MEASUREMENT OF SPECIFIC GRAVITY

Abstract: The specific gravity of a liquid (12) in a container (10) is measured by using an ultrasonic transmitter (14) and receiver (16) and associated circuitry (18-22) to measure the velocity of sound in the liquid (12). This information is combined with liquid temperature measured by a sensor (28) to calculate the specific gravity. Other embodiments are described for use with multi-phase solutions and solutions containing solids or gas bubbles.

Description: "Measurement of Specific Gravity" This invention relates to a method and apparatus for measuring the specific gravity (s.g.) of liquids. It is of particular utility in relation to brewing, distilling and other processes for producing material for human consumption, but is not limited thereto. Measurement of s.g. is of great interest in monitoring and controlling brewing and distilling processes. Conventionally, such measurement is done by physically removing a sample for test in a hydrometer or a more sophisticated instrument. It 0 i s desirable to replace such measurement by an on-line measurement which could be made part of a process control loop. One object of the present invention is to make this possible. Accordingly, one aspect of the invention provides a method of measuring the s.g. of a liquid, comprising measuring the velocity * ■ of sound in the liquid, measuring the temperature of the liquid, and deriving from these the s.g. of the liquid. From another aspect, the invention provides apparatus for measuring the s.g. of a liquid, comprising an ultrasonic pulse transmitter and receiver separated by a known path length, means 20 for establishing the transit time of pulses between the transmitter and receiver, a temperature sensor for measuring the temperature of the liquid, and computing means arranged to derive from the transit time and the temperature the s.g. of the liquid. In preferred forms, the transmitter, receiver and temperature ^ sensor are positioned on the exterior of the liquid container, which may be a storage container such as a tank, or a conduit in which the liquid is flowing. A relatively simple form of the invention is useful in measuring the s.g. of single-phase solutions such as water/ethanol, 30 for example in whisky distilling. Other embodiments of the invention have utility in measuring the s.g. of more complex solutions and solutions containing gas bubbles and/or solids, for example in brewing. It will be understood that the apparatus may be used to * • > give a direct reading of a parameter related to s.g., such as the concentration of an acid. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic block diagram of one apparatus embodying the invention; Fig 2 is a set of graphs representing the relationship between original gravity and specific gravity for beers and the speed of sound therein; Fig 3 is a schematic diagram of an apparatus for diluting beer incorporating a monitor embodying the invention; Fig 4 is a cross-section of the monitor used in Fig 3; Fig 5 is a graph, similar to Fig 2, illustrating the operation of the apparatus of Fig 3, and Fig 6 shows graphs of temperature and s.g. against time in a brewing process. / Referring to Fig 1, a tank 10 holds a liquid 12 the s.g. of which is to be measured. The apparatus includes an ultrasonic transmitting transducer 14 and receiving transducer 16. A pulse generator 18 is connected to drive the transmitting transducer 5 14 immediately after the preceding pulse has been received at 16. The arrangement is such that the time taken for a given number of pulses to traverse across the fixed space between the transducers 14 and 16 can be measured from which a measure of ■ J ^ Q the velocity of sound in the liquid 12 can be derived. For this purpose the pulse generator drives a counter 20 to give an output when the predetermined number of pulses has traversed the circuit. The output of the counter 20 is used to reset a counter 22 receiving clock pulses from a clock circuit 24. Thus the output of counter 15 22 on line 26 is a measure of the time taken for transit of this number of pulses, which is representative of the sonic velocity. Alternatively, the transmitter and receiver transducers could be located together, or a single transducer used for both purposes, with the pulses being reflected from the opposite wall 20 of the tank. The apparatus further includes a temperature sensor 28 giving a signal on line 30 representative of the temperature of the liquid 12. The temperature sensor 28 is suitably a thermistor. It has been found that an accurate measure of the temperature 25 of the contents can be made by a thermistor applied to the exterior of the tank provided that the tank wall of sufficiently low thermal conductivity material and is relatively thin (one example being 1/8 inch stainless steel) and the thermistor is enclosed in thermally insulating