Superconductor, method for manufacturing superconductor, liquid hydrogen level sensor, and liquid hydrogen level gauge

Abstract

The objective of the present invention is to realize a superconducting material that can be used in liquid hydrogen level gauges and that can be manufactured simply and reproducibly. [Solution] A superconductor with a critical temperature (Tc) that can be used in a liquid hydrogen level gauge is made of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, does not contain SiC, and has the critical temperature (Tc) at which electrical resistance becomes substantially zero, ranging from 29 K to 32 K. This makes it possible to realize a liquid hydrogen level gauge that can be manufactured simply and with high reproducibility.

Description

[Technical Field]

[0001] The present invention relates to a superconductor, a method for manufacturing a superconductor, a liquid hydrogen level sensor, and a liquid hydrogen level gauge. [Background technology]

[0002] There is a growing global awareness that global warming is a common issue that we must address now and in the future, but a major challenge is the movement toward decarbonization, which reduces fossil fuel consumption. One option is the use and utilization of hydrogen. In Japan, the "Basic Policy for Realizing GX (Green Transformation)" was approved by the Cabinet in February 2023, and while promoting the introduction of hydrogen, there is a need for accurate measurement of the amount of liquid hydrogen during its transportation, storage, and consumption.

[0003] Liquid hydrogen is an extremely low-temperature substance with a boiling point of approximately 20 K under normal pressure, and there has been a demand for a material that exhibits superconductivity at temperatures higher than the temperature of liquid hydrogen as a sensor material for liquid hydrogen level gauges that utilize changes in electrical resistance. At the beginning of the 21st century, the relatively high-temperature superconductivity of MgB2 (magnesium diboride) was reported as an intermetallic compound, and efforts are underway to develop liquid hydrogen level gauges using MgB2.

[0004] Patent Document 1 describes that the compound is an intermetallic compound made of magnesium (Mg) and boron (B), and that the critical temperature (Tc) at which the electrical resistance becomes substantially zero is 39K. Patent Document 2 describes a liquid hydrogen level gauge that is constructed using a long superconductor in which some of the B atoms in MgB2 are replaced with C atoms. Patent Document 3 describes a method of manufacturing a superconducting wire by filling one end of a metal sheath with a first powder made by mixing Mg and B with an average particle size of less than 2 μm, filling the other end of the metal sheath with a second powder made by mixing Mg and B with an average particle size of 2 μm or more and less than 15 μm, and then performing wire drawing and heat processing. [Prior art documents] [Patent documents]

[0005] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-211916 Patent Document 2: Japanese Patent Application Laid-Open No. 2000-175034 Patent document 3: Re-table 2014 / 162379 publication Summary of the Invention [Problem to be solved by the invention]

[0006] However, the intermetallic compound described in Patent Document 1 has a high critical temperature (Tc) of 39 K, which is too high for a superconductor to be used in a level gauge for liquid hydrogen, whose boiling point under normal pressure is approximately 20 K.

[0007] Furthermore, the superconductor described in Patent Document 2, in which some of the B atoms of MgB2 are replaced with C atoms, has the problem that some of the B atoms must be replaced with C atoms, making the manufacturing method complicated and making it difficult to achieve reproducibility.

[0008] Furthermore, the superconducting wire described in Patent Document 3 also has a problem in that the manufacturing method is complicated because two types of B with different average particle sizes are used.

[0009] An object of the present invention is to solve the above problems and to realize a superconducting material that can be used in a liquid hydrogen level gauge and that can be manufactured by a simple and reproducible method. [Means for solving the problem]

[0010] In order to solve the above problems, the present invention provides a superconductor having a critical temperature (Tc) that can be used in a liquid hydrogen level gauge, It is made of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, and does not contain SiC. The critical temperature (Tc) at which the electrical resistance becomes essentially zero is 29K or higher. 31.5K The present invention provides a superconductor characterized by:

