Abstract:
A sensor for measuring density of a liquid that comprises a float unit having a sealed hollow casing that contains a first magnet and a strain-gauge unit having a sealed hollow casing that contains a strain gauge and a second magnet arranged coaxially to the first magnet. Coaxiality of the magnets is provided by means of a guide rod installed on the casing of the strain-gauge unit and used to guide the float unit by inserting the guide rod into the central opening of the float unit casing. A characteristic feature of the sensor is that changes in the density of the liquid that cause displacement of the float cause detectable deformations of the strain gauge via forces of magnetic interaction between the first and second magnets without physical contact between the magnets. Since the elements of the sensor are located in sealed casings, they are not subject to damage and do not require maintenance.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a device for measuring density of a liquid. More specifically, the invention concerns a device and method for measuring the content of a substance that changes the density of a liquid, e.g., the content of sugar in a grape must during fermentation in a winemaking process. In particular, the aforementioned device and method may find application in conjunction with a wine press that is equipped with means for controlling temperature and sugar content of the must in a winemaking process. 
       BACKGROUND OF THE INVENTION 
       [0002]    Although the subsequent description relates mostly to the content of sugar in a wine, it should be understood that reference to the wine is given only as an example and that the same principle applies to measuring the content of any other substance that is contained in a liquid, the density of which depends on the content of the aforementioned substance. 
         [0003]    Winemaking is a very old and complex process. For about 5,000 years man has used grapes to make wines by fermentation. In fact, wine can be defined as a liquid obtained as a result of partial or complete fermentation of the juice of grapes, other fruits, or berries. However, grape is the only fruit with a sufficiently high level of natural sugar and with the proper balance of acid and nutrients to sustain natural fermentation to dryness with stable results. Fermentation of other fruits and berries may require addition of sugar, acid, or various yeast nutrients to avoid spoiling. 
         [0004]    Although there have been improvements in various aspects of the winemaking process over the years, these improvements have been primarily in the equipment used in the processing of the grapes. The basic reaction by which grapes are transformed into wine remains unchanged. Typically, grapes are crushed to release the juice into a fermentation vessel. When the fermentation is complete, the wine is pressed to separate the liquid from the stems, skin, pips, and pulp. Wine is then stored to age and clarify. 
         [0005]    Ripe grapes naturally have yeast cells residing on their surface that aid and abet the reaction of grapes into wine. When the yeast comes into contact with grape juice, the yeast begins to feed on the juice. The yeast contains the enzyme, zymase, which converts sugar in the grape juice to alcohol and carbon dioxide and which releases heat. The reaction continues naturally until the sugar has been converted or the yeast dies off or weakens. 
         [0006]    Fermentation usually takes about three weeks. During the first few days of the process, which is frequently referred to as “aerobic fermentation,” the reaction usually produces more yeast through reproduction of the yeast cells. This first step is followed by anaerobic fermentation which produces the most alcohol. Fermentation may be permitted to continue until there is no residual sugar, or the reaction may be terminated at some point during the process to vary the level of sweetness. The reaction is usually terminated by killing or removing the yeast cells. This may be accomplished by adding alcohol to raise the level to 15% or more, adding sulfur dioxide or sorbate (sorbic acid), by filtering through a sterile filter, or by chilling the must and filtering out the yeast cells. 
         [0007]    The color of the wine comes from contact of the grape juice, which is clear, with the skin of the grapes. The greater the color of the skin, plus the amount of time the juice is in contact with the skin, increases the color of the wine. Different steps in winemaking can cause variations in taste and bouquet; for example, juice separated from the must before pressing usually has less bitterness and oxidation. The speed and pressure of the press may also affect the wine. Too much pressure in the press may cause bitter tannins to leach from the seeds. 
         [0008]    There is also a second fermentation that occurs in most winemaking. This is called malolactic fermentation. In this type of fermentation, bacteria, i.e., lactobacillus, converts some of the malic acid naturally present in grapes into lactic acid along with the resultant byproduct of carbon dioxide. Malolactic fermentation usually has the effect of softening the wine, i.e., taking some of the edge off the wine. 
