SYSTEM FOR MEASURING THE LEVEL OF A SURFACE

The present invention refers to a system for measuring the level of a surface, aimed in particular to verify the measure of a surface level with respect to a geoid.

It is known that, in modern industrial contexts, and in particular in the field of industrial buildings and laying of big-sized machines that are sensibile to positioning accuracy, such as for example measuring machines or machine tools, it is particularly important that the planarity is verified for surfaces, pavements or installation basements, that ofter have big sizes.

In order to perform such checks, the art evolved by providing several arrangements, that range from the simple use of mechanical levels (such as for example the classical bubbles) or electronic laser levels. While the former ones have obvious accuracy limits, this making now very rare their use in technologic fields requiring on the contrary extremely high measuring accuracies, the latter ones have an extremely complex and long application, requiring the intervention of specialised technicians that are rare to find, and consequently costly.

Therefore, object of the present invention is solving the

above prior art problems by providing a system for measuring the level of a surface that can be more accurately, quickly, practically and economically applied.

Another object of the present invention is providing a measuring system that is able to determine a level difference between at least two points placed in the space by measuring the hydrostatic thrust of a measuring fluid.

The above and other objects and advantages of the invention, as will appear from the following description, are reached with a system for measuring the level of a surface as described in claim 1. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims .

It will be immediately obvious that numerous variations and modifications (for example related to shape, sizes, arrangements and parts with equivalent functionality) can be made to what is described without departing from the scope of the invention as appears from the enclosed claims.

The present invention will be better described by some preferred embodiments thereof, provided as a non-limiting example, with reference to the enclosed drawings, in which:

- Figure 1 shows a diagram that schematically shows the components of a preferred embodiment of the measuring system according to the present invention;

- Figure 2 shows a diagram that schematically shows the

components of another preferred embodiment of the measuring system according to the present invention;

- Figure 3 shows a diagram that schematically shows the components of another preferred embodiment of the measuring system according to the present invention;

- Figure 4 shows a diagram that schematically shows the components of another preferred embodiment of the measuring system according to the present invention;

- Figure 5a shows a longitudinally sectional view of a preferred embodiment of a component of the measuring system according to the present invention;

- Figure 5b shows an orthogonally sectional view of the component of Figure 5a;

Figure 6a shows a longitudinally sectional view of another preferred embodiment of a component of the measuring system according to the present invention;

- Figure 6b shows an orthogonally sectional view of the component of Figure 6a;

- Figures 7a and 7b show a practical example of an embodiment of the measuring system according to the present invention of Figure 1 in two different measuring situations;

- Figures 8a and 8b show a practical example of an embodiment of the measuring system according to the present invention of Figure 2 in two different measuring situations;

- Figures 9a and 9b show a practical example of an embodiment of

the measuring system according to the present invention of Figure 3 in two different measuring situations; and - Figures 9c and 9d show other two practical example of embodiments of the measuring system according to the present invention of Figure 3.

The system 1 for measuring the level of a surface according to the present invention, aimed in particular to verify the measure of a level of at least one surface with respect to a horizontal plane or geoid comprises means for measuring the hydrostatic thrust of at least one measuring fluid F, against the pressure action of the external environment, typically the atmospheric pressure, in at least one hydraulic circuit preferably made, as will be described below in more detail, as an airtight circuit with constant temperature and shape able to determine the level difference between at least two measuring points Pl, P2, ..., Pn.