[0011] This configuration makes it possible to realize a superconductor that can be used in a liquid hydrogen level gauge and that can be manufactured simply and with high reproducibility. Here, since the temperature of liquid hydrogen is 20K at normal pressure, the superconductor for the liquid hydrogen level gauge must have a critical temperature (Tc) above 20K. Furthermore, under high pressure (1 MPaG), the temperature of liquid hydrogen rises to 30 K. In this case, the superconductor for the liquid hydrogen level gauge must have a critical temperature (Tc) above 30 K. In other words, the superconductor must have a critical temperature (Tc) that is equal to or higher than the boiling point of liquid hydrogen at the pressure inside the tank that is used. As will be described later, the applicant has determined that the average particle size is 38 μm for 1 Mg. m's By mixing boron (B) at a molar ratio of 2, MgB2 was produced, and Tc=29 K's Superconductors can be realized By mixing magnesium (Mg) with boron (B) of an average particle size of 19.25 μm in a molar ratio of 2 to produce MgB2, a superconductor with Tc=31.5K can be realized. We found that this is the case. They discovered that there is a very strong linear relationship between the average particle size (μm) of boron and the critical temperature (Tc) (see Figure 4).

[0012] In order to solve the above problems, the present invention provides a method for manufacturing a superconductor having a critical temperature (Tc) that can be used in a liquid hydrogen level gauge, comprising the steps of: Magnesium (Mg) and average particle size 38 μm 19.25 μm a mixing step of mixing the above-mentioned boron (B) with the above-mentioned boron (B); a firing step of firing the mixture of Mg and B mixed in the mixing step; By executing The present invention provides a method for producing a superconductor characterized by using MgB2.

[0013] This configuration makes it possible to realize a superconducting material that can be used in a liquid hydrogen level gauge and that can be manufactured by a simple and reproducible method.

[0014] In order to solve the above problems, the present invention provides a metal compound containing no SiC and having Mg and B in a molar ratio of 1:2, The critical temperature (Tc) at which electrical resistance becomes essentially zero is 29K. 31.5Kand a coating metal covering the surface of the superconductor.

[0015] This configuration makes it possible to realize a liquid hydrogen level sensor that can be used in a liquid hydrogen level gauge and that includes a superconducting material body that can be manufactured simply and with high reproducibility.

[0016] In order to solve the above problems, the present invention provides a metal compound containing no SiC and having Mg and B in a molar ratio of 1:2, The critical temperature (Tc) at which electrical resistance becomes essentially zero is 29K. 31.5K a liquid hydrogen level sensor having a long superconductor and a coating metal covering the surface of the superconductor; a heater for heating the liquid hydrogen level sensor; a power source for supplying current to the liquid hydrogen level sensor; a voltmeter for measuring a voltage at the liquid hydrogen level sensor; The present invention provides a liquid hydrogen level gauge comprising:

[0017] This configuration makes it possible to realize a liquid hydrogen level gauge that can be manufactured simply and with high reproducibility. [Effects of the Invention]

[0018] The superconductor and the method for manufacturing a superconductor of the present invention make it possible to realize a superconductor that can be manufactured simply and reproducibly, and therefore liquid hydrogen level sensors and liquid hydrogen level gauges can be easily realized. [Brief explanation of the drawings]

[0019] [Figure 1] 1 shows measurement data of the critical temperature (Tc) of a superconductor in Example 1 of the present invention (including comparative data). [Figure 2] FIG. 1 is a diagram illustrating a liquid hydrogen level sensor according to a first embodiment of the present invention. [Figure 3]FIG. 1 is a diagram illustrating a liquid hydrogen level gauge according to a first embodiment of the present invention. [Figure 4] 1 is a graph showing the relationship between the average particle size of boron and the critical temperature in Example 1 of the present invention. DETAILED DESCRIPTION OF THE INVENTION [Example]

[0020] Example 1 of the present invention will be described with reference to Figs. 1 to 4. Fig. 1 shows measurement data of the critical temperature (Tc) of the superconductor in Example 1 of the present invention (including comparative examples). Fig. 2 is a diagram illustrating a liquid hydrogen level sensor in Example 1 of the present invention. Fig. 3 is a diagram illustrating a liquid hydrogen level gauge in Example 1 of the present invention. Fig. 4 is a graph showing the relationship between the average particle size of boron and the critical temperature in Example 1 of the present invention.