         [0009]    Acids are a natural component of wine. However, if a wine is too low in acid, the wine tastes too flat and dull. If the wine has too great an amount of acid, the wine tastes too tart and sour. As a result, the winemaker frequently manipulates acidity in wine. The principal acids formed in grapes and therefore in wine are tartaric acid, potassium hydrogen tartrate, and malic acid and potassium hydrogen malate. The relative amounts of acid depend on the grape variety used to make the wine. In addition, the growing temperature of the grapes can also affect the amount of acidity in a wine. There are other factors that may affect the taste of wine, such as climate of the area in which the grape grows, barrels used for cellaring, exposure to UV light, etc. 
         [0010]    Among all factors listed above, the content of sugar is the main factor that affects the taste of wine and that is used as a criterion for controlling the degree of readiness of the wine as a result of fermentation. 
         [0011]    In the United States the so-called Brix scale measures the sugar content of grapes and wine. The term “Brix” stems from the name of a nineteenth-century German inventor. Normally, Brix (sugar content) is determined by a hydrometer. 
         [0012]    The hydrometer is a simple instrument that measures the weight, or gravity, of a liquid in relation to the weight of water. Because the relation of the gravity to water is specified, the resulting measure is called a specific gravity. A hydrometer floats higher in a heavy liquid, such as one with a quantity of sugar dissolved in it, and lower in a light liquid, such as water or alcohol. In truth, the average winemaker has no interest in the specific gravity of a must, per se, but has a very keen interest in the amount of sugar dissolved in it because yeast converts sugar into carbon dioxide and alcohol. By knowing how much sugar one starts with and ends with, one can easily calculate the resulting amount of alcohol. 
         [0013]    There are many types of hydrometers. Some have only one scale, some two, and some three. The typical hydrometer measures three things: specific gravity (SG), potential alcohol (PA), and sugar. 
         [0014]    The specific gravity scale usually reads from 0.990 to 1.120. The SG of water is 1.000. If one fills a test jar (a deep chimney-shaped vessel that holds from one half to two cups of juice) with water and floats a hydrometer in it, the water surface should rest at the 1.000 mark. As sugar dissolves in water, the hydrometer floats to a higher level. One pound of sugar dissolved in one US gallon (3.7853 liters) of water floats a hydrometer to the level of 1.045. (Please note that one pound of sugar dissolved in one gallon of water is not the same as one pound of sugar added to one gallon of water. In the first instance, there is one gallon of liquid. In the second instance, there is one gallon and ten fluid ounces of liquid.) 
         [0015]    Table wines are generally started at an SG of 1.090 or higher and are fermented to dryness, i.e., 0.990 to 1.000. Sweeter wines are started at a higher SG and are stabilized at the desired sweetness, or, more commonly, started at 1.090 or higher, fermented to dryness, stabilized, and sugar added back to the wine to sweeten it. 
         [0016]    Sugar can be measured in ounces per gallon or in degrees Balling, or Brix. Ounces per gallon are measured on a numeric scale in which an SG of 1.045 equals 16 oz. (one pound) sugar per U.S. gallon. Brix is measured as a percentage of sugar by which pure water has a Brix of 0 (or 0% sugar), an SG of 1.045 equals a Brix of 11.7 (11.7% sugar), and an SG of 1.095 equals a Brix of 23.1 (23.1% sugar). 
         [0017]    The potential alcohol scale typically reads from 0 to 16%. Using the standard hydrometer, one cannot measure the alcohol in a finished wine. But one can measure the PA before the yeast is added and again after fermentation is complete. By simple subtraction, the PA lost is the percentage of alcohol in the finished wine. 
         [0018]    Other methods and devices are available for measuring sugar content in a liquid, e.g., in wine. One such method is the use of floats immersed into the liquid and then changing the position of the float in the liquid in response to the change in liquid density. 
