IT201800003588A1 - Cap for Dewar jars using ultrasonic sensors for monitoring the level of cryogenic liquids, and related cryogenic system - Google Patents

Description

Cap for Dewar jars using ultrasonic sensors for monitoring the level of cryogenic liquids, and related cryogenic system

[0001] The present invention relates to the field of cryogenics systems, in particular of Dewar vessels and of the systems which use them.

[0002] State of the art

[0003] The use of Dewar vessels in cryogenics systems is known and now widespread.

[0004] With reference to Fig. 1, a "Dewar jar" (or simply "Dewar") is a container or tank or cylinder that keeps its contents thermally isolated from the external environment thanks to a cavity in which it is kept the void. The vacuum is used only for thermal insulation. The content, liquid and / or solid, is not under vacuum.

[0005] Dewar are often used to store liquid substances which would become gaseous at room temperature, such as nitrogen, helium, argon, carbon dioxide, oxygen, ammonia. For these liquids, the progressive increase in temperature inside the container can give rise to boiling, therefore usually the container is not pressurized but equipped with a cap simply resting on its mouth. The cap, also called "Dewar's cap", generally in the shape of a mushroom made of insulating material, is simply placed on the mouth of the jar under the force of its own weight and at the minimum internal pressure it lets the evaporating gas vent.

[0006] The content of the Dewar therefore gradually decreases in level and this makes continuous and systematic monitoring necessary, especially when it comes to the tank of a cryogenic liquid which must guarantee the functionality of some device. In this case the level must always be kept within a certain range in order not to discover the cooling pipes that branch off from the Dewar (not shown).

[0007] Referring to Fig. 2, the level measurement can be carried out manually by inserting a graduated rod 5 through the neck of the Dewar jar (Fig. 2 (c)), once the cap 2 has been removed. The rod at room temperature with the liquid 4 at a very low temperature causes a sudden boil, signaling that it has come into contact with the liquid level. This manual method provides imprecise measurements and reduces the shelf life of the jar contents, both due to immediate losses due to boiling 6 and due to the temperature change in the internal microclimate caused even by the opening of the cap. In fact, there is a suction of ambient air 3 inside the Dewar every time the long insulating cylinder of the cap is removed from the neck of the vessel, and then again part of the internal atmosphere 3 will be pushed out like a piston when, on closing of the cap, the cylinder will be reinserted into the neck (Fig. 2 (b)).

[0008] The alternative to the graduated auction is felt but far from being available with a simple, reliable and effective, as well as economic, structure. The sensors available on the market, in fact, are for the most part inadequate at very low temperatures or have very high costs. Below is an overview of the possibilities, divided into three types of measurement, with the relative disadvantages.

[0009] With regard to the measurements made from the outside, the outer casing of the Dewar, typically metallic, and its vacuum cavity do not allow to carry out probes from the outside with capacitive or ultrasonic measurements.

[0010] Regarding the internal measurements without contact, it is noted that by definition cryogenic liquids are considered to have a boiling temperature lower than -73 ° C. At these temperatures, both the liquid and the evaporation gas that saturates the interior of the Dewar do not allow the use of common electronic technologies for "contactless" distance measurement (such as ultrasound or infrared) in which sensors and / or electronics they must necessarily be immersed at least in the gas. In addition, there are other disadvantages: laser meters have high precision even at great distances but do not allow measurements at the required short distance (<50 cm). Radar sensors, in addition to the problem of distance, can transmit heat inside even if only by thermal conduction through the antenna. They are also very expensive.

[0011] Coming to internal measurements in direct contact, the remaining alternative is direct measurement with immersion probes, but this approach also requires ad hoc technologies. It should also be considered that a liquid of common use such as nitrogen (used in the tests of the prototype under consideration), being electrically insulating, even excludes the possibility of using immersion resistors. What is currently on the market are long immersion sensors for very low temperatures based on the variation of the capacitance measured between two tubes immersed in the liquid (variation in the dielectric). This is impractical in a cap that has to be opened often. Other attempts based on the difference in pressure have known drawbacks (e.g. loss of liquid) due to the tiny cross-section of the tubes.