With reference in particular to Figures 1 to 4, it is possible to note that the measuring means of the measuring system 1 according to the present invention comprise: at least two means for measuring the pressure 3 and/or 5 arranged on at least two measuring points Pl, P2, ..., Pn that are different and mutually connected through first fluid connection means 6 containing at least one measuring fluid F, such means for measuring the pressure 3 being adapted in particular to evaluate the hydrostatc pressure exerted by such measuring fluid

F in the corresponding measuring point Pl, P2, ..., Pn against the pressure action of the external environment, typically the atmospheric pressure: such means can be made as pressure sensors 3, whose ranges and accuracies are compatible with the measures that have to be detected; the pressure-sensitive means can also be made as vases 5, containing at least the measuring fluid F in liquid form, adapted to be directly abutted in at least one measuring point Pl, P2, ..., Pn of the surfaces to be verified;

- pressure/volume regulating/compensating means 7 connected through second fluid connection means 8 to the first fluid connection means 6, possibly by interposing control valves 12, such means 7 being adapted to regulate the pressure inside the first fluid connection means 6 and to compensate the volume by changing temperature and shape, consequently keeping the pressure-sensitive means 3 and/or 5 within the appropriate measuring field to avoid positive and negative saturation; and

- means for displaying the measuring results performed by the pressure-sensitive means, and in particular by the pressure sensors 3.

Obviously, the pressure sensor 3 can be of any type suitable for the purpose, namely meant as meters or feelers with electrical, mechanical operation, etc. : in particular, in the various possible embodiments of the present invention, normal or differential pressure sensors 3 can be used. The nature of the pressure sensor 3 (with plunger, membrane, etc..) applied to a

capacitive, inductive, resistance transducder, etc., that can be with a load cell, a spring, a mechanical pointer (micrometric, etc.), with electronic pressare converter or the like, depends solely on the degree of measure accuracy and on pratical needs of a user of the system 1 according to the present invention.

The pressure sensors 3 are adapted to be directly abutted in a measuring point Pl, P2, ..., Pn on the surface to be verified: preferably, in order to make the measure easy, practical and accurate, it is advisable to accurately fasten each sensor 3 to its related supporting container, ground in such a way as to guarante a safe, accurate and steady contact with the reference or measuring points of the abutment surface: in addition, such supporting container could be connected to an adapter of a various shape and nature, that allows being able to measure points or surfaces with particular workings or shapes such as, for example, the surface at the bottom of a blind hole, the bottom of a recess, prism-shaped, ball-shaped surfaces or surfaces with other irregular shapes.

In parallel, the vases 5, meant as sensors or feelers, in the various possibile embodiments of the present invention, could be both with free surface and with a membrane. The vases 5, moreover, having a similar operation to the above pressure sensors 3, can use the same arrangements of these latter ones, such as, for example, the supporting container and/or the adapters. Obviously, the measuring fluid F in liquid form is

compatible with used materials and suitable for the temperature range in which one operates.

To guarantee the correct and accurate operation of the measuring system 1 according to the present invention, the value of the atmospheric pressure must be equal in all measuring points Pl, P2, ..., Pn in which the pressure-sensitive means 3 and/or 5 are arranged; if instead the measuring points Pl, P2, ..., Pn have different atmospheric pressure values, the pressure- sensitive means, being both pressure sensors 3 and/or vases 5, must be mutually connected through at least one equalising circuit 15, containing therein an equalising fluid, preferably in gaseous form, in connection with an environment at atmospheric pressure or another different pressure, for example by interposing at least one control valve 16. Alternatively, always if the measuring points Pl, P2, ..., Pn have' different atmospheric pressure values, such pressure differential must be integrated in the reading of the measuring results.

Obviously, the pressure/volume regulating/compensating means 7 are not necessary during the measure with the system 1 according to the present invention, but are used as expansion vase during transport, storage, filling, compensation and calibration of the system 1 itself. The pressure/volume regulating/compensating means 7 can be used with a measuring fluid F both in liquid form and in gaseous form. The pressure/volume regulating/compensating means 7 could further be

fluidically connected, through third fluidic connection means 10 and, possibly, by interposing at least one control valve 11, with at least one vessel 9 containing the measuring fluid F, if in liquid form, to allow filling or topping-up the further fluidic connection means 6, 8 of the measuring system 1 according to the present invention. Also the presence of the vessel 9 has obviously no influence as regards the measuring.