         [0019]    U.S. Pat. No. 6,026,683 issued in 2000 to Buyong Lee describes a liquid-level measuring system using a strain-gauge load cell. The system includes a buoyancy weight standing upright in liquid held in an object to be measured in a long bar shape, and having an average density substantially the same as that of the liquid; the strain-gauge load cell producing the change in the buoyancy weight varies with the liquid level when there is a change in an electrical signal; an amplifier amplifying a voltage signal detected from the strain-gauge load cell; an analog/digital converter converting an amplified voltage signal into a digital signal; a central processing unit applying a driving pulse to the strain-gauge load cell and average-operating a plurality of digital signals to find an average value of the liquid level; and a display displaying the average value of the liquid level as a number. A disadvantage of this device is the limited area of use since it is intended for a specific application. The measurement element, i.e., the rod that connects the float with the strain gauge and the strain gauge, itself, is exposed to the liquid to be controlled or to its vapors. Therefore the device is difficult to maintain. 
         [0020]    UK 1,165,451 Patent published in 1969 to W. Becker, et al., discloses a float for the accurate measurement of a liquid level that is provided with a temperature-responsive element, such as an expansion body, which moves an inductor disk vertically to compensate for deeper immersion of the float in the liquid due to a decrease in liquid density with a rise in temperature. The temperature-responsive element may be bimetallic strips used in conjunction with a sleeve of magnetic material within the float, or the element may be a fluid expanding a bellows supporting a cylinder with the float. 
         [0021]    Russian Patent No. 2193181 issued in 2002 to M. Kagan describes a device for measuring density of a liquid, in particular for continuous density measurement of petroleum products directly in storage tanks. The principle of the invention consists of converting movements of a float into an electric signal. A signal converter comprises an autogenerator of RF oscillations. One of the oscillation circuits is a coaxial line vertically submerged into the liquid being controlled, along with a surrounding float that supports an external, annular permanent magnet. This magnet interacts with another magnet that is located inside the coaxial line. When density of the liquid changes, the outer magnet moves in the appropriate direction due to change in buoyancy and causes movement of the inner magnet. The latter is connected to a short-circuiting plunger made from a non-magnetic material and having sliding galvanic contact with both tubes of the coaxial line. When the plunger moves, it changes the active length of the coaxial line and thus changes the frequency of the signal generated by the signal generator in response to the density of the liquid. The device of this invention is complicated in structure and is difficult to maintain. 
         [0022]    European Patent EP 0073158 published in 1983 (inventors R. Guay, et al.) describes an electronic hydrometer for determining the density of a liquid. The hydrometer comprises a housing having a liquid-receiving chamber and means to insert a liquid, the density of which is determined in the chamber. A float having a known weight is disposed in the chamber. A connecting shaft is secured to the float and is retained for guided displacement along its longitudinal axis in response to introduction of a predetermined quantity of liquid in the chamber. A sensor is associated with a shaft for detecting a reference position of the shaft. A permanent magnet is mounted near the shaft and has a magnetic core and a displacement coil surrounding the core. The coil is secured to the shaft. An electronic circuit means automatically adjusts the valve of the current flowing in the coil to displace the coil and the shaft relative to the reference position. The circuit calibrates the value of the adjusted current in order to generate an output signal that indicates the density of the liquid in the chamber. 
         [0023]    Thus, the common disadvantage of the known sensors of a float-and-strain-gauge type is their complicated construction or inconvenient maintenance. 
       SUMMARY 
       [0024]    It is an object of the invention to provide a sensor for measuring density, which is simple in construction, inexpensive to manufacture, reliable in operation, highly sensitive, and easy to maintain. Another object is to provide the aforementioned sensor in which a force proportional to change in density of the liquid is transmitted to the strain gauge in a manner that requires no contact. Another object is to provide a sensor of the aforementioned type for measuring the content of sugar in a winemaking must where the force is magnetically transmitted from the float to the strain gauge. Another object is to provide a method for measuring the density of a liquid by means of a sensor with magnetic interaction between a magnet installed in a float and a magnet installed in a strain-gauge unit. 