[0012] Other systems of the prior art are based on floats. The simplest type is the one used to monitor pressure vessels (not Dewar) where a minimum excursion (like 1 cm) is required to simply report the loss of the standard level. Some devices are based on the measurement of the excursion of a float with technology created specifically for very low temperatures. The floats should be built in a thermally insulating way, otherwise they conduct heat inside. The floats have too little excursion (such as a long electric button) or have a large footprint such as a long retractable cable or a long metal tube. In the case of rigid tube floats, such as those with permanent magnet or the magnetostrictive type, the structure, which should remain integral with the cap, is bulky, delicate and expensive. These are impractical solutions for a cap that must be opened periodically to top up the liquid. Furthermore, both magnetic floats and magnetostrictive floats do not have a continuous excursion but two level ranges (maximum and minimum, with two floats on the same rod). In this case, they are not suitable for making a prediction of the autonomy of the tank that can adapt in real time, with continuous measurements, to changes in ambient temperature.

[0013] The need is felt for a cap having an integrated level sensor for the contents of a generic container, and in particular of a Dewar jar.

[0014] Following the check of the liquid level in the Dewar, if the level is too low, it must be topped up.

[0015] For topping up the level of the liquid in the dewar, pressure vessels are commonly used where the liquid is kept longer, because the closure is hermetic, the pressure prevents boiling and the coating is in any case insulated. Referring to Fig. 3, for manual topping up an appendix or "spout" 14 is used applied through an insulated metal tube 13 to the "tapping head" 11 (comprising a manual valve 12 and a cap 15) of a cylinder 10 through a screw joint. This appendix is made with a common insulated copper tube (typically 3/8 inch) and is modeled according to the shape and length necessary for the user to reach the mouth of the Dewar. In a typical case, a cylinder can provide liquid for more than two top-ups over a period of 10 or 15 days. At least two cylinders must alternate in a month.

[0016] The topping up in this way has the disadvantage of a continuous human presence for those Dewar that serve specific functional cooling purposes. Furthermore, the entire filling level monitoring process is inefficient, expensive and does not guarantee full cooling functionality.

[0017] The need is also felt for an automatic refilling system for a container, in particular for a Dewar jar. In this regard, even using a free-wire, non-rigid float, an automatic top-up could not be implemented, as the transfer of the liquid would cause agitation of the liquid surface, preventing the measurement with the float during refueling. Without this measurement in real time it is not possible to close the valve at the right time and therefore to carry out an automatic top-up.

[0018] Purpose and object of the invention

[0019] The object of the present invention is to provide a cap which solves all or part of the problems and overcomes the drawbacks of the prior art.

[0020] A further object of the present invention is a fluid release system equipped with an automatic refilling system, in particular using the Dewar cap which is the object of the present invention.

[0021] The object of the present invention is a cap according to the attached claims.

[0022] A further object of the invention is a fluid release system according to the attached claims, in particular a release system for a gas for cryogenics.

[0023] Detailed description of examples of implementation of the invention

[0024] In the following, the invention according to the present description will be illustrated with reference to a cryogenic system using a Dewar vessel, but it should be understood that it can be applied to any system for releasing a fluid from a tank, in any field application.

[0025] List of figures

[0026] The invention will now be described for illustrative but not limitative purposes, with particular reference to the drawings of the attached figures, in which:

- figure 1 shows a Dewar vase seen in section (a) and in its three-dimensional external appearance (b), in (b) the Dewar cap applied to the vase is also shown below as in section (a);

- figure 2 shows in (a) a typical Dewar cap resting on the Dewar jar under the force of its own weight; in (b) the internal pressure of the Dewar vessel slightly raises it allowing the degassing of the liquid which evaporates slowly; in (c) the measurement of the level of the liquid in the Dewar vessel by means of a graduated rod, which causes the sudden boiling of the liquid on contact, dispersing the contents more quickly in the external environment;

- Figure 3 shows a traditional configuration for topping up the Dewar jar, with a pressurized cylinder for liquid nitrogen: the tapping head allows the liquid to be extracted thanks to the pressure of its own steam (with the same principle as a coffee maker). An insulated tube is used to pour liquid into the neck of the Dewar jar (not shown, in capless configuration) when topping up is required;