material, to minimise the effect of ambient air temperature, 3Q The signals on lines 26 and 30 are supplied to a computing means such as a microprocessor 32 which is arranged to compute the specific gravity of the liquid 12. The speed of sound in the liquid is a function of temperature, specific gravity, and compressibility, and since compressibility is for all practical <- purposes constant for a liquid the s.g. can be computed given knowledge of the sonic velocity and temperature. The output signal from the microprocessor 32 representing s.g. can be displayed or can be used in a process control loop. It will be noted that in the drawing the transducers 14 and 16 are shown as being external to the tank 10. This is desirable when one is dealing with substances for human consumption or with substances which are dangerous or corrosive. A technique for permitting such transducers to function via the wall of the tank is fully described in our patent publication W084/01233. The above embodiment may be modified to minimise the effect of drift and ambient variations in the electronics. As described in our patent publication EP 0037196 such modification makes use of a reference path through a medium other than the liquid 12 of interest. It has been found that the above embodiment provides excellent results in measuring the s.g. of pure liquids and single-phase solutions such as ethanol and water. However, determining the s.g. of multi-phase solutions (eg water/ethanol/sugar) and liquids containing gas bubbles or solids presents difficulties. One example of this is beer. When fermenting beer one starts with a material which is essentially a solution of sugars in water. Fermentation converts all, or more commonly only some, of the sugars to ethanol, and thus during fermentation a three- " phase solution is present. Additionally, during fermentation gas is evolved at a varying rate and yeast solids are present * in varying amounts. The above embodiment has been found not to give useful measurements of actual s.g. in these circumstances. The applicants have, however, made the unexpected discovery that the above technique can provide an accurate measure of the original gravity of a beer after fermentation. The term "original gravity" (o.g.) denotes the specific gravity of the liquor before fermentaion, and is a measure of the amount of sugars present and thus also of the potential alcohol content if these sugars were to be fully fermented to alcohol. A typical beer o.g. would be 1.035, which in the United Kingdom would conventionally be described as "o.g. 1035". A further term used hereinafter is "product gravity" (p.g.) which refers to the specific gravity of the beer after brewing. Referring to Fig 2, there is shown in full lines a family of curves showing the s.g. for beers of various o.g. as they progress from unfermented to fully fermented (assuming constant temperature). As one example, the curve 30 represents a beer with an o.g. of 1.035 which, if totally fermented, results in a beer of s.g. 0.97 approximately. If, however, fermentation is stopped at the point 32, a beer of s.g. approximately 1.008 is obtained, some sugars remaining unfermented. The dashed lines in Fig 2 represent constant sonic velocities (in the absence of gas and solids) and it will be seen that these follow s.g. closely, but not precisely. Thus, if sonic velocity alone is known, it is possible to derive the o.g. of the liquid but not the actual s.g. which may lie anywhere along the curve. The small differences between the constant o.g. and the constant sonic velocity curves may readily be allowed for by building up a library of curves by empirical methods. This data may suitably be set up in computer memory and the computer programmed to make the correction, using interpolation techniques where necessary. Normally, the o.g. of a brew is known since this is measured by hyrdrometer before fermentation is started; the apparatus of the present invention can equally be used to measure o.g. Thus, the relationship illustrated in Fig 2 allows the known o.g. and the measured sonic velocity to be used in principle to derive the s.g. at the time of measurement. Embodiments of the invention making use of this will now be described. Fig 3 illustrates an embodiment in which the above factors are utilised to control a dilution or "cutting" process for beer. It is commercially attractive to brew beer to a high strength and thereafter dilute it, but it is necessary to control the dilution to give a product equivalent to a stated o.g. In Fig 3, beer of a known o.g. in tank 40 is mixed with water from tank 42 in a mixer 44 of known type. The flows are controlled by respective valves 46, 48, the beer control valve 46 normally being set at a constant flow rate and the water control valve 48 being varied as part of a feedback loop. The product passes from the mixer 44 through a flow meter 50 and monitor 52. The monitor 52 acts to measure the temperature and sonic velocity of the product, generate a temperature-compensated sonic velocity, and supply this as a feedback signal to the valve 48. 