         [0025]    The liquid-density control sensor of the invention (hereinafter referred to as a sensor) consists of two main units, i.e., a float unit and a strain-gauge unit. Both aforementioned units are independently sealed, spaced apart from each other, and magnetically interactive via magnets installed in both units. The entire sensor, i.e., the float unit and the strain-gauge unit, are completely submerged into a liquid, the density of which is to be controlled. More specifically, the strain-gauge unit consists of a strain-gauge holder that is made in the form of an elongated strip or a bar of a substantially flexible material that supports the strain gauge cemented to its surface and a first permanent magnet of certain polarity. The strip or bar that carries the strain gauge and magnet is placed into a sealed casing, and the strip or bar is—in a cantilevered manner—attached to a stationary object, e.g., a wall of the aforementioned box. The vertical position of the sealed casing may be adjusted by moving it along the guides provided on the inner wall of the container. The lead wires of the strain gauge are guided to the outside of the container in a sealed manner, e.g, via feedthrough devices, and are further guided to a measurement instrument, e.g., a potentiometer or a Wheatstone bridge, or a similar device for accurate measurement of changes in electrical resistance. The upper surface of the casing supports a vertically arranged guide rod which is coaxial with the position of the first magnet. This guide rod slidingly supports the aforementioned float unit that consists of a sealed hollow body provided with a through-central opening into which the guide rod is inserted without violation of the hermetic conditions inside the sealed body of the float. Attached to the bottom of the hollow, sealed float body is a second permanent magnet with a polarity that provides magnetic interaction with the first permanent magnet. 
         [0026]    The forces acting on the components of the sensor are the following. In addition to hydrostatic pressure that uniformly and from all directions acts on the float surfaces, the float unit immersed into the liquid experiences its own gravity force. A force that acts on the float immersed into the liquid is a well known Archimedean force that is equal to the weight of the liquid displaced by the float. It is understood that when the Archimedean force is equal to the weight of the float, the latter is maintained in a freely floating state, i.e., in the so-called weightless condition in the liquid. 
         [0027]    Let us assume that under a predetermined density of the liquid, a given float is maintained in a weightless condition in the liquid. If the float is rigidly connected to a strain gauge, then under the aforementioned weightless condition of the float, the strain gauge does not detect any force. However, if density of the liquid increases, the float experiences a positive buoyancy that tends to raise the float to a higher level. In this case, the strain gauge will detect a force that is proportional to the variation in the liquid density. Similarly, if the density of the liquid decreases, the float experiences negative buoyancy that tends to move the float to a lower level. In this case as well, the strain gauge will detect a negative force that is proportional to the variation in the liquid density. If the float is designed with initial negative buoyancy, this negative buoyancy can be compensated through a certain coupling between the float and the gauge. In the present invention, in the case of negative buoyancy of the float, such a coupling is realized in the form of a repelling force between two magnets, one of which is connected to the float unit and another to the strain-gauge unit. If the float is designed with initial positive buoyancy, this positive buoyancy can be compensated through the use of attractive forces between the magnets. Since the force of interaction between the magnets to a great extent depends on the distance between the magnets, even slight changes in the position of the float will create well measurable deformation in the strain gauge. It is understood that changes in the liquid density will change buoyancy of the float and thus cause deformations on the strain gauge. 
         [0028]    Provision of the vertical guide rod on which the float slides in the vertical direction restricts the float from movements in the transverse direction. This makes it possible to improve accuracy of measurement since the interactive forces between the magnets strongly depend on the degree of their coaxiality. 
         [0029]    Since fermentation is accompanied by formation and accumulation of gas bubbles (CO 2 ) on the surface of the float, which is undesirable, it is recommended that the surface of the float be spherical for surface minimization. Another factor is minimal roughness of the float surface. Furthermore, the surface of the float should be sufficiently large in order to exclude the effect of the bubbles on the buoyancy of the float. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a vertical sectional view of the device according to one embodiment of the invention in which the magnets of the float unit and the strain-gauge unit have opposite poles of the same polarities (repelling force between magnets). 