- Figure 4 shows several attempts made by the inventor of this description to use an ultrasonic sensor in a Dewar cap: all are affected by low temperature. From left: (a) sensor at the base of the cap; (b) sensor out of the cap through two open channels; (c) split sensor with capsules at the base and electronics outside the cap; (d) sensor outside the cap with channels filled with thermal insulation;

- Figure 5 shows an embodiment of the cap according to the present description, resting on the neck of a Dewar jar in an adherent and enveloping way, also acting as a base for a control electronics housing box; - Figure 6 shows in (a) a bottom view and in (b) a top view of an embodiment of the cap according to the present description;

Figure 7 shows a vertical sectional view of the cap according to the present description;

Figure 8 shows a diagram of an automatic refilling system of a Dewar jar or other container which uses a Dewar cap according to an embodiment of the present description;

Figure 9 shows a more detailed diagram of an automatic refilling system for a Dewar jar or other container which uses a (Dewar) cap according to a different embodiment of the present description;

- Figure 10 shows in (a) a bottom view and in (b) a top view of an embodiment of the cap according to the present description used in Fig. 9; And

Figure 11 shows a vertical sectional view of the cap according to the present description used in Fig. 9.

[0027] It is specified here that elements of different embodiments of the present description can be combined together to provide further embodiments without limits while respecting the technical concept of the invention, as the average person skilled in the art understands without problems from what has been described.

[0028] The present description also refers to the known art for its implementation, with regard to the detailed characteristics not described, such as for example less important elements usually used in the prior art in solutions of the same type.

[0029] When introducing an element, it always means that it can be “at least one” or “one or more”.

[0030] When a list of elements or characteristics is listed in this description, it is understood that the invention according to the invention "includes" or alternatively "is composed of" such elements, or it can individually comprise the listed elements.

[0031] Embodiments

[0032] In the following, aspects of the invention according to the present description will be illustrated with reference to prototypes made and tests carried out, but it is to be understood that such references are not limitative of the device or system or process described.

[0033] A preliminary experiment was carried out on a Dewar containing 40 liters of nitrogen, the most common cryogenic liquid because it is inert. The maximum temperature (boiling point) for nitrogen is -195.82 ° C. The gas temperature in the Dewar at the beginning of the neck (under the cap, mouth of the neck) was found to remain more or less constant at -70 ° C.

[0034] Ultrasonic sensors have been positioned in various ways, such as the sonar HC-SR04, typically used on Arduino microcontrollers. The sensors available today have measurement ranges and accuracy suitable for the purpose but only work at room temperature. Sometimes the same sensors are sold declaring a minimum operating temperature of -20 ° C, which is insufficiently low for the purposes of the solution according to the present description.

[0035] Referring to Fig. 4, the numerous tests performed on the Dewar 100 cap, with similar 120 sonar of different manufacture, have shown the ineffectiveness of some initial solutions designed by the Inventor.

[0036] A first solution consisted in positioning the sensor 120 and the detection electronics 110 both at the base of the cap 100 as in Fig. 4 (a), or at the end 102 which is the end facing the inside of the container towards the contents (e.g. liquid). In this case, after about 10 minutes the measurement freezes on a fixed number.

[0037] A second solution consisted in positioning sensor 120 and detection electronics 110 of the sensor 120 at the top 101 of the cap 100 as in Fig. 4 (b), outside the neck of the Dewar, by making two channels 130 in the insulating cap 100. In this case, the sensors freeze after about 30 minutes. The top 101 is the opposite side to the base or end 102, and faces the outside of the Dewar cap. Between the two ends 101 and 102 develops the Dewar plug, typically along an axis which is also an axis of the neck of the Dewar jar.

[0038] A third solution, on the other hand, consisted in separating the ultrasonic capsules 120 from the detection electronics 110, which contains, among other things, the oscillating crystal, in particular by positioning the capsules 120 at the base 102 of the cap 100 and the detection 110 just outside the top 101 of cap 100. The result in this case does not change much: within 20 minutes the measurement stops (because the sensor 120 and / or the electronics 110 drop in temperature), albeit on a value different from those of the previous solutions described.