[0021] (Superconductor) The superconductor in Example 1 can be used in a liquid hydrogen level gauge. If the pressure inside a container filled with liquid hydrogen is normal, the critical temperature (Tc) at which the electrical resistance of liquid hydrogen becomes substantially zero is 20 K, so to accommodate this, Tc = 29 K to 32 K was achieved.

[0022] However, if the pressure inside a container filled with liquid hydrogen is high (1 MPaG), the critical temperature (Tc) at which the electrical resistance of liquid hydrogen becomes essentially zero is a maximum of 30 K, so to accommodate this, Tc must be set to 31 K to 32 K.

[0023] In Example 1, whether a superconductor with Tc=29K-32K for normal pressure use or a superconductor with Tc=31K-32K for high pressure use is used can be controlled by the average particle size of the boron (B) raw material, MgB2. Also, for high pressure use, a lower temperature is desirable in order to make the heater power supply more compact and to prevent evaporation loss of liquid hydrogen, and Tc=30.5K-31.5K may be used. Similarly, for normal pressure use, 29K-31.5K may be used.

[0024] That is, the superconductor in Example 1 does not use SiC, but is made of magnesium (Mg) and boron (B), and is produced by filling a mixture of magnesium (Mg) and boron (B) in a molar ratio of 1 to 2 into a hollow metal tube, drawing the mixture, and then heat-treating it in an argon atmosphere at approximately 600°C to produce an intermetallic compound. As shown in the experimental data in Figure 1, if the average particle size of the raw material boron (B) is 38µm, Tc = 29.2K, and if the average particle size of boron (B) is 14µm, Tc = 32.4K.

[0025] In Figure 1, the horizontal axis represents temperature (K) and the vertical axis represents output voltage (mV). When the output voltage becomes substantially zero, the resistance also becomes zero, and the temperature at this point is the critical temperature (Tc). In Figure 1, wire A is a superconductor manufactured with an average boron (B) particle size of 0.7 μm, and its critical temperature Tc is 35.8 K. Similarly, wire B had an average grain size of 1 μm and Tc = 35.5 K, wire C had an average grain size of 2 μm and Tc = 34.3 K, wire D_1 had an average grain size of 14 μm and Tc = 32.4 K, wire E had an average grain size of 36 μm and Tc = 27.3 K, wire F had an average grain size of 38 μm and Tc = 29.2 K, wire G had an average grain size of 39 μm and Tc = 26.9 K, wire D_2 had an average grain size of 14 μm and Tc = 32.4 K, and wire D_3 had an average grain size of 14 μm and Tc = 32.4 K. The magnesium used in this study was 325 mesh (manufactured by Alfa Aesar, grain size approximately 44 μm).

[0026] The graphs for wires A, B, C, E, and G in Figure 1 are experimental data (comparison data) in which the critical temperature (Tc) did not fall within the range of 29K to 32K. As mentioned above, if the pressure inside the liquid hydrogen container is normal, the superconductors of wires D_1, D_2, D_3, and F can be used as sensors for a liquid level gauge. Here, wires D_1, D_2, and D_3 all have an average grain size of 14μm, but are different individuals, and all of them achieved a Tc of 32.4K, demonstrating high reproducibility in achieving a specified critical temperature depending on the average grain size of boron (B).

[0027] Wires D_1, D_2, and D_3 can be used even at high pressure (1 MPaG), but wire F cannot be used because its Tc is 29.2 K. In other words, the superconductors that can be used at normal pressure or high pressure are different, and these can be selected by the average particle size of boron (B).

[0028] The experimental data in Figure 1 uses commercially available boron (B), so the selectable average particle size is limited, but if the average particle size can be controlled, it is possible to manufacture a superconductor with the desired critical temperature Tc. Figure 4 is a graph showing the relationship between the average particle size of boron and the critical temperature in Example 1 of the present invention, as will be described later. For example, when manufacturing a superconductor with Tc = 31K, the average particle size 22 μm of boron (B) is sufficient for manufacturing a superconductor with Tc=30K. 27 μm boron (B) can be used.

[0029] Here, we will explain how to determine the average particle size and critical temperature (Tc) shown in the experimental data. The average particle size was determined by measuring using Shimadzu's SALD-2300 (laser diffraction particle size distribution analyzer). The critical temperature (Tc) was not determined according to JISH7309, but rather by approximating the measurement data (25K to 45K) of a short sample using a cryostat device with a sigmoid curve (transition curve) using the least squares method, and the value at 50% of the height was taken as Tc.