           [0031]      FIG. 2  is a diagram of forces acting on the float of the device in  FIG. 1 . 
           [0032]      FIG. 3  is a vertical, sectional view of the device according to another embodiment of the invention in which the magnets of the float unit and the strain-gauge unit have opposite poles of different polarities (attracting force between magnets). 
           [0033]      FIG. 4  is a diagram of forces acting on the float of the device in  FIG. 3 . 
           [0034]      FIG. 5  is an embodiment of the invention with the position of the sensor adjustable in the vertical direction relative to the level of the liquid in the container. 
           [0035]      FIG. 6  is a side sectional view of the device according to the embodiment of the invention where the strain gauge casing is arranged above the level of the liquid. 
           [0036]      FIG. 7  a side sectional view of the device according to the embodiment of the invention where the guide means for the floating unit is made in the form of a tubular body into which the floating unit is inserted with a sliding fit. 
           [0037]      FIG. 8  is a sectional view of a device according to another embodiment of the invention, wherein means for maintaining the first magnet and the second magnet in coaxial alignment comprise flexible elements that support the floating unit in a spaced position above the strain gauge unit in a bridge-like manner. 
           [0038]      FIG. 9  is an example of a graph that was obtained for a magnetically coupled sensor of the invention and that shows dependence of an electrical signal that corresponds to the density of the liquid (in mV units) from the content of sugar in the wine must (in Brix scale units). 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0039]    A sectional schematic view of the device of the invention is shown in  FIG. 1 . It can be seen that a liquid-density control sensor  20  of the invention (hereinafter referred to as a sensor) consists of two main units, i.e., a float unit  22  and a strain-gauge unit  24 . Both aforementioned units  22  and  24  are independently sealed, spaced apart from each other, and magnetically interactive via magnets  26  and  28  rigidly installed in both units, respectively. 
         [0040]    The entire sensor  20 , i.e., the float unit  22  and the strain-gauge unit  24 , are completely submerged into a liquid  30 , the density of which is to be controlled. The liquid  30  is held in a container  32 , only a wall  34  of which is shown in  FIG. 1 . The liquid  30  may comprise any liquid medium, the density of which may change. For example, this may be a salt solution, the density of which changes depending on the concentration of salt; or it may be a wine must, the concentration of which may vary depending on the percentage of sugar. For the sake of example only, let us consider the case of a wine must, the density of which changes in the course of fermentation during the winemaking process. 
         [0041]    More specifically, the strain-gauge unit consists of a strain-gauge holder  36  that is made in the form of an elongated strip or a bar of a substantially resilient non-conductive and non-magnetic material that supports a strain gauge  38  cemented to the surface of the holder. 
         [0042]    The holder  36  also supports a first permanent magnet  28  of certain polarity. Preferably, the magnet should have a symmetrical shape, e.g., round or square, and should have one of the magnetic poles on the surface of the holder and the opposite magnetic pole on the surface perpendicular to the first pole. A preferable type of the magnet  28  suitable for the invention is a magnetic rod or a disk with opposite poles on opposite end faces of the rod or the disk. 
         [0043]    The holder  36  that carries the strain gauge  38  and the aforementioned magnet  28  is placed into a sealed casing  42 , and the holder  36  is attached in a cantilevered manner to a stationary object, e.g., a wall of the aforementioned casing  42 . In the embodiment shown in  FIG. 1 , the casing  42  is rigidly attached to the wall  34  of the container  32  by means of a connection device  44  with a feedthrough device  46  for guiding lead wires  48  to the measurement unit (not shown in  FIG. 1 ), such as a potentiometer or a Winston bridge, or a similar device for accurate measurement and registration of changes in electrical resistance. 
         [0044]    The upper surface  50  of the casing  42  supports a vertically arranged guide rod  52 , which is coaxial with the position of the first magnet  28 . This guide rod  52  slidingly guides the aforementioned float unit  22  that consists of a sealed hollow body  54  provided with a through-central opening  56  into which the guide rod  52  is inserted without violation of the hermetic conditions inside the sealed body  54 . Attached to the bottom of the hollow, sealed float body  54  is the aforementioned second permanent magnet  26  with a polarity that provides magnetic interaction with the first permanent magnet  28 . In order to adjust the weight of the float unit  22  and to fix the second magnet  26  inside the float body  54 , the latter may be filled with a light material such as foam plastic  23 . 