Moreover, other pressure/volume regulating/compensating means 7 and their related vessel 9 could be connected to the equalising circuit 15 to change the internal pressure of the equalising fluid and, in thi way, change the ambient pressure operating on the measuring fluid F and according to which the pressure-sensitive means 3 and/or 5 operate.

The fluidic connection means 6, 8, 10 and/or 15 are made as hydraulic or pneumatic circuits, preferably undeformable, composed of connection ducts compatible with the form, liquid or gaseous, of the measuring fluid F used in the measuring system 1 according to the present invention. In particular, the connection ducts can be, according to the application, of a rigid, semirigid, flexible, metallic, plastic type etc., with suitable diameters to avoid the capillary effect; to avoid different behaviours of the system 1, it is preferable to use a single type of pipe in the same circuit.

In order to increase stability, accuracy and repeatability of the performed measure of the system 1 according to the

present invention, with reference to Figures 5a and 5b, it is possible to note that a preferred embodiment of the fluidic connection means 6, 8, 10, and/or 15 comprises at least one connection duct 20 composed of at least one external layer made of insulating material 21 and at least one first internal duct 22 for passing a first measuring fluid F, between the external layer made of insulating material 21 and the first internal duct 22 being interposed at least one hollow space 24 for passing at least one conditioning fluid adapted to keep indirectly stable the temperature of the measuring fluid F passing inside the first internal duct 22. As alternative, with reference to Figures 6a and 6b, the connection duct 20 could comprise inside it at least one second internal duct 23 for passing, for example, the equalising fluid: in this case, therefore, the connection duct 20 could transport two mutually different measuring fluids, for example in liquid form inside the first internal duct 22 and in gaseous form inside the second internal duct 23, keeping both conditioned by the conditioning fluid passing inside the hollow space 24. Obviously, the number of internal ducts of the connection duct 20 could also be greater than two, depending on specific application needs of the measuring system 1 according to the present invention.

In addition, always to increase stability, accuracy and repeatability of the performed measure of the system 1, the valves, the pressure sensors 3 and/or the vases 5 could be

provided with at least one insulating layer, in order to prevent that, by operating in environments with zones at different temperatures, they are affected when measuring.

In a preferred embodiment thereof, the displaying means could be individual, analogue or digital displays, each arranged on the respective pressure-sensible means 3: in this case, each pressure-sensible means 3 would be equipped with an autonomous electric current supply through a cable or through at least one electric battery. Instead, in case of direct coupling pressure sensors 3, for example with mechanical, pneumatic or hydraulic coupling, it is possibile to obtain a direct reading of the measure: in this case, the displaying means could be at least one comparator, with manometer or another instrument with adequate sensitivity cooperating with the sensor 3 itself. If the pressure sensors 3 are of the electronic type and with analogue output, the displaying means could be made as a power supply and a tester with wnich measures can be directly performed on the sensors 3 themselves. Alternatively, the displaying means could comprise at least one data acquiring device 14 connected to each pressure sensor 3 included in the used configuration of the measuring system 1 according to the present invention, possibly by interposing at least one analogue/digital or digital/analogue converter 13 if the data acquiring device 14 is arranged for a different data input from the sensors. Instead, in case of sensors directly compatible

with the acquisition device 14, the converter 13 is therefore not necessary. In particular, the data acquiring device 14 can be with PC with a suitable software, with "data logger" or another adequate electronic device, compatible with the needs of the user of the measuring system 1 according to the present invention .

Obviously, when manufacturing and before using the measuring system 1 according to the present invention, pressure sensors 3, fluidic connection means 6, 8, vases 5, pressure/volume regulator/compensator 7 and related valves, must be accurately filled-in with the measuring fluid F suitable for characteristic temperatures of the environments in which it is desired to operate and for used materials, making carefully vent air present inside them and afterwards proceeding to reset or calibrate the system 1.