[0030] From the above experimental data, we learned that the smaller the average particle size of boron, the higher the critical temperature (Tc), and the larger the average particle size, the lower the critical temperature (Tc). Next, we investigated what the average particle size of boron should be to achieve a critical temperature (Tc) of 29K to 32K. The graph shown in Figure 4 shows the average particle size of boron (μm) on the horizontal axis and the critical temperature (Tc) on the vertical axis, and seven data points from the above experimental results are plotted on this graph. As can be seen from Figure 4, these data have a very strong linear approximation relationship as shown in Equation 1. Equation 1: Tc temperature = -0.2 x average particle size (μm) + 35.35

[0031] The average particle size of boron required to obtain a critical temperature (Tc) of 29K to 32K, obtained from this approximation line, is 38μm to 14μm. In other words, if one part magnesium is mixed with two parts boron with an average particle size of 38 to 14μm, a superconductor with a critical temperature (Tc) of 29.2K to 32.4K can be obtained.

[0032] In Example 1, the particle size of boron was expressed as an average particle size. The average particle size is a representative diameter obtained by averaging the measurements of the SALD-2300 described above, and is calculated by multiplying and adding up the particle size on the horizontal axis of the histogram by the frequency on the vertical axis. However, if the particle size distribution, which represents the particle size distribution of the particulate matter, differs, the critical temperature (Tc) may differ.

[0033] (Method of manufacturing superconductors) First, a mixing process is carried out in which 1 part magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) is mixed with 2 parts boron (B) with an average particle size of 14 μm. Next, a firing process is carried out in which the mixture of Mg and B mixed in the mixing process is fired in an argon atmosphere at 600°C for 1 hour to complete an MgB2 superconductor. As will be described later, to make a long liquid hydrogen level sensor, the fired MgB2 can be powdered and placed in a long tube and then wiredrawn, or the mixed Mg and B powder can be wiredrawn and then sintered.

[0034] Here, boron (B) was purchased and used as sold by Nilaco with a particle size of 40 μm, but the average particle size measured using the above-mentioned laser diffraction particle size distribution analyzer was 14 μm. Magnesium (Mg) was purchased and used as sold by alfa Aesar with a 325 mesh (particle size of approximately 44 μm).

[0035] Magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) was mixed with boron (B) of an average particle size of 14 μm in a molar ratio of 1 part, and the resulting wire was drawn and fired. The critical temperature (Tc) of the resulting superconductor was 32.4 K (see Figure 1).

[0036] In addition, a mixing process is carried out in which boron (B) with an average particle size of 38 μm is mixed with magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 1 part to 2 parts. Next, a firing process is carried out in which the mixture of Mg and B mixed in the mixing process is fired in an argon atmosphere at 600°C for 1 hour.

[0037] Here, boron (B) was purchased and used as sold by nanogaraphi technology with a particle size of 10 μm, and the average particle size measured using the laser diffraction particle size distribution analyzer mentioned above was 38 μm. Magnesium (Mg) was purchased and used as sold by alfa Aesar with a 325 mesh (particle size of approximately 44 μm).

[0038] Magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) was mixed with boron (B) of an average particle size of 38 μm in a molar ratio of 1:2, and the critical temperature (Tc) of the sintered superconductor was 29.2 K (see Figure 1).

[0039] In Example 1, the magnet was manufactured without adding any substance other than MgB2, but this is not necessarily limited to this and can be changed as appropriate. For example, the magnet may contain SiC or other substances that do not affect the critical temperature (Tc).

[0040] After mixing boron (B) and magnesium (Mg), and before the firing process, the Mg and B powder particles are in contact at their contact points. When the firing process is carried out in this state, MgB2 crystal nuclei are generated from the contact points and these nuclei grow. The number of MgB2 crystal nuclei generated is affected by the number of contact points, which varies depending on the particle size ratio of the powders being mixed. When there are many crystal nuclei (many contact points), the overall reaction is fast and adjacent crystals collide at the interface, fuse and grow, resulting in a complete crystal with no unreacted powder that can become an impurity.

