Abstract:
The present invention provides a method and apparatus for measuring the level of a liquid cryogen. The invention provides for use of a current source ( 52 ), a length, or multiple lengths, of superconducting filament ( 40 ) situated within a cryostat ( 1 ) containing a cryogenic fluid ( 2 ) and a voltmeter ( 51 ) for measuring the voltage drop across the filament ( 40 ). The invention further provides a method for using said apparatus.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to methods and devices used to measure the liquid levels in a cryogenic environment. More specifically, the invention relates to a new and improved method and apparatus for assembling a liquid helium level sensor that is used for measuring the liquid helium level in a cryogenic cooler. 
   Commonly used liquid helium sensors employ a string of superconducting NbTi filament in either rigid or flexible tubing. Generally, filament sizes range from 0.0005 inch to 0.002 inch in diameter. The sensor operates by measuring the resistance of the portion of the superconductor filament that is above the liquid helium level. The portion that is submerged in the liquid will not contribute measurable resistance because it is in the superconducting state at the temperature of liquid helium. Very thin NbTi filament is employed so that the resistance generated by that portion of the filament above the liquid helium surface produces a measurable voltage drop across the entire filament even with a current as small as 50 mA to 100 mA. The liquid helium level is then inversely proportional to the voltage drop measured, given a constant excitation current. 
   In general, the sensitivity and accuracy of the sensor increases as the resistance of the superconducting filament increases. Normally, there are two ways to increase the resistivity of the filament. The first way is to reduce the diameter of the filament. The second method involves etching off the copper matrix from the NbTi—Cu filament, because copper has very low resistivity. 
   In practice, very thin filament is difficult to handle, easy to break and is so thin that is extremely difficult to manufacture. Also, very thin filament size reduces the current carrying capacity of the superconducting filament, thus reducing the voltage output. This causes a reduction of the signal to noise ratio and reduces the accuracy of the measuring device. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a novel technique for assembling superconducting filaments and heaters to form a single object for use as a cryogenic fluid indicator in a cryogenic environment. The present invention provides a high degree of sensitivity and reliability in comparison to previous methods. 
   The present invention provides a thicker NbTi filament that is connected in series to increase the output voltage. Increasing the output voltage increases the sensitivity to changes in the level of liquid helium and improves the accuracy for measuring the voltage drop. The present invention can also be modified from a single filament level sensor to a multiple filament level sensor by a relatively simple change in the connection scheme. 
   The present invention provides better sensitivity to the liquid helium level due to the longer resistive filament. The present invention also achieves better accuracy because of the higher resistance of the filament used in the present invention. The present invention is also more reliable than previous methods because larger diameter filament can be used. Lastly, manufacturing a very thin filament presents a high probability of breakage during the manufacturing process. Therefore, using a thicker filament reduces this risk and the cost associated with the manufacturing process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic showing a liquid helium level sensor installed in a liquid helium reservoir. 
       FIG. 2  is a block diagram of the circuitry of a liquid helium level sensor. 
       FIG. 3  is a schematic showing the liquid helium level sensor of the present invention. 
       FIG. 4  is another schematic showing another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Reference is now made to the drawings wherein like numbers represent like elements throughout.  FIG. 1  is a schematic of the liquid helium level sensor of the present invention showing the cryostat  1 , the cryogenic fluid  2 , the liquid-gas interface  3 , the filament  40  and the superconductive solder  10 . Also shown in  FIG. 1  is an electric current source  52 , current switches  53  and positive and negative voltage terminals  61 ,  62 . 
     FIG. 2  is a schematic block diagram of a voltage measurement device  51 , a rate sensor  54 , a current source  52 , and a current switch  53 . As shown, the current source  52  is connected to the filament  40  and heater  20  through the current switch  53 . A voltmeter  61  is then connected to measure the voltage drop across the length of the filament  40 . The voltmeter measures the voltage drop at voltage terminals  61 ,  62 . 
   In normal operation of this general embodiment of this invention, the filament  40  and heaters  20  are placed within a rigid or flexible tube  30  and lowered into the cryostat  1  until at least a portion of the filament  40  is submerged. As discussed above, the extremely low temperature of the liquid phase of the helium  2  makes the filament  40  a superconductor. 
   Current from the current source  52  is then applied through the switch  53  to the heater  20  and filament  40 . Normally, between 50 and 200 mAmps are used. Initially, there will be no voltage drop generated through the filament  40  because it is all superconducting. However, as the heater  20  heats the filament  40 , it will produce a region of normal resistance in the filament  40 , and thus a measurable voltage drop across voltage terminals  61 ,  62 . In general, it takes between 0.1-1 second to warm that portion of the filament  40  situated above the liquid helium  2  to the point that it provides some measurable electrical resistance. The heat from the heater  20  will progress down the filament  40  to the gas-liquid interface  3 , at which point it ceases to have an effect on the filament  40 . Thus, that portion of the filament  40  situated above the gas-liquid interface  3  will offer normal electrical resistance while that below the gas-liquid interface  3  will be superconductive. In general, the resistance offered by the filament  40  above the superconducting portion will produce a voltage drop of up to approximately 50 volts. 