With reference to Figures 1 to 4, some preferred embodiments of the measuring system 1 according to the present invention wil now be analysed, using different configurations and arrangements of the pressure-sensitive means, in particular of the pressure sensors 3 and/or of the vases 5, and of the first and second fluid connection means, respectively 6 and 8.

With particular reference to Figure 1, it is possible to note that the measuring system 1 according to the present invention comprises a pressure sensor 3, indifferently of the normal or the differential type, arranged on a first measuring

point Pl and connected through a connection duct 6, that makes the first fluid connection means, to a vase 5, indifferently of the type with free surface or with a membrane, arranged on a second measuring point P2. The pressure/volume regulating/compensating means 7 and the vessel 9 can be connected to the connection duct 6 through the second fluid connection means, also shaped as a connection duct 8, possibly by interposing a control valve 12. It must be noted that the position of the connection duct 6 that connects the sensor 3 and the vase 5 arranged in the measuring points Pl and P2 must not necessarily be horizontal, but can assume infinite forms in the space, provided that the connection duct 6 remains with constant temperature and shape. In this case, the measuring fluid F is preferably in liquid form. This configuration of the system 1 is accurate under constant conditions for temperature and shape of the connection duct. In this case, the evaluation of the level difference between the first measuring point Pl and the second measuring point P2 is performed by a computation that takes into account the pressure value, returned by the pressure sensor 3, compensated by the measuring fluid F density, the scale factor and, possibly, the different atmospheric pressure values between the first measuring point Pl and the second measuring point P2 (if the pressure sensor 3 and the vase 5 are not mutually connected through the equalising circuit 15) . There must further be the chance of inverting the position of the pressure sensor 3

and of the vase 5 when measuring or calibrating. With reference to Figures 7a and 7b, it is possibile to note the operating principle of the measuring system 1 of Figure 1: merely as an example, in no way a limiting one, in Figures 7a and 7b the pressure sensor 3 is shown as a substantially known mechanical meter comprising at least one plunger 3a directly in contact with the measuring fluid F and counterbalanced by the thrust of dynamometric springs 3b, such plunger cooperating with displaying means made as a mechanical comparator 3c, possibly with a customised and compensated measuring scale, and related feeler 3d. The embodiment of the vase 5, comprising at least one plunger 5a directly in contact with the measuring fluid F and counterbalanced by the thrust of dynamometric springs 5b, is substantially similar to the one of the pressure sensor 3, but lacks the mechanical comparator 3c.

Figure 7a shows the behaviour of the measuring system 1 of Figure 1 if the pressure sensor 3 and the vase 5 are at the same level. The comparator 3c and the feeler 3d must under all aspects be equivalent to a dynamometric spring. In this case, the balance condition is reached when the hydrostatic thrust exerted by the measuring fluid F that is generated in the measuring points Pl and P2 is counterbalanced by the pressure exerted by the load of springs 3b and 5b and the equal pressure Pa in the two points.

Figure 7b instead shows the behaviour of the measuring

system 1 of Figure 1 when the measuring points Pl and P2 are not at the same level. Every level variation necessarily implies that a new balance condition is reached.

The forces acting on the measuring system 1 and that keep it balanced are: the barometric pressure Pa that operates equally on the upper face of the plungers 3a and 5a; the force exerted by the comparator 3c and by the elasticity of springs 3b and 5b; the hydrostatic pressure, as function of the measuring fluid F density, the level difference and the force of gravity operating on the lower face of plungers 3a and 5a.

The level difference δL between the first measuring point Pl and the second measuring point P2 is function of the pressure measure performed in the measuring point Pl at the elasticity of springs 3b and 5b, at the measuring fluid F density and the pressure Pa if different in the two measuring points Pl and P2 and if they are not mutually connected through the equalising circuit 15. The plunger 3a of the pressure sensor 3 and the plunger 5a of the vase 5 will never be levelled but will assume a level difference δP position that can change according to the rigidity of springs 3b and 5b. When there are very elastic springs 3b and 5b, the level difference δP tends to be cancelled while with very rigid springs 3b and 5b, it approaches the level difference δL .