[0039] A fourth solution consisted in carrying out measurements with the sensors 120 and the electronics 110 outside (in particular across the end line 101 as illustrated) and filling the communication channels 130 with an insulating material 135 to heat but more transmissive for acoustic radiation (eg glass wool). All attempts have failed, and an explanation given is that precisely the characteristics that generally make a material thermally insulating make it sound insulating if not at least sound absorbing.

[0040] It should also be considered that in the illustrated solutions the electronics 110 can be adequately protected while the capsules 120 which emit the ultrasonic signal must necessarily be immersed in the gas but have a mechanics whose rigidity must be preserved by ensuring that they work as if were at room temperature.

[0041] Another explanation for the above failures is that the gas under pressure, having to escape, crosses the communication channels 130 with the sensors 120 and in any case envelops their membranes. Furthermore, the cap 100, weighed down by electronics 110 and cables (not shown), can no longer perform its slow degassing function by rising slightly and the low-temperature gas tends to pass through the sensors 120 and then escape.

[0042] A solution to these problems encountered as previously tested was that of maintaining a constant microclimate in the sensor channels by creating an exhaust duct (with a diameter of 3 mm, for example, but also higher or preferably lower) for the cold gas with a vent that releases it at a lower level than the sensors. By level here we can mean the level relative to a plane perpendicular to the aforementioned development axis of the cap.

[0043] With reference to Fig. 5, a first aspect of a stopper (e.g. of Dewar) 200 positioned to close the neck of a jar (e.g. of Dewar) 300 is described.

The cap of Dewar 200 according to one aspect of the present description comprises a body 250 which includes a preferably elongated central portion 251, configured and adapted to be inserted into the neck 310 of the jar of Dewar 300, and two side portions 252 and 253 which extend beyond the neck 310 (limit line 311) of the jar of Dewar 300. This extension beyond the neck 310 is only optional, but is preferred as it improves the watertightness of the cap 200.

[0045] As can be seen from Fig. 5, in the central part 251 there are two channels.

[0046] The first channel 230 preferably crosses the entire cap 200 and is closed at the opposite end to that which enters the package 310 by a sensor 220 and the relative detection electronics. The channel or duct 230 may not even cross the entire cap, as long as it outlets on the lower end 254 of the cap 200.

[0047] A second channel 240 (or "vent channel" or "vent channel"), starting from the lower end 254 of the cap 200, curved at 241 (angled or arched) to exit laterally to the cap at 242 ( on the external surface of the cap). However, nothing prevents the second channel 240 from making a different path opening in any other point of the external surface of the cap 250, which is however lower than the top 201 of the cap.

[0048] From the outlet of the second channel 240 in 242 the gas 235 formed by the liquid content 320 of the Dewar 300 jar comes out.

While operating with any reciprocal configuration between the first 230 and the second channel 240, laboratory tests have shown that the Dewar cap has an optimal behavior with a distance D1 between the side outlet 242 and the top 201 of the cap 200 ( or in any case any point of the sensor), opposite the base 254, at least 3cm, preferably at least 4cm, even more preferably at least 6cm. The effective distance D1 in the cap can vary according to a series of parameters on its structure and composition (and also for example it depends on the level of the liquid in the jar, because the actual distance from the surface of the liquid or solid changes), but it is in any case easily determined with a limited series of tests once the structure described above is known. Furthermore, a vertical distance D2 is also given between the side outlet 242 and the mouth 312 of the neck 310 of the Dewar 300. This distance can be any, preferably minimized in order not to increase too much the overall height D of the cap with respect to the level of the mouth 312 of the neck 310.

[0050] With the Dewar plug structure just described, the nitrogen gas (or other cryogenic liquid) 320 is made heavier than air by its very low temperature and under these conditions it overflows from the vent 240,241,242 to fall back down (gas 325 ) and substantially does not penetrate further into the conduits 230 reserved for ultrasounds (there is substantially no more flow in the conduit 230 which can also be provided as a series of conduits). The cap can therefore be made adherent and "enveloping" with respect to the neck of the Dewar jar, in order to have the stability necessary to act as a support and container for all the accessory technology (for example the control electronics 210), while the gas circulation is necessarily forced in the special conduit 240 due to the lack of other outlets.

