Abstract:
An improved liquid cryogen dewar for storing cryobiological materials is disclosed having an inner tank with a reservoir holding a pool of liquid cryogen. A rotatable tray is contained within the inner tank, containing a platform, vertical dividers and a thermally conductive cylindrical outer sleeve, which contains a skirt extending into the pool of liquid cryogen. The sleeve transfers entering heat away from the cryobiological materials and into the liquid cryogen pool. The sleeve maintains a consistently low temperature throughout the vertically disposed shelves of the cryobiological materials contained within the inner tank.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to the field of cryogenic storage devices and, more particularly, to an improved cryogenic dewar having a tray for holding specimens, the tray including a thermally conductive, cylindrical sleeve, containing a skirt which is at least partially immersed in liquid cryogen. 
     Vapor phase liquid cryogen freezers have been used for several decades for long term storage of biological specimens, which are heat sensitive. Normally, a frozen specimen is placed into a storage container, which is stored in a dewar. A typical dewar  10 , shown in FIG. 1, contains an outer shell  12  housing inner tank  14 , separated from inner shell  16  by vacuum-insulated space  18 . Inner tank  14  is closed using lid  34 . 
     A stainless steel turn tray  30  holds a number of stainless steel storage racks  32  with shelves  33 , where vials of biological specimens are placed in boxes on the shelves  33  for storage. The racks  32  rest on a circular, stainless steel, turn tray platform  26  welded to the remainder of the tray. Vertical dividers  24  separate turn tray  30  into sections, each of which may hold one or more racks  32 . For example, four dividers may be used to separate tray  30  into quadrants. 
     A cylindrical sleeve  36 , made of stainless steel and welded to the edges of dividers  24 , surrounds tray  30 . Sleeve  36  and dividers  24  cooperate to help maintain the storage racks  32  placed between dividers  24  in an upright position by keeping the racks  32  from tipping over within their particular sections. The sleeve  36  of the prior art dewar extends upwardly from platform  26  to the top of vertical dividers  24 . 
     Dividers  24  and platform  26  are welded to a stainless steel central tube  28  to allow tray  30  to rotate within inner tank  14 . To access storage racks  32 , a user rotates tray  30  using handles  20  attached to the top edge of dividers  24 , until a desired rack  32  is positioned underneath lid  34 , whereby a desired specimen may be acquired by removal of the rack  32 . 
     The bottom of the inner tank is a reservoir for a pool of liquid cryogen  40 , such as liquid nitrogen. As the nitrogen receives heat transferred from outside of dewar  10 , via inner shell  16  and lid  34 , a portion of the nitrogen evaporates to produce a cold vapor, which surrounds the storage racks  32 . This type of cold storage, known as “vapor phase” storage, prevents cross-contamination of the biological specimens stored within dewar  10 . The nitrogen vapor passes through apertures  25  within dividers  24  and platform  26 . 
     A primary concern of such vapor phase storage is maintaining a desired, low temperature at the storage racks, particularly at the upper shelves. While liquid nitrogen at the bottom of the dewar remains at a constant temperature (about −196° C.), and while vapor near the liquid nitrogen approaches this temperature, ambient heat entering from the walls and lid of the container warm the vapor above the liquid pool. This warmer vapor migrates to the upper portions of inner tank  14 , and thus to the specimens contained on the upper shelves. A temperature gradient of as much as 100° C. can exist from the bottom of the dewar to the top. This difference is significant, because it is accepted that diffusion within biological specimens can begin to occur at temperatures as warm as −132° C. Keeping the temperature of the specimens under this threshold is thus a significant concern. Storage below −150° C. is generally accepted by the industry as safe since it is below the threshold for diffusion by a safe margin to allow for temperature fluctuation in the freezer. 
     Past efforts to decrease the temperature gradient, and thereby lower the upper shelf temperatures, fall into two categories. The first is improving the insulation efficiency of the dewars, which indeed lowers the temperature gradient for a closed dewar. However, once the lid of the dewar is opened, heat enters the dewar, adversely affecting, the top shelves. The top shelves can get quite warm (about −50° C.), and there is a slow recovery time for the shelves to revert to a cooler temperature. 
     A second solution is making the shelving and rack out of aluminum or a similar metal with high thermal conductivity. While at steady state temperatures, with the lid closed, this method appears to solve the problem, but it is actually worsened when the dewar lid is opened to add or remove samples. As heat enters the dewar through the open lid, the aluminum shelving and rack transfer significant heat to the lower shelves. This is because the nitrogen vapor is a poor thermal conductor and doesn&#39;t effectively transfer the heat to the liquid nitrogen pool below. 