Before proceeding with the level measure, it is necessary that the measuring system 1 is calibrated depending on the ambient temperature, performing a double level measure (inverting the position between pressure sensor 3 and vase 5) : in this way, in addition to know the level of the measuring points Pl and P2, it is also possible to obtain a field recalibration of the system 1, necessary when the thermal conditions change. During a level check, such calibration can be repeated every time the thermal stability of the system 1 appears compromised. If the pressure sensor 3 is with pressure- static detector, the measure of the level difference δL occurs by compensating the pressure value, returned by the sensor, with the density of the measuring fluid F, the force exerted by the springs 5b of the vase 5 and the pressure Pa, if different in the measuring points Pl and P2 and if they are not connected.

With reference instead to Figure 2, it is possibile to note that the measuring system 1 according to the present invention comprises a pressure sensor 3 of a differential type connected, through different connection ducts β making the first connection means, to two vases 5, indifferently of the type with free surface or with a membrane, at least each one of the vases 5 being arranged on a different measuring point Pl and P2. Also in this case, the pressure/volume regulating/compensating means 7 and the vessel 9 can be connected to the connection ducts 6 through the second fluid connection means, also shaped as a

branched connection duct 8 by interposing respective control valves 12. It must be noted that the positions of the connection ducts 6 that connect the sensor 3 with the vases 5 arranged in the measuring points Pl and P2 must not necessarily be horizontal, but can assume infinite forms in the space, provided that the connection ducts 6 remain at constant temperature and shape. In this case, the measuring fluid F is preferably in liquid form. It must further be noted that the pressure sensor 3 of the differential type can be abutted onto the surface to be verified or onto another place: anyway, for precision measures, it is advisable that the sensor 3 is symmetrical from the hydraulic point of view, therefore allows the connection to the two symmetrical connection ducts 6 with constant volume. This configuration of the system 1 is therefore accurate provided that the two vases 5, the sensor 3 and the respective connection ducts 6 are at constant temperature and shape; the temperature can symmetrically change but the connection ducts 6 must be symmetrical. In this case, the evaluation of the level coincidence between the first measuring point Pl and the second measuring point P2 is performed through a computation that takes into account the differential pressure value returned by the pressure sensor 3, compensated by the measuring fluid F density, by the scale factor and, posibly, by the different atmospheric pressure values between the first measuring point Pl and the second measuring point P2 (if the two vases 5 are not mutually

connected through the equalising circuit 15) . There must further be the chance to invert the position of the vases 5 when measuring or calibrating. ^'

With reference to Figures 8a and 8b, it is possibile to note the operating principle of the measuring system 1 of Figure 2 : merely as a non-limiting example, in Figures 8a and 8b the pressure sensor 3 is shown as a substantially known mechanical differential meter comprising two chambers, respectively X and Y, mutually not communicating, each one of which connected with one of the vases through its own connection duct 6 containing the measuring fluid F, each one of such chambers being equipped with a plunger 3a; the plungers 3a, directly in contact with the measuring fluid F, are mutually cooperating through an equaliser mechanism 3f, an end of which cooperates with at least one dynamometric spring 3b, and the other end cooperates with the feeler 3d of the displaying means made as mechanical comparator 3c. In particular, the physical construction of the pressure sensor 3 of the differentialy type provides that the two chambers X and Y, though being individually with variable volume, are mutually constrained, for example mechanically through the equaliser mechanism 3f and the related plungers 3a, so that the sum of the two variable volumes of the due chambers X and Y generates a constant volume: every volume variation of chamber X is therefore inversally created on the volume of chamber Y and vice versa.

The vases 5 are similar to those described with reference to previous Figures 7a and 7b.