[0051] The diameter of the vent duct 240 can conveniently be comparable to the interstitial area of an original cap. In place of an original cap with a diameter of 6 cm, it has been experimentally verified that a vent duct with a diameter of 2-4 mm (for example an average diameter) keeps the evaporation rate of liquid nitrogen unchanged (typically about 4 cm per day) without building up pressure.

Referring to Fig. 6 (a) a bottom view of the cap of Dewar 200 according to one aspect of the present description is shown, with the holes of the first 230 and of the second channel 240. Also seen is the ring recess 255 which rests on the upper edge 312 of the neck 310 of the Dewar jar 300. In Fig. 6 (b) the upper outlets of the two channels 230 are seen (in the specific case of a sensor with an emitter separated from the receiver). Fig. 7 shows a vertical section of the cap, ie along a plane passing through the axis of the duct 230.

In all embodiments, the sensor 220 is any distance sensor, for example a time-of-flight sensor, such as for example an infrared or laser or ultrasonic sensor.

[0054] The solution described for the cap is particularly suitable for supporting an automatic refilling system according to the present description.

[0055] With reference to Fig. 8, an automatic refilling system 400 is illustrated which includes a servo valve 410 installed on a refilling pipe 430 and connected on one side to one or more liquid reservoirs S1, S2, and from the other part to a control unit 420 of the servovalve 410 connected to the sensor 220. The top-up tube 430 passes through the cap 200 passing through its channel 260. In this way, the cap 200 will no longer need to be opened and it will be possible to keep under control the filling level 321 of the Dewar 300 jar, with consequent advantage in terms of human presence and safety of the cooled devices.

[0056] Fig. 9 illustrates the entire level control system and automatic refilling of the Dewar vessel in more detail according to an aspect of the present description.

[0057] With respect to the system of Fig. 8, the vessel of Dewar 300 is also illustrated, a servovalve 410 which is connected to a control unit 420 integrated in the electronics 210 of the sensor 220. The connection is provided for example through the conductor 411 which in particular passes through a cover 270 (optional) of the upper part of the cap 200 so as to protect the electronics 210 and the sensor 220. According to an aspect of the present description, the servovalve 410 can also be integrally constituted with a control unit at outside the cover 270. The copper pipe 430 is connected to the servo-valve 410 through a system of hydraulic joints 435. The servo-valve, on the other side, is this time connected to connectors applied on three tanks S1, S2, S3. This connection is generally indicated with the reference 440. According to an aspect of the present description, it is possible to provide a manual valve for releasing the pressure in the lowest point of the system 440, so as to facilitate the replacement of the Dewar 310 vessel without having to necessarily disconnect the hydraulic coupling system 435.

[0058] The three tanks S1, S2, S3 can be connected in parallel as a single container. When the sensor 220 detects a lower critical level of the liquid, the control unit (eg Arduino) can open the solenoid valve 410 letting the liquid present in the tanks pour into the Dewar 300 vessel. When the sensor 220 detects a higher critical level of the liquid, the control unit 420 can close the solenoid valve 410 ending the supply of the Dewar 300 vessel. The pressure vessels S1, S2, S3 can be disconnected to be filled periodically. For this automation, the variant necessary to the shape of the cap with respect to the other embodiments is an additional through hole 260. The cap 200 itself provides insulation to the section of pipe 430 that passes through it. The presence of the through hole 260 may make it necessary to vary the position of the ducts 230 (it is preferable that they are centered but they can be moved more or less significantly). It is noted here that the filling duct 260 can cross the cap axially starting from the external end 201 or it can start laterally (according to a variant not shown) and open on the insertion end 254.

[0059] The inventors have created, among other things, a prototype in extruded polystyrene. This material has a thermal insulation equivalent to the expanded polystyrene of the original Dewar cap but is better suited to be worked and sanded with cutting tools (lathe, cutter, sandpaper, grinding wheels for power tools). In general, any insulator can be used.