         [0045]    The forces acting on the components of the sensor  20  are shown in  FIGS. 1 and 2 , where  FIG. 2  is a view that for convenience of explanation shows only the float unit  22 . The forces acting on the float unit  22  shown in  FIGS. 1 and 2  relate to the case of negative buoyancy of the float  22 . 
         [0046]    In addition to the hydrostatic pressure shown in  FIG. 2  by arrows H that acts uniformly from all directions on the float surfaces, the float unit  22  immersed into the liquid  30  experiences its own gravity force P. Another force that acts on the float unit  22  immersed into the liquid  30  is the well known Archimedean force F A  that is equal to the weight of the liquid displaced by the float unit  22 . It is understood that when the Archimedean force F A  is equal to the weight of the float, the latter is maintained in a freely floating state, i.e., in the so-called weightless condition in the liquid. 
         [0047]    Let us assume that under a predetermined density of the liquid  30 , a given float unit  22  is maintained in a weightless condition in the liquid. If the float unit  22  were rigidly connected to the holder  36  of the strain gauge  38 , then under the aforementioned weightless condition of the float unit  22 , the strain gauge  38  would not detect any force. If, in this case, density of the liquid increases, the float unit  22  experiences positive buoyancy that tends to raise the float to a higher level. The strain gauge  38  then detects a force that is proportional to the increase in the liquid density. Similarly, if the density of the liquid  30  were decreased, the float would experience negative buoyancy that tends to move the float to a lower level. In this case as well, the strain gauge  38  detects a force that is proportional to the decrease in the liquid density. If the float is designed with certain initial negative buoyancy, this negative buoyancy is compensated through a certain coupling between the float unit  22  and the strain gauge  38 . 
         [0048]    In contrast to the above condition with a mechanical kinematic link between the float unit and the strain-gauge holder, in the case of the present invention and, in particular, of the embodiment shown in  FIG. 1  with negative buoyancy of the float unit  22 , the aforementioned link between the float unit  22  and the strain-gauge holder  36  is realized in the form of a repelling force F M  ( FIG. 2 ) between two magnets  26  and  28  ( FIG. 1 ), one of which ( 26 ) is connected to the float unit  22  and another ( 28 ) to the holder  36  in the strain-gauge unit  24 . 
         [0049]    In the case of negative buoyancy and arrangement of the magnets  26  and  28  in the manner shown in  FIG. 1 , the float unit  22  assumes a position on the guide rod  52  that is determined by the following balance of forces: F A +F M =P. 
         [0050]    Since the interaction between the magnets  26  and  28  to a great extent depends on the distance between them, even slight changes in the position of the float  22  will create well measurable deformation in the strain gauge  38 . It is understood that changes in the density of the liquid  30  will change buoyancy of the float unit  22  and thus cause deformations on the strain gauge  38 . 
         [0051]    Provision of the vertical guide rod on which the float slides in the vertical direction restricts the float from movements in the transverse direction. This makes it possible to improve accuracy of measurement since the interactive forces between the magnets strongly depend on the degree of their coaxiality. 