Figure 8a shows the behaviour of the measuring system 1 of Figure 2 if the two vases 5 arranged in the two measuring points Pl and P2 are at the same level. In this case, the comparator 3c of the differential pressure sensor 3, for example placed on the right side of the sensor, must generate for the feeler 3d a thrust whose characteristics are identical to the dynamometric spring 3b placed on the left side (stroke function of pressure and zero friction) . The comparator 3c and the feeler 3d must be, under all aspects, equivalent to a dynamometric spring. The balance condition is reached when the hydrostatic thrust that is generated in the measuring points Pl and P2 is counterbalanced by the pressure exerted by the springs load and by the equal pressure Pa in the two measuring points.

Figure 8b instead shows the behaviour of the measuring system 1 of Figure 2 when the measuring points Pl and P2 are not at the same level. Every level change necessarily implies that a new balance condition is reached.

The forces operating on the measuring system 1 and that keep it balanced are: the barometric pressure Pa that equally operates on the upper face of the plungers 5a of the vases 5; the force exerted by comparator 3c, by sensor and vases springs, respectively 3b and 5b;

the hydrostatic pressure, as function of the measuring fluid F density, of the level difference and the force of gravity that operates on the lower face of plungers 3a and 5a.

The measuring system 1 under this configuration can proceed with the measure independently from ambient temperature, provided that the temperatures in the two circuits have the same value. When there are different temperatures for the two circuits, it is necessary that the system 1 is calibrated by performing a double level measure, inverting the position of the two vases 5: in this way, in addition to know the level difference between the measuring points Pl and P2 (the level difference is the result of a computation performed on two samplings), it is also possible to obtain a field recalibration of the system 1, necessary when thermal conditions change. During a level check, such calibration can be repeated every time the thermal stability appears compromised. The evaluation of the level difference δL is ^' performed through a computation that takes into account the measure of the comparator 3c, the pressure exerted by the set of springs of the sensor 3 and of the vases 5, the measuring fluid F density and the pressure Pa if different in the measuring points Pl and P2 and if they are not mutually connected through the equalising circuit 15. In case of a pressure-static detector, the evaluation occurs by compensating the differential pressure value, returned by sensor 3, with the measuring fluid F density, the pre-loads of the

springs of the vases 5 and the pressure Pa if different in the measuring points Pl and P2 and if they are not mutually connected through the equalising circuit 15.

With reference instead to Figure 3, it is possible to note that the measuring system 1 according to the present invention comprises at least two pressure sensors 3, indifferently of the normal or differential type, mutually connected through a shared, suitably branched connection duct 6, that generates the first fluid connection means, each one of such pressure sensors 3 being arranged on a different measuring point Pl, P2, ..., Pn. Also in this case, the pressure/volume regulating/compensating means 7 and the vessel 9 can be connected to the connecting duct 6 through the second fluid connection means, also shaped as a connection duct 8, possibly by interposing at least one control valve 12. In this case, the measuring fluid F is preferably in liquid form. This configuration of the system 1 is accurate even if the ducts shape changes and the temperature can steadily change; there can further be lengths of duct at different temperatures, but in this case both temperature and height of columns of possible siphons, mentioned below, must remain constant. In this case, the evaluation of the level differences between the various measuring points Pl, P2, ..., Pn is performed by a computation that takes into account the difference between the pressure values, returned by two pressure sensors 3, one of which is defined as reference sensor, compensated by the

measuring fluid F density, the scale factor and the different atmospheric pressure values between the measuring points Pl, P2, ..., Pn (if the pressure sensors 3 are not mutually connected through the equalising circuit 15) . If the pressure sensors 3 are more than two, one proceeds with the remaining sensors taking into account the difference with the reference sensor.