     Accordingly, an improved cryogenic freezer, which has a lower temperature gradient from the bottom to the top, and that can keep the top shelves at a relatively constant temperature, below a desired threshold, is needed. 
     It is an object of the invention to provide a liquid cryogen freezer that prevents specimens stored within the freezer from exceeding a desired threshold temperature. 
     It is a further object of the invention to provide a dewar that significantly lowers the temperature gradient. 
     It is a further object of the invention to provide a liquid cryogen freezer that allows a user to quickly and easily access desired biological specimens within the freezer, while maintaining a safe temperature for the specimens. 
     It is a further object of the invention to provide a liquid cryogen freezer that reduces the time of the dewar to recover to steady state storage conditions after the lid has been opened. 
     It is a further object of the invention to provide a liquid cryogen freezer that maintains a nearly uniform steady state temperature within the dewar, even when the lid of the dewar is opened. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the shortcomings of the prior art, and consists of a dewar with an improved turn tray having a sleeve made of a thermally conductive material. The sleeve of the improved tray has a thermally conductive skirt extension, which extends below the floor of the tray so as to be at least partially immersed in the pool of liquid nitrogen contained in the inner tank. The sleeve extends upwardly to a level substantially even with the top of the storage racks. 
     The sleeve is in direct contact with the liquid nitrogen and is an excellent thermal conductor. Heat entering the tank through the lid is rapidly transferred into the liquid nitrogen pool below via the sleeve instead of into the nitrogen vapor surrounding the stored specimens. This, in turn, increases evaporation of the liquid nitrogen producing additional cool vapor that reaches the top storage shelves more quickly than in prior art dewars thereby decreasing the time required for the dewar to recover to steady state conditions. As a result, the temperature gradient is significantly decreased and the upper storage shelves are maintained at a safe temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art dewar, cut away so as to show the inner tank and turn tray. 
     FIG. 2 is a perspective view of a dewar using the improved turn tray of the present invention, cut away so as to show the inner tank and turn tray. 
     FIG. 3 is a side sectional view of a dewar using the improved turn tray of the present invention. 
     FIG. 4 is a graph showing a relationship between temperature and shelf location for a prior art dewar having a stainless steel tray and stainless steel shelving and storage racks. 
     Fig.5 is a graph showing a relationship between temperature and shelf location for a prior art dewar having a stainless steel tray and aluminum shelves and storage racks, at steady state. 
     FIG. 6 is a graph showing a relationship between temperature and shelf location for a prior art dewar having a stainless steel tray and aluminum shelves and storage racks, with the dewar lid opened. 
     FIG. 7 is a graph showing a series of relationships between temperature and shelf location for dewars representing different embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more particularly to FIG. 2, according to a preferred embodiment of the invention, the invention consists of a dewar  100 , having an improved turn tray  130  with an outer sleeve  136  which is formed of a thermally conductive material, such as aluminum or copper. The turn tray  130  is rotatably housed within dewar  100 . Sleeve  136  contains a thermally conductive skirt extension  138  which extends downwardly below turn tray platform  126  so as to be at least partially immersed in the liquid nitrogen pool  140 . It thus acts as a heat conduit or heat sink in conjunction with the liquid nitrogen. 
     Sleeve  136  is typically formed by rolling a sheet of aluminum into a cylinder so as to surround turn tray  130 . Sleeve  136  is welded onto the outer edges of vertical dividers  124  and the outer periphery of platform  126 . Skirt extension  138  is typically an integral portion of sleeve  136 , as skirt  138  and sleeve  136  are usually one piece. Sleeve  136 , including skirt  138 , preferably has a thickness of about one-sixteenth inch. 
     In order to effectively weld sleeve  136  onto vertical dividers  124 , the dividers are preferably formed of aluminum. For the same reason, turn tray platform  126  and turn tray tube  128 , which are welded components of tray  130 , are similarly constructed of aluminum. The rack  132  and shelves  133  are constructed of stainless steel. 
     As shown in the enlarged portion of FIG. 3, thermally conductive skirt extension  138 , part of sleeve  136 , extends below platform  126  into the pool of liquid nitrogen  140 , for conductive transfer of heat between sleeve  136  and pool  140 . For a sleeve having a height of twenty-six inches, for example, the skirt  138  may extend downwardly beneath platform  126  about three inches. 