         [0052]      FIG. 3  illustrates another embodiment of the device of the invention that is substantially the same as the embodiment of  FIG. 1 , except that the poles of the magnets generate a force of mutual attraction instead of a repellant force. Since the majority of the parts of a sensor  120  shown in  FIG. 3  is identical to those shown in  FIG. 2 , they will be designated by the same reference numerals with an addition of 100, and their detailed description will be omitted.  FIG. 4  is similar to  FIG. 2 , except that force F M  has a direction opposite to one shown in  FIG. 2 . In accordance with this embodiment, the permanent magnets  126  and  128  have magnetic poles of opposite signs whereby the following balance of forces can be written, as shown in  FIG. 4 : F A =P+F M , where F A , P, and F M  have the same meanings as defined above. It is understood that when density of the liquid decreases and causes the float unit  122  to move toward the strain-gauge unit  124 , the distance between the magnets will be reduced. However, this will increase the force attraction between the magnets and at a predetermined position of the float will violate the condition of balance of forces acting on the float unit  122 . In other words, the force of mutual attraction will tend to bring the magnets  126  and  128  into mutual physical contact. Nevertheless, the sensor of  FIG. 3  will work in a predetermined range of forces F A , P, and F M , the misbalance of which can be prevented by a stopper  129  formed on the guide rod  152 . This stopper  129  is located on the rod  152  in a position that provides substantial balance between the force to satisfy the equation F A =P+F M  ( FIG. 4 ). Since the sensor  120  of the embodiment of  FIG. 3  can operate under limited conditions, use of the embodiment in  FIG. 2  is preferable. 
         [0053]    The lead wires of the strain gauge are guided to the outside of the container  132  in a sealed manner, e.g, via a connector  144  with a feedthrough device  146 . 
         [0054]    In the case of both embodiments, the lead wires are guided further to a measurement instrument (not shown), e.g., a potentiometer or a Wheatstone bridge, or a similar device for accurate measurement of changes in electrical resistance. The obtained analog signal is converted into a digital form and is either registered in a computer, CPU, or the like, or is used for controlling the process through a feedback line (not shown). The measurement system is conventional and is beyond the scope of the present invention. 
         [0055]    Since some technological processes may be accompanied by variations in temperature, which, in turn, may affect the data detected by the sensor, the aforementioned data can be corrected to compensate deviations caused by variations in temperature of the liquid. A good example of such a process is fermentation of a must in the production of wine. In this process, the temperature of the must varies in the range of 10° C. to 35° C. During fermentation, the must density changes in the range of 1.10 to 0.99 or about 10%. Therefore, in order to provide optimal operation of the sensor in the aforementioned density change range, the following calibration procedure is carried out. A liquid is selected, the density of which is known at a predetermined temperature. Following this, the temperature of the liquid is changed at a certain temperature interval, e.g., 2° C., and the density is measured for each temperature within the expected temperature change interval. This procedure is repeated for the entire density change interval. As a result, certain data are accumulated and are stored in a memory device of the measurement system. Since in the controlled process the temperature of the liquid is measured independently, e.g., by a thermocouple located in the liquid, the density measurement data can be corrected. 
         [0056]      FIG. 5  illustrates another embodiment of the sensor  220  of the invention, according to which the entire sensor unit  222  can be shifted in the vertical direction on a guide  223  formed on the wall  234  relative to the level L of the liquid  230  in the container  232 . Positions of the sensor  220  on the guide  223  can be fixed by a screw  225 . The lead wires  248  are guided from the casing  242  to the outside of the container  232  first via a feedthrough  246   a  to the container  232  and then via a feedthrough  246   b  to the outside of the container  232  to the measurement instrument. The portion  227  of the lead wires  248  that is located in the liquid  230  is isolated by a sealed coating and is wound into a loop in order to compensate for displacements of the sensor unit  222 . Reference numeral  252  designates a guide rod;  226  is a magnet of the float unit, and  228  is a magnet of a strain gauge unit. 
         [0057]      FIG. 6  illustrates another embodiment of the invention wherein the parts similar to those of the embodiment of  FIG. 1  are designated by the same reference numerals with an addition of  200  and a prime sign. For example, in the embodiment of  FIG. 6  the strain gauge unit  24  of  FIG. 1  will be designated by reference numeral  224 ′. The float unit  22  of  FIG. 1  will be designate in  FIG. 6  by reference numeral  222 ′, etc. In general, the embodiment of  FIG. 6  is similar to the embodiment of  FIG. 3  and differs from it in that the strain gauge unit  224 ′ is arranged above the level of the liquid  230 ′ and that the guide means, i.e., the guide rod  252 ′, is attached to the lower side of the casing  242 ″ and is directed in the downward direction towards the sealed hollow body  254 ′ of the float unit. Similar to the embodiment of  FIG. 1 , the guide rod  252 ′ passes in a sealed manner through the central opening  256 ′ of the sealed hollow body  254 ′, and the magnets  226 ′ and  228 ′ face each other with the magnetic poles of the same polarity. The embodiment of  FIG. 6  is convenient in that it provides more convenient excess to the strain gauge for its maintenance, replacement, or repair. The hollow casing  242 ′ need not be sealed as it is not immersed into the liquid. 