The configuration of the measuring system 1 of Figure 4 is substantially similar to the one of Figure 3, apart that the pressure sensors 3 are exclusively of the differential type and are all mutually stabily connected through the equalising circuit 15. Also in this case, the measuring fluid F is preferably in liquid form. This configuration of the system 1, with respect to the one in Figure 3, is not sensitive to atmospheric pressure Pa varations that can exist between a measuring point and another. In this case, the evaluation of the level differences between the various measuring points Pl, P2, ..., Pn is performed through a computation that takes into account the difference between pressure values, returned by the two pressure sensors 3, one of which is defined as reference sensor, compensated by the measuring fluid F density and the scale factor. If the pressure sensors 3 are more than two, one proceeds with the remaining sensors taking into account the difference with the reference sensor.

In the configurations of Figures 3 and 4, the measuring system 1 is advantageously able to simultaneously detect the

leveling of a plurality of measuring points Pl, P2, ..., Pn placed in the space and not necessarily mutually connected by a plane junction surface. Moreover, it must be noted that in the systems 1 of Figures 3 and 4, the hydraulic diagram can assume infinite construction forms, provided that the following items do not change : the measuring points Pl, P2, ..., Pn, must be mutually connected in order to form an airtight, rigid or flexible (with constant or variable volume) hydraulic circuit, deformable because subjected to thermal dilatation and shape variation. The volume variation of the circuit is not influent as regards the accuracy of the differential level measure between two or more measuring points; the connection of the measuring points Pl, P2, ..., Pn must not be necessarily horizontal, but can assume infinite forms in the space.

With reference to Figures 9a, 9b, 9c and 9d, it is possibile to note the operating principle of the measuring system 1 of Figure 3: merely as a non-limiting example, in Figures 9a, 9b, 9c and 9d, the pressure sensor 3 is like the one used and described depending on the previous Figures 7a and 7b and the system 1 comprises only two pressure sensors 3 arranged on respective measuring points Pl and P2.

Figure 9a shows the behaviour of the measuring system 1 of Figure 3 if the two pressure sensors 3 are at the same level. In

this case, the comparator 3c of each pressure sensor 3 must generate for the feeler 3d a thrust whose characteristics are identical to the spring ones (stroke function of pressure and zero friction) . The comparator 3c and the feeler 3d must be under all aspects equivalent to a dynamometric spring. The hydraulic circuit therefore assumes an appearance similar to a balance in which the rod joining the plungers 3a of the pressure sensors 3 is replaced by the connection duct 6.

The balance condition is reached when the hydrostatic thrust exerted by the measuring fluid F that is generated in the measuring points Pl and P2 exerted on the plungers 3a of the pressure sensors 3 counterbalances the pressure exerted by the load of the springs 3b and the pressure Pa equal in the two points. In fact, when the measuring point Pl is at the same level of the measuring point P2, the plungers 3a of the two pressure sensors 3 are levelled because subjected to the same hydrostatic pressure and counterbalanced by the same pressure of the springs 3b summed to the barometric pressure Pa.

Figure 9b instead shows the behaviour of the measuring system 1 of Figure 3 when the measuring points Pl and P2 are not at the same level. Every level variation necessarily implies that a new balance condition is reached.

The forces operating on the measuring system 1 and that keep it balanced are: the barometric pressure Pa that equally operates on the

upper face of the plungers 3a; the force exerted by the comparators 3c and the elasticity of the springs 3b of the pressure sensors 3; the hydrostatic pressure, as function of the measuring fluid F density, of the level difference and of the force of gravity that operates on the lower face of the plungers 3a.

The level difference value δL between the first measuring point Pl and the second measuring point P2 is computed depending on the pressure measure difference performed in the measuring points Pl and P2 by the respective pressure sensors 3, the elasticity of the springs 3b, the measuring fluid F density and the pressure Pa if different in the two measuring points Pl and P2 and if they are not mutually connected through the equalising circuit 15. The plungers 3a of the pressure sensors 3 will never be levelled, but will assume the level difference δP position that can change according to the springs 3b rigidity. When there are very elastic springs 3b, the level difference δP tends to be null, while with very rigid springs 3b, it approaches the level difference δL .