5 A similar monitor 54 may be provided to check that beer flowing from the tank 40 is of the stated o.g. Fig 4 shows in greater detail the monitor 52 mounted on a pipe 54. The monitor comprises an ultrasonic transmitter transducer 56, an oppositely-disposed receiver transducer 58, and a thermistor 60, all secured to the JO exterior of the pipe 54 and enclosed in thermal insulation such as an expanded polystyrene sleeve 62. The relationship between the wall thickness and ultrasonic frequency and the mode of use of the ultrasonic transducers is as described in the above-mentioned publications. 15 Fig 5, which is a graph similar to Fig 2, illustrates the operation of the apparatus of Figs 3 and 4. The beer in tank 40 has a known o.g. and a known s.g. (measured, for example, by hydrometer) and is thus defined by point 64. Dilution with water causes the characteristics of the product to move along 20 the chain-dotted line 66. By maintaining the temperature-corrected ultrasonic count within the curves 68 and 70, a product is achieved which is equivalent to a beer of o.g. at area 72 and alcohol content at area 74. It will be appreciated that Fig 3 shows only the apparatus 25 used when operating as a continuous process. For starting up the process, additional apparatus (not shown) may be required, such as means for feeding product back to the mixer until a relativel stable control feedback is achieved. A further application of the present invention is to the 30 brewing process itself. In Fig 6, the curve 70 shows s.g. against time and the curve 72 temperature against time durinε the brewing process (which typically proceeds for a period of3-5 days). It is conventional to monitor temperature continuously and to apply cooling to the fermenting vessel during the period 72a 35 to limit the maximum temperature. It is also conventional to remove samples from the vessel from time to time to measure s.g. by hydrometer; when a desired s.g. has been reached, the temperature is forced down by additional cooling (period 72b) to terminate fermentation. Previous attempts to measure s.g. during fermentation by ultra¬ sonic techniques without drawing off samples have been unsuccessful, owing to the presence of gas bubbles and yeast and other solids. An embodiment of the present invention overcomes these problems by means of a statistical technique. The fermentation period is divided into a number of relatively short time periods, and within each time period a relatively large number of measurements are taken. Suitably the apparatus of Fig 1 is used and each recorded measurement is the number of pulses passing in a predetermined time interval; such number of pulses is referred to hereinafter as a "count". In the presently preferred embodiment for use with beer, the fermentation is divided into time periods of four minutes, and in each period eight thousand counts are taken. Fig 7 illustrates typical statistical distributions for three time periods at different points in the fermentation process. Curve 82 is at an early stage and shows a sharply defined peak. Curve 84 is at a later stage with a high degree of gas generation, and shows a much greater variance. Curve 86 is towards the end of the process where gas evolution has almost ceased, and the variance is again much smaller. In each case, however, the applicants have established that the distribution contains information which can be used to derive actual s.g. at that time period, and other data. It has been found that the minimum point of the distribution is a function of both s.g. and o.g., and of temperature. Given the relationship between s.g. and o.g. discussed above, and given that o.g. for a particular liquor is known, it is therefore possible to determine s.g. at each time period as fermentation proceeds. The relationships between these variables can be established empirically by building up a library of data by measuring s.g. by hydrometer for various o.g., temperature and ultrasonic count values. Once such a library is available, it is possible to SUBSTITUTE . ε:j£ET measure s.g. automatically in an on-line and non-intrusive manner. The distribution information also provides the possibility of automatically measuring other factors during brewing. As indicated above, the spread of the distribution is affected by outgassing; it is also affected by suspended solids. Similar empirical techniques may be used to quantify these effects, and thus provide on-line data relating to these factors. It will be apparent that the counts can be stored in digital form and their minima and spread derived in a computer by techniques which are well-known and will therefore note be described here.

