determined value of the free volume and from data supplied via the data input means a parameter related to the free volume of the container.

1 3. Apparatus as claimed in claim 12, wherein said logic element is arranged to calculate the state of fill of said container.

14. Apparatus as claimed in any one of claims 7 to 13, which is in portable form and which further comprises a duct communicating with said chamber of known volume, said duct acting as an extension of said chamber and supporting within it said diaphragm means, wherein (a) the duct terminates in a coupling for attachment to the volume which is to be measured; and (b) said first, pressuresensitive transducer is arranged and located so as to detect pressure variations within said duct between the diaphragm means and said coupling.

1 5. Apparatus as claimed in any of claims 7 to 14, which futher includes means for flushing out the contents of said chamber of known volume.

1 6. Apparatus as claimed in claim 15, which comprises a by-pass duct located as a by-pass around said diaphragm means; a oneway valve located in said by-pass duct so as to admit gases or vapours from upstream of the diaphragm means into the chamber of known volume downstream of the diaphragm means; and a pump for withdrawing gas or vapour from said chamber of known volume.

1 7. Apparatus as claimed in any one of claims 7 to 16, wherein said drive means for moving said diaphragm means in accordance with a predetermined waveform is a square wave generator.

1 8. Apparatus for measuring the free volume within a confined space or a container, substantially as hereinbefore described with reference to the accompanying drawings.

19. A method of measuring the free vol ume within a confined space or container, substantially as hereinbefore described.