[0041] Therefore, in the particle size range of several μm to several tens of μm, the smaller the particle size of B is compared to the particle size of Mg, the more contact points there are, and the generated MgB2 becomes a perfect crystal, so the critical temperature (Tc) becomes higher. This supports the relationship between the average particle size of B and the critical temperature (Tc) explained in Figure 4 above.

[0042] The above steps have made it possible to produce reproducible superconductors, but to ensure even greater reproducibility, it is necessary to strictly control the heat treatment (rate and time of temperature increase, constant temperature, and temperature decrease), the particle size and particle size ratio of Mg and B, and particle size distribution, etc., to ensure the same conditions during production.

[0043] (Liquid hydrogen level sensor) The liquid hydrogen level sensor 10 in Example 1 is composed of a superconductor 11 and a metal coating 12, and forms a long core wire with a length of approximately 1 m and a diameter of approximately 0.3 mm, as shown in Figure 2. The superconductor 11 is made by mixing magnesium (Mg) of 325 mesh (manufactured by Alfa Aesar, particle size approximately 44 μm) with boron (B) of an average particle size of 14 μm in a molar ratio of 1:2 (mixing process), which is then drawn in a wiredrawing process and then sintered to form an MgB2 superconductor. In the wiredrawing process, the mixture of Mg and B obtained in the mixing process is powdered and placed in a tube approximately 1 m long, which is then drawn into a long shape.

[0044] In Example 1, the mixing step, wire drawing step, and firing step are performed in this order, but this is not necessarily limited to this and can be modified as appropriate. For example, the firing step may be performed after the mixing step, followed by a powdering step to form the material into a powder, and finally the wire drawing step. The heat treatment step may also be performed multiple times.

[0045] In Example 1, a long core wire having a length of about 1 m and a diameter of about 0.3 mm is formed, but this is not necessarily limited to this and can be changed as appropriate. For example, the length may be about 5 m or 0.5 m, and may be prepared according to the depth of the liquid hydrogen. In addition, although the diameter is set to approximately 0.3 mm in Example 1, it is not necessarily limited to this and can be changed as appropriate. For example, it may be 0.5 mm or 0.2 mm, and the diameter may be selected in consideration of the critical temperature (Tc).

[0046] Furthermore, the metal coating 12 may be any metal having high conductivity. Although cupronickel was used in Example 1, other metals such as stainless steel may also be used.

[0047] In Example 1, boron (B) having an average particle size of 14 μm was mixed with magnesium (Mg) having a mesh size of 325 (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 2, but this is not necessarily limited to this and can be changed as appropriate. For example, boron (B) having an average particle size of 38 μm may be mixed with magnesium (Mg) having a mesh size of 325 (manufactured by Alfa Aesar, particle size approximately 44 μm) in a molar ratio of 2.

[0048] The resistance value of the liquid hydrogen level sensor 10 in the first embodiment at room temperature is 1 Ω / m to 6 Ω / m, but is not limited to this and can be changed as appropriate.

[0049] In addition, in the first embodiment, the length of the liquid hydrogen level sensor 10 is set to approximately 1 m, but this is not necessarily limited to this and can be changed as appropriate. For example, the length may be set to approximately 5 m or approximately 0.5 m. It is sufficient if the length is at least long enough to measure the liquid hydrogen level.

[0050] (Liquid hydrogen level gauge) The configuration of the liquid hydrogen level gauge 100 in Example 1 is shown in Figure 3. A tank T is filled with liquid hydrogen M, and a liquid hydrogen level sensor 10 is installed vertically in the tank T. The liquid hydrogen level sensor 10 is placed deep in the liquid hydrogen M.

[0051] The liquid hydrogen level sensor 10 in the room temperature area above the liquid surface can generate resistance heat by passing a current from the heater power supply 21 through the heater 20, thereby heating the liquid hydrogen level sensor 10. This eliminates the influence of the temperature of the liquid hydrogen M and makes it possible to maintain a normal conducting state above the liquid surface.