In practical measures of the level difference δL, it is almost impossibile to perform flexible hydraulic connections lacking a siphon. Figures 9c and 9d therefore show, as an example, the measuring system 1 of Figure 3 equipped with a connection through the first fluid connection means composed of a connection duct 6 comprising respectively a siphon in the

lower part (Figure 9c) and a siphon in the upper part (Figure 9d) . The siphons, both in the upper and in the lower parts, introduce problems related to the measuring fluid F column weight inside when the temperature changes. If the two siphon branches are not at the same temperature, the column weight is not the same and therefore hydrostatic forces are summed that operate differently on the pressure sensors 3, impairing the level measure. Briefly, the above problem will be dealt with in detail with reference only to the embodiment in Figure 9d, but treated concepts are also valid for the embodiment of Figure 9c.

Figure 9d therefore shows the measuring system 1 of Figure 3 when the two pressure sensors 3 ar at the same level and the connection duct 6 comprises a siphon 20 oriented upwards: it can be noted that the pressure sensor 3 on the right side in Figure 9d can have a connection opening 3g with a possible further pressure sensor.

The introduction of one or more siphons 20 along the connection duct 6 does not impair the level measure if the two measuring fluid F columns in liquid form enclosed in vertical lengths of the siphon 20 have the same weight. The hydraulic circuit behaves in a wholly similar way to a pulley to which weights are applied that simulate the liquid columns weight. When the amount of the weights in the left branch equals the one in the right branch, forces Fl and F2 are the same and mutually cancel and the system remains balanced. In this case, forces Fl

and F2 do not interfere in the level measure.

The weight force of a hydrostatic column can be expressed as product of the specific weight of the liquid by the column height multiplied by gravity acceleration. Column height and gravity acceleration operate in the same way on the siphon branches and therefore do not impair the system. In order to make the hydrostatic thrust (weight force) of the two branches •equivalent, it is necessary to keep the liquid density in the siphon equal.

The specific weight variation of a liquid depends on pressure and temperature since both can modify its volume. The volume variation due to the pressure is neglectable and anyway symmetric (uncompressible liquid) , while the variation due to temperature is relevant and is further amplified when it is multiplied by the height.

Summarising, it is possible to state that the presence of siphons 20 along the connection duct 6 does not impair the level measure provided that the same fluid density is ensured on the two measuring fluid F columns in liquid form in the siphon branches, and this can be obtained (when there is a homogeneous measuring fluid F in liquid form) only if the temperatures in the columns have the same value or the same gradient depending on the height. In practice, it is surely difficult to keep the hydraulic circuit at the same temperature, therefore the level measure will be the more accurate, the more constant the

temperature and the minimum siphons height will be kept.

If the measuring system 1 according to the present invention is interfaced with a data processor, the level difference will be able to be quickly acquired, even when the temperatures in the siphons are not the same, performing a double level measure (inverting the position of the sensors without modifying the height position of the central point of the siphon/s) : in this way, in addition to know the measuring .points level (the value is function of a computation performed on two sampligs) , it is also possible to obtain a field recalibration of the system 1 that is necessary when the thermal conditions change. During a level check, such calibration can be repeated every time the thermal circuit stability appears compromised. It must then be noted that, advantageously, the measuring system 1 according to the present invention can quickly operate also when there are siphons that are not thermally homogeneous.

Concluding, the Applicant has discovered on the field, by means of experimental tests, that due to the measuring system 1 according to the present invention as previously described, by interfacing it with adequate sensors, the measuring resolution between two measuring points Pl, P2, ..., Pn equal to ±0.001 mm can reach an accuracy equal to +0.02 mm as measure uncertainty referred to the level condition of two or more measuring points in space. Such uncertainty represents the sum of all physical and practical errors, that occur in the system 1.