Abstract:
Apparatus, systems and methods are disclosed for efficient elimination of door and frame condensation at widely utilized commercial freezer/refrigeration display cases. The apparatus includes a local controller unit connected with an array of case sensors including an internal case temperature sensor, an external case frame temperature sensor and a dew point sensor. The case frame temperature sensor and dew point sensor are housed to thermally isolate sensing elements thereof from effects of frame temperature changes and ambient air temperature in the vicinity of a sensing element, and to limit heat transfer by the housing to the sensing elements thereby improving sensing accuracy and apparatus performance.

Description:
RELATED APPLICATION  
       [0001]     This application is a divisional application of U.S. patent application Ser. No. 11/069,378 Filed on Mar. 1, 2005 by Murray et al. and entitled “Refrigeration Unit Condensation Prevention” which will issue as U.S. Pat. No. 7,207,181 on Apr. 24, 2007. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to frame and/or door heating to remediate condensation build-up in refrigerated display cases, and, more particularly, relates to sensing and operating devices and methods for activation and deactivation of such heating.  
       BACKGROUND OF THE INVENTION  
       [0003]     Condensation build up on commercial refrigeration display case doors can cause door damage and presents a safety hazard if runoff from the door(s) and/or case frames accumulates on adjacent floors. Currently, in most commercial installations, in order to prevent excessive condensation build-up at display case doors and frames, the doors and frames are heated utilizing internal frame heaters operating at 100% duty cycle time (and incurring correspondingly high energy costs).  
         [0004]     Devices have been heretofore suggested and/or utilized to control condensation without running heaters at 100% duty cycle times. One approach has utilized a detector to directly sense the presence of condensation on the freezer door or frame and, responsive thereto, activate the internal frame/door heaters when condensation is sensed. The heaters then are run for a fixed duration or until moisture has evaporated. These devices have, however, not always proven successful. For example, detector failure due to environmental contamination or poor manufacturing tolerances of moisture sensors in general is common. Such detector failure can result in either frame/door heaters remaining off (thus failing to inhibit dangerous condensation build-up and runoff) or the heaters remaining on (thereby achieving no energy savings). Moreover, such devices are merely reactive, activating heaters only after potentially damaging and dangerous condensation has formed. (see the DOOR MISER XP by Door Miser, LLC and U.S. Pat. No. 5,899,078, for examples).  
         [0005]     Other devices have suggested condensation control utilizing dew point calculation. Monitoring air temperature, relative humidity and surface temperatures to initiate condensation control events has been utilized in a variety of applications including refrigeration (see, for example, U.S. Patent Publication No. US 2004/0050072 A1 and U.S. Pat. Nos. 6,470,696, 5,778,689, 5,778,147, and 4,127,765). Some such devices or systems have utilized temperature sensing of both cold surfaces and the surrounding ambient air in condensation control response calculations. As is known, when a cold surface temperature is equal to or lower than the dew point of the ambient air, condensation forms on the cold surface.  
         [0006]     Such devices and systems have not proven altogether successful, however, due to inaccuracy of temperature readings, particularly where both cold and warm environments are adjacent one another such as is found at commercial freezer/refrigeration display cases. Failure to configure such systems to enhance accuracy of readings has resulted in erratic condensation control, on some occasions wasting energy unduly heating frames/doors, on other occasions responding late or otherwise inadequately to condensation formation, and/or on still other occasions failing to control appropriate heater cycling (i.e., shut-off) with consequent loss of efficiencies. Some such systems, utilized in fields not related to the problem of condensation at freezer doors, have addressed the problem of erratic temperature readings by artificially cooling the temperature sensors while heaters are powered on to counter the affect. This approach, however, is not feasible in large commercial freezer display installations where size and space are limited and where the cost for such counter measures are not easily absorbed (and not likely to be tolerated). Finally, most heretofore known systems employing dew point sensors for condensation control remain reactive, not proactively based on anticipation of condensation events. Further improvement could thus still be utilized  
       SUMMARY OF THE INVENTION  
       [0007]     The purpose of the apparatus, systems and methods of this invention is to efficiently eliminate door and frame condensation at widely utilized commercial freezer/refrigeration display cases. The invention is proactive in preventing condensation from forming by activating frame/door heaters before the dew point temperature at frame and door surfaces is reached. Power is then cycled off at a set point above dew point to save energy. This is accomplished by configuring apparatus of this invention to provide highly accurate sensing of frame temperature and dew point by thermally isolating temperature sensors and constantly updating data to reliably provide the ability to accurately anticipate condensation at case surfaces. The invention eliminates door damage and runoff safety hazards due to condensation build-up, efficiently cycles duty time of frame/door heaters thus lowering the installation&#39;s energy costs, and requires no expensive countermeasures to maintain accuracy and efficacious performance.  
         [0008]     The condensation control apparatus of this invention includes a frame temperature sensing unit adapted for monitoring temperature of either the door or case of a display case. The frame temperature sensing has temperature sensor mounted on a carrier. A dew point sensing unit is adapted for monitoring ambient air temperature and relative humidity outside of the display case and includes a carrier having at least one dew point sensing component thereat.  
         [0009]     A control unit is connected with the sensing units and includes a processor connected for activating the display case frame/door heater when monitored display case door or frame temperature drops below a preselected set point above a dew point value derived from monitored ambient temperature and relative humidity. The processor also deactivates the frame/door heater when monitored display case door or frame temperature rises above a second preselected set point above the dew point value. Housing for locating the sensing units includes structure establishing thermal isolation of the sensing units by minimizing heat transfer contacts with the housing. The temperature sensor and the dew point sensing component are located amid insulating air pockets formed by the structure of the housing. The housing and control unit are mountable at the display case.  
         [0010]     The system of this invention further includes the sensing and control apparatus located at each display case in an array of display cases. An internal case temperature sensing unit is positionable inside the display case and connected with the control unit processor. The processor stores data related to readings at the sensing units and operation of the frame/door heater, and a communication control unit associated with the processor enables coordination of programming and data download access to the processor.  
         [0011]     The method of this invention is for condensation control at a refrigerated display case having at least one door, a frame and at least one frame/door heater. The steps of the method include placing a temperature sensing unit in contact with the display case frame to monitor case frame temperature and mounting a dew point sensing unit on the display case to monitor ambient air temperature and relative humidity outside the display case. The monitored case frame temperature, ambient air temperature and relative humidity are utilized to anticipate formation of condensation at the display case and activate and deactivate the frame/door heater responsive thereto.  
         [0012]     It is therefore an object of this invention to provide apparatus, systems and methods for efficient elimination of door and frame condensation at widely utilized commercial freezer/refrigeration display cases.  
         [0013]     It is another object of this invention to provide apparatus, systems and methods for elimination of door and frame condensation at freezer/refrigeration display cases that proactively prevents condensation from forming by activating frame/door heaters before dew point temperature at frame and door surfaces is reached.  
         [0014]     It is still another object of this invention to provide apparatus for efficient elimination of door and frame condensation at freezer/refrigeration display cases that are configured to accurately sense frame temperature and dew point by thermally isolating temperature sensors and that are constantly data updated to reliably provide the ability to accurately anticipate condensation at case surfaces.  
         [0015]     It is yet another object of this invention to provide apparatus, systems and methods utilized with freezer/refrigeration display cases that eliminate door damage and runoff safety hazards due to condensation build-up, efficiently cycle duty time of frame/door heaters thus