[0052] A power supply 30 and a voltmeter 40 are connected to both ends of the liquid hydrogen level sensor 10. Then, a current is passed from the power supply 10 to the liquid hydrogen level sensor 10, and the voltage across the liquid hydrogen level sensor 10 is measured by the voltmeter 40 and stored in the processing unit 50. Then, the liquid level position is calculated from the stored voltage based on a predetermined known relational expression.

[0053] As described above, in Example 1, a superconductor having a critical temperature (Tc) that can be used in a liquid hydrogen level gauge is used. It is made of a metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, and does not contain SiC. A superconductor characterized by having a critical temperature (Tc) of 29K to 32K at which the electrical resistance becomes substantially zero can be used in a liquid hydrogen level gauge, and can be manufactured in a simple and reproducible manner.

[0054] In addition, in Example 1, a method for manufacturing a superconductor having a critical temperature (Tc) that can be used for a liquid hydrogen level gauge is provided, A mixing step of mixing magnesium (Mg) with boron (B) having an average particle size of 38 μm to 14 μm; a firing step of firing the mixture of Mg and B mixed in the mixing step; By executing By using a method for manufacturing a superconductor characterized by using MgB2, it is possible to realize a superconductor for a liquid hydrogen level gauge that can be manufactured simply and reproducibly.

[0055] In Example 1, the metal compound does not contain SiC and has Mg and B in a molar ratio of 1:2. A liquid hydrogen level sensor comprising a long superconductor having a critical temperature (Tc) of 29K to 32K at which the electrical resistance becomes substantially zero, and a metal coating covering the surface of the superconductor, can realize a liquid hydrogen level gauge that can be manufactured simply and with high reproducibility.

[0056] Furthermore, in Example 1, the metal compound does not contain SiC and has Mg and B in a molar ratio of 1:2, a liquid hydrogen level sensor having a long superconductor having a critical temperature (Tc) of 29K to 32K at which the electrical resistance becomes substantially zero, and a coating metal covering the surface of the superconductor; a heater for heating the liquid hydrogen level sensor; a power source for supplying current to the liquid hydrogen level sensor; a voltmeter for measuring a voltage at the liquid hydrogen level sensor; By using the liquid hydrogen level gauge having the above features, it is possible to realize a liquid hydrogen level gauge that can be manufactured simply and reproducibly. [Industrial Applicability]

[0057] The superconductor, the method for manufacturing the superconductor, the liquid hydrogen level sensor, and the liquid hydrogen level gauge of the present invention can be widely applied in the field of liquid hydrogen level gauges. [Explanation of symbols]

[0058] 10: Liquid hydrogen level sensor 11: Superconductor 12: Coated metal 20: Heater section 21: Heater power supply 30: Power supply 40: Voltmeter 50: Processing section 100: Liquid hydrogen level gauge

Claims

1. A superconductor having a critical temperature (Tc) that can be used in a liquid hydrogen level gauge, A metal compound of magnesium (Mg) and boron (B) in a molar ratio of 1:2, not containing SiC, A superconductor characterized in that the critical temperature (Tc) at which the electrical resistance becomes substantially zero is 29K to 31.5K.

2. A method for manufacturing a superconductor having a critical temperature (Tc) that can be used in a liquid hydrogen level gauge, comprising: A mixing step of mixing magnesium (Mg) with boron (B) having an average particle size of 38 μm to 19.25 μm; a firing step of firing the mixture of Mg and B mixed in the mixing step; By executing A method for producing a superconductor, characterized in that the superconductor is MgB2.

3. a metal compound containing no SiC and having Mg and B in a molar ratio of 1:2; A liquid hydrogen level sensor comprising: a long superconductor having a critical temperature (Tc) of 29K to 31.5K at which the electrical resistance becomes substantially zero; and a coating metal covering the surface of the superconductor.

4. a metal compound containing no SiC and having Mg and B in a molar ratio of 1:2; a liquid hydrogen level sensor comprising a long superconductor having a critical temperature (Tc) of 29K to 31.5K at which the electrical resistance becomes substantially zero, and a coating metal covering the surface of the superconductor; a heater for heating the liquid hydrogen level sensor; a power source for supplying current to the liquid hydrogen level sensor; a voltmeter for measuring a voltage at the liquid hydrogen level sensor; A liquid hydrogen level gauge comprising:

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