Claims: CLAIMS 1. A method of measuring the s.g. of a liquid, comprising measuring the velocity of sound in the liquid, measuring the temperature of the liquid, and deriving from these the s.g. of the liquid. 2. The method of claim 1, in which the velocity of sound is measured by measuring the transit time of ultrasonic energy between known locations. 3. The method of claim 2, in which the transit time is measured by measuring the time taken for a number of pulses to transit, successive pulse transmissions being triggered by reception of the preceding pulse. 4. The method of claim 1, in which the transit time is measured by measuring the number of ultrasonic pulses passing between fixed locations in agiven time, successive pulse transmissions being triggered by reception of the preceding pulse. 5. The method of claim 1 for use with a fermentable liquor the method including establishing a set of data defining the relationship between constant sonic velocities and liquors of varying original gravities. 6. The method of claim 5, for use in determining the s.g. of said liquor during fermentation, beginning from a known o.g., in which:- The fermentation is divided into a number of time periods each of which is short in relation to the time required for fermentation, in each said time period a statistically significant number of measurements of sonic velocities are made, the maximum sonic velocity during said time period is established, and the s.g. is derived form the known o.g. and said maximum velocity. 7. The method of claim 6, in which the statistical distribution of said measurements is analysed and the spread of said distribution is used as a measure of solids content and/or outgassing. 8. The method of any preceding claim, in which the liquid is contained within a tank or conduit, and the sonic velocity is measured by means of one or more ultrasonic transducers positioned on the exterior thereof. suBsτ_τϋ * . - ε:~:zετ 9. Apparatus for measuring the s.g. of a liquid, comprising an ultrasonic pulse transmitter and receiver separated by a known path length, means for establishing the transit time of pulses between the transmitter and receiver, a temperature sensor for measuring the temperature of the liquid, and computing means arranged to derive from the transit time and the temperature the s.g. of the liquid. 10. Apparatus according to claim 9, in which the transmitter and receiver are attached to the exterior of a tank or conduit containing the liquid. 11. Apparatus according to claim 9 or claim 10, in which the temperature sensor is attached to the exterior of a tank or conduit containing the liquid and within a thermal insulation. 12. Apparatus according to claim 9, in which the computing means comprises a memory in which is stored data defining the relationship between fermentable liquors of varying o.g.'s and constant sonic velocities in said liquors. 13. Apparatus according to claim 12, for use in measuring the s.g. of a fermentable " liquor of known o.g. during the fermentation thereof, and in which the computing means is arranged to store a value related to transit time for each of a statistically significant number of measurements during a time period which is short in relation to the fermentation process, to derive from the stored data a measure of maximum sonic velocity, and to compute from said measure and said stored data the s.g. of the liquor during said time period. 14. Apparatus for diluting fermented liquor, comprising a mixer, means for supplying said fermented liquor to said mixer at a known rate, means for supplying water to said mixer at a controlled rate, an output conduit from said mixer, and a monitor associated with said output conduit for controlling the water supply means, the monitor comprising apparatus with claim 12.

Inventors: GRAHAM JOHN ALEXANDER, MITCHINSON JAMES COLLIER, SKRGATIC DAMIR MIROSLAV JOSIP, GRAHAM, JOHN, ALEXANDER, MITCHINSON, JAMES, COLLIER, SKRGATIC, DAMIR, MIROSLAV, JOSIP
Applicants: ACUMET PRECISION INSTR LTD, ACUMET PRECISION INSTRUMENTS LIMITED
Application year: 1986
Main classification: G01N9
IPC codes: G01N9, G01N29, G01N33