         [0058]      FIG. 7  illustrates another embodiment of the invention, in which the guide means  352  for guiding the magnet  326  of the float unit  322  in alignment with the magnet  328  of the strain gauge unit  324  is made in the form of a tubular body attached to the upper surface  350  of the of the casing  342 . The sealed hollow body  354  of the float unit  322  has a spherical shape and is slidingly guided in the tubular guide  352 . In order to ensure buoyancy of the spherical floating unit in the guide tube  35  which is immersed into the liquid, the tubular guide  352  has a plurality of perforations  353   a ,  353   b , . . .  353   n  in its side wall. The rest of the device for measuring density of liquid is the same as in other embodiments. 
         [0059]      FIG. 8  is a sectional view of a device according to another embodiment of the invention, wherein means for maintaining the first magnet  426  and the second magnet  428  in coaxial alignment comprise flexible elements  427  and  429  that support the floating unit  422  in a spaced position above the strain gauge unit  424 . The flexible elements  427  and  429  may comprise, e.g., leaf springs flexibility of which resists the force of magnetic interaction. As can be seen from  FIG. 8 , the flexible elements have one ends attached to the supports  423  and  425  that project in a vertical directions from the upper surface of the strain gauge casing  424  while other ends of the flexible elements are attached to the opposite sides of the sealed casing of the floating unit  422  so that the floating unit is supported above the strain gauge casing in a bridge-like manner. Depending on whether the magnets face each other with poles of the same or opposite polarity, the flexible elements  427  and  429  should comprise leaf strings of compression or expansion nature. 
         [0060]      FIG. 9  is an example of a graph that was obtained for a magnetically coupled sensor of the invention and that shows dependence of an electrical signal that corresponds to the density of the liquid (in mV units) from the content of sugar in a wine must (in Brix scale units). 
         [0061]    Thus, it has been shown that the invention provides a sensor for measuring density which is simple in construction, inexpensive to manufacture, characterized by improved conditions for maintenance, able to transmit the force proportional to the change in density of the liquid to the strain gauge without contact, reliable in operation, highly sensitive, and suitable for measuring the content of sugar in a winemaking must wherein the force is magnetically transmitted from the float to the strain gauge. The invention also provides a method for measuring density of a liquid by means of a sensor with magnetic interaction between a magnet installed in a float and a magnet installed in a strain-gauge unit. 
         [0062]    The invention has been shown and described with reference to specific embodiments, which should be construed only as examples and do not limit the scope of practical applications of the invention. Therefore, any changes and modifications in technological processes, constructions, materials, shapes, and components are possible, provided these changes and modifications do not depart from the scope of the patent claims. For example, the liquid is not necessarily a winemaking must but may comprise a salt solution, juice, oil, petrochemical product, etc. The principle of the invention can be used for measuring the level of a liquid in a container or other characteristics that may be determined by measuring variations in electrical resistance of a strain gauge caused by displacement of a float. Guiding and/or magnetic repulsion can be done by electromagnets. The casing need not be hollow and can have positive or negative buoyancy. The strain gauge and housing may also be the float with the magnet fixed. The alignment can also be maintained by a highly flexible thin beam or any low friction coupling method. The strain gauge may be attached to the upper or to the lower surface of the elongated holder. In an embodiment where the floating casing is supported above the strain gauge casing by the flexible elements in a bridge manner, the strain gauge may be attached to the aforementioned flexible element instead of the beam which supports the magnet.