[0060] Referring to Figs. 10-11, the profile of the cap according to the embodiment that is being described has been modeled according to a preferred embodiment with the addition of the through hole 260 for the filler tube 430. The design of the prototype has been sized to an ultrasonic sensor model HC-SR04 on an ORTEC 30 liter Dewar jar with an internal diameter of about 6 cm at the neck (to be precise it was the cryogenic liquid tank of a mass spectrometer).

[0061] Examples of sensors that could be used with this cap are listed below:

- HC-SR04 ultrasound;

- JSN-SR04T ultrasonic waterproof;

- infrared GP2Y0A21YK0F;

- KY-032 infrared; And

- LIDAR-Lite laser.

[0062] The ultrasonic sensors used in the experimental tests are traditional sensors which adopt a pair of identical capsules to perform the two functions of emitter and receiver separately. Of course, even a single capsule can be used to perform the two functions alternately. In that case the plug will simply have a single duct 230. The characteristic of the difference in height with respect to the vent duct can preferably remain unchanged.

[0063] The cap constructed in this way allows the perfect operation of the ultrasonic sensor, which has for example been connected to a suitably programmed Arduino Yun control unit and also equipped with a thermocouple and a loudspeaker. This device is able to measure in the required range (19-40 cm) the liquid level with a measurement stability of / - 1 mm. The measurement is continuous and the control unit can provide a series of acoustic alarms to suggest the need for topping up when the level of attention is reached, to give an urgent need to top up near a limit level or to give the alarm if it has not been topping up within a safety deadline / limit. In addition, through the Arduino console (not shown), all the data and the autonomy forecast can be made available on the network to anyone who has the relative access rights to the information. The same data can be made available on the web.

[0064] Advantages of the invention

[0065] The invention has the advantage of allowing the use of time-of-flight sensors, for example ultrasonic, infrared, laser, to measure the filling level of a Dewar vessel or other generic container of liquid and / or solid at a negligible cost compared to solutions on the market that use immersion probes. It is also equipped with greater simplicity and practicality than the known technique, since it does not occupy space inside the Dewar vase.

[0066] Compared to a graduated rod, the measurement without contact, without the need to open the cap, therefore in a maximally conservative manner with regard to the contents of the jar.

[0067] Again with respect to immersion probes, the use of the cap according to the present description then has the advantage of being able to provide more stable average measurements even during liquid refilling (which is difficult to obtain with floats or condensers).

[0068] The use of this cap is made possible according to the present description in a liquid / gas supply system, for example in a liquid nitrogen cooling system, including periodic refilling of the tank (by means of a servo valve) and a possible request via the network to supply the liquid / gas to the supplier. In this case it would no longer be a monitoring / alarm system but an automation thanks to which human assistance is no longer required except for the periodic replenishment, typically monthly, of the liquid / gas supply.

[0069] Ultimately, with respect to any other solution built specifically for very low temperature applications, the invention of the present description allows a better result to be obtained at a significantly lower cost thanks to the use of sensors which are already widespread today (for temperatures up to -20 ° C). The invention also offers the prospect of being able to use other low-cost technologies, such as the microcontrollers of the Arduino series, for an unlimited range of developments ranging from remote monitoring to any possible type of alarm or maintenance notice, from acoustic to '' notification via SMS or e-mail, up to the total automation of the routine maintenance of the system. Obviously, even sensors and controllers of greater complexity and quality can be used according to the present description.

[0070] Among the advantages of the cap according to the present description compared to a traditional cap there are:

- the thermal insulation and gas tightness, therefore the evaporation rate of the liquid / solid, may not change compared to the typical Dewar cap or other cap;

- the average cost may not change compared to the typical Dewar cap or other cap;

- the operation of common remote measurement sensors (eg ultrasound) is preserved, allowing their use in the Dewar environment;

- the fact of being able to measure the cryogenic liquid level with low-cost technologies for applications ranging from monitoring / alarm to automated refilling.

[0071] The cap according to the present description, in particular ultrasonic, can perform measurements of millimeter or submillimetric precision and can remotely provide the maintenance technician of the system with a forecast of the interventions to be carried out in the times and in the manner necessary to maintain the safety of the system.