     As shown in FIG. 2, sleeve  136  may also extend upwardly over the top edges of vertical dividers  124  and towards the top of storage shelves  133 , to a level substantially even with the top of the racks  132 . To reinforce sleeve  136  at the section of the sleeve above the vertical dividers  124 , an aluminum, circular stiffener band  152  is rolled and tack welded to the inner tank  114  at the top edge of sleeve  136 , extending around the circumference of the sleeve  136 . 
     In operation, sleeve  136  transfers incoming heat from lid  134  and inner shell  116 , drawing the heat away from shelves  133 , into the liquid nitrogen pool  140  below thus acting as a heat sink. Evaporation of liquid nitrogen is thereby increased, producing additional cold vapor which flows to the top of inner tank  114  more quickly than in the prior art. This decreases the time it takes for the dewar to return to steady state conditions. 
     As a result of the action of sleeve  136 , temperature at the top of inner tank  114 , and thus at the top of shelves  133 , is lowered. In doing so, the temperature gradient between the top and bottom of the storage shelves is similarly reduced. 
     In addition to the exemplary structure above, additional embodiments are possible which include an upright, vertically disposed, thermally conductive element such as a rod extending into the pool of liquid nitrogen. Alternatively, the vertical dividers  24  may contain one or more extensions into the liquid nitrogen pool. Or, tube  128  may contain a fin extending into the nitrogen, in thermal connection with a series of fins near the top of the inside of the dewar. These additional embodiments, and others, are contemplated to be within the scope of this invention. 
     The benefits of the improved dewar are illustrated in the graphs shown in FIGS. 4-7. In each of these graphs, the horizontal axis represents the relative location of a shelf within an inner tank of a dewar. The vertical axis represents a maximum measured temperature at a particular shelf. 
     FIG. 4 shows the temperatures at the top, middle, and bottom shelves of a prior art dewar having stainless steel shelves and racks with a stainless steel turn tray, without the improvements of the present invention. There is a significant temperature differential of 37° C., resulting in a large temperature gradient. Also the temperature at the top shelf rises to −146.4° C., which is above the threshold temperature where diffusion of cryobiological specimens can occur. 
     FIG. 5 shows a temperature relationship for a similar prior art dewar having a stainless steel tray, but with aluminum shelves and racks. Here the temperature gradient, at steady state, is acceptably small. However, this graph does not account for the instance when the dewar lid is opened. This is shown in FIG.  6 . 
     FIG. 6 shows the results of opening the lid for a period of time (at least several hours) until equilibrium (stability) is reached within the tank. The gradient is much larger. Also the temperature at the top and middle shelves are significantly above the “safe” storage temperature for cryobiological materials, and that even on the bottom shelf, the temperature is barely below the safe level. This is because the heat from the ambient air is quickly absorbed by the upper shelves, and transferred to the lower shelves. The surrounding vapor (nitrogen, typically) is a poor thermal conductor and does not transfer the heat into the nitrogen pool below rather it warms the sleeves and racks. For a dewar that is frequently opened by a user, this dewar configuration may compound the problem. 
     FIG. 7 shows temperature relationships for two embodiments of dewars of the present invention, both having an aluminum turn tray with a thermally conductive aluminum skirt extension immersed within a pool of liquid nitrogen. A first curve, indicated by diamonds, shows a dewar at steady state having an aluminum turn tray and aluminum shelves and racks. The overall temperatures, as well as the gradient, have decreased slightly as compared to the dewar of FIG. 5. A second curve, indicated by squares, shows a preferred dewar, which has an aluminum tray and stainless steel racks. Here, the temperatures are slightly above those of the dewar having aluminum shelves and racks (FIG.  5 ), at least at steady state. However, the gradient has been lowered, and the temperatures are all significantly below the safe level of −150° C. The third curve, indicated by asterisks, shows the preferred dewar with stainless steel shelves and racks with the lid open at steady state. The gradient is much smaller than the gradient shown in FIG. 6, and the temperature at the top shelf is safely below −150° C. This improved dewar allows for safer and consistently cooler storage of cryobiological material. 
     It will be apparent to those skilled in the art that other modifications and variations can be made to the method and system of the instant invention without diverging from the scope, spirit, or teaching of the invention. Therefore, it is the intention of the inventors that the description of the instant invention should be considered illustrative and that the invention is to be limited only as specified in the claims and equivalents thereto.