   The voltage drop across the filament  40  increases in a generally linear manner, the rate of change being dV/dt. As the gas-liquid interface  3  is approached, the voltage becomes constant, or the rate of increase of voltage changes until dV/dt=0 as the normal resistive state of the filament  40  has reached the gas-liquid interface  3 . 
   At a low value of dV/dt, the rate sensor  54  causes the switch  53  to open. Current then stops flowing through the filament  40  and the heater  20 . Therefore, current only flows during the time necessary to warm the filament  40  above the gas-liquid interface  3 . This minimizes the evaporation of liquid helium  2  due to heat added while measuring the level of the liquid helium  2 . The current could also be cycled to reduce liquid helium  2  evaporation. 
   The present invention uses a combination of superconducting filament  40  and heater  20  to improve accuracy and sensitivity in liquid helium  2  level measurement. The present invention also employs a thicker NbTi filament  40  connected in series to increase the output voltage. The NbTi filament  40  used will range between 0.001 in. and 0.005 in. Use of a thicker filament increases the sensitivity to changes in helium level and increases the accuracy of measurement. This new and unique invention provides for higher sensitivity to changes in the liquid helium  2  level because the present invention provides for a longer resistive region. Further, the amount of voltage drop increases linearly with the amount of resistance. Therefore, the greater the voltage drop, the more easily it is measured, and thus, the more accurately it is measured. Additionally, the filament  40  used in the present invention can be thicker than what is presently used. The filament  40  used in the present invention is more reliable and less subject to breakage. Thicker filaments  40  are far easier to manufacture than filaments  40  that are presently being used. The filament  40  employed is common commercial NbTi filament of approximately 46% to 48% titanium by weight. 
   The present invention also provides for the ability to use a multifold superconducting filament  40  in the level sensor in contrast to a conventional, single filament  40 . Multifold configurations generally consist of filament  40  connected using a superconductive solder  10 . A multifold configuration, as further provided for in this disclosure will increase the sensor accuracy by a factor of two, as illustrated in  FIG. 3 , or by a factor of four, as shown in FIG.  4 . Obviously, the concept can be extended to as many folds as are required to further enhance accuracy. 
     FIG. 3  shows a two length filament  140  wherein the filament  140  is comprised of a first length  141  having a first end  241  connected to the voltage terminal  161  and a second end  242  and a second length  142  having first end  243  connected to the second end  242  of the first length  141  and a second end  244  connected to a voltage terminal  162 .  FIG. 4  illustrates an example of the use of four lengths  341 ,  342 ,  343 ,  344  of superconducting filament  340  wherein the superconducting filament  340  is comprised of a first length of superconducting filament  341  having a first end  441  connected to the voltage terminal  361  and a second end  442 , a second length of the superconducting filament  342  having first end  443  connected to the second end of the first length  442  and a second end  444 , a third length of superconducting filament  343  having a first end  445  connected to the second end  444  of the second length  342 , and a fourth length of superconducting filament  344  having a first end  447  connected to the second end of the third length  446  and a second end  448  connected to a second voltage terminal  362 . 
   Accordingly, an improved device for measuring the level of liquid helium  3  in a cryogenic environment has been disclosed. The device of the present invention a longer and thicker filament  40  that provides a greater measurable voltage drop and thus a more accurate measurement of the level of liquid helium  2  present in the cryostat. Further, a thicker filament  40  improves performance and is less complicated to produce than the thinner filaments  40  of the prior art. 
     FIG. 3  shows an embodiment in which current would enter the cryostat  1  at  63  and leave at  64 . The voltage drop would be measured at voltage terminals  61  and  62 .  FIG. 4  is identical with the exception of the increased filament  40  length. 
   Although we have very specifically described the preferred embodiments of the invention herein, it is to be understood that changes can be made to the improvements disclosed without departing from the scope of the invention. Therefore, it is to be understood that the scope of the invention is not to be overly limited by the specification and the drawings, but is to be determined by the broadest possible interpretation of the claims. 
   PARTS LIST 
   
       
         1  cryostat 
         2  liquid cryogen 
         3  interface between liquid and gas phase 
         10  connector and epoxy 
         20  heater 
         30  flexible or rigid tubing 
         40  filament 
         51  voltmeter 
         52  current source 
         53  current switch 
         54  rate sensor 
         61  positive voltage terminal 
         62  negative voltage terminal 
         63  positive current source 
         64  negative current source 
         140  two length filament 
         141  first length of filament  140   
         142  second length of filament  140   
         161  voltage terminal 
         162  voltage terminal 
         241  first end of  141   
         242  second end of  141   
         243  first end of  142   
         244  second end of  142   
         340  four length filament 
         341  first length of filament  340   
         342  second length of filament  340   
         343  third length of filament  340   
         344  fourth length of filament  340   
         361  voltage terminal 
         362  voltage terminal 
         441  first end of  341   
         442  second end of  341   
         443  first end of  342   
         444  second end of  342   
         445  first end of  343   
         446  second end of  343   
         447  first end of  344   
         448  second end of  344