[0072] Considering that the expensive scientific equipment to which a cryogenic plant is dedicated risks breaking if there is no cooling, and that the evaporation rate of the Dewar vessel varies with the external temperature, prior to the solution according to the present description it was It was necessary for an operator on site to check the level with the graduated rod almost every day. Through the solution of this description, only an occasional remote control is required to know the day and time when the operator will have to intervene on site.

[0073] In the foregoing the preferred embodiments have been described and variants of the present invention have been suggested, but it is to be understood that those skilled in the art will be able to make modifications and changes without thereby departing from the relative scope of protection, such as defined by the attached claims.

Claims (13)

CLAIMS 1) Cap (200) of container (300) for a liquid and / or solid substance (320), comprising: - a body (250) with: - an insertion end (251), which can be inserted, in use, in a neck (310) of said container (300) having a mouth (312) which defines an opening of the container (300); - an external end (201), opposite to said insertion end (251); And - a lateral surface extending between said insertion end (251) and said outer end (201); the cap (200) being characterized in that: - includes a distance sensor (220); - the distance sensor (220) is configured to measure a distance between the same distance sensor (220) and a level (321) of said liquid and / or solid substance (320); - the distance sensor (220) is inserted in at least one detection duct (230) which passes through the cap (200) in whole or in part and flows, in use, into the container (300) through said insertion end (251) ; And - comprises a vent duct (240) for a gas generated by said liquid and / or solid substance (320), different from said detection duct (230) which has a first outlet end on said insertion end (251) and a second outlet end (242) on said lateral surface, the second outlet end (242) being at an intermediate level between said insertion end (251) and said outer end (201) measured substantially parallel to a through axis for said detection duct (230). 2) Cap (200) according to claim 1, wherein a distance D1 between said outer end (201) and said second outlet end (242) is at least 3 cm. 3) Cap according to claim 2, wherein the distance D1 is at least 4 cm. 4) Cap according to claim 3, wherein the distance D1 is at least 6 cm. 5) Cap according to one or more of claims 1 to 4, which further comprises a refilling duct (260) which passes through the cap (200) and is configured for the insertion of a refilling tube (430) of said container (300). 6) Cap according to one or more of claims 1 to 5, wherein said distance sensor (200) comprises a first element or "capsule" for sending a signal and a second element or "capsule" for receiving a signal, respectively inserted in two detection ducts (230). 7) Cap according to one or more of claims 1 to 6, wherein said distance sensor comprises or consists of an ultrasonic sensor. 8) Cap according to one or more of claims 1 to 7, wherein the second duct (240) has an average diameter of between 2 and 4 mm. 9) Cap according to one or more of claims 1 to 8, in which a cover (270) for the protection of the sensor (220) is mounted on the outer end (201). 10) Cap according to one or more of claims 1 to 8, wherein the cap (200) is configured to close a Dewar jar. 11) System (400) for the supply of a liquid and / or a gas, comprising: - a vessel (300) having a neck (310) with a mouth (312) and containing a liquid and / or gaseous substance (320); - a plug (200) configured to close said mouth (312); wherein the cap is the cap (200) defined according to one or more of claims 1 to 10. 12) System (400) according to claim 11, further comprising: - one or more refilling tanks (S1, S2, S3) containing the same liquid and / or the same gas present in the vessel (300) and fluidically connected through pipes (430) to said vessel (300) through the cap (200) ; - a servo-valve (410) installed on said pipes (430) and configured to start or stop a flow of fluid from said one or more filling tanks (S1, S2, S3) to said vessel (300); - a control unit (420) connected to said servo valve (410) and to said distance sensor (220); in which the control unit (420) is configured to command the opening or closing of the servo valve (410) as a function of the distance of the liquid and / or gas detected by said distance sensor (220), and in which said plug (200) comprises a refilling duct (260) through which one end of said pipes (430) is inserted. System (400) according to claim 11 or 12, wherein the liquid is nitrogen and the vessel is a Dewar vessel, the system supplying nitrogen gas for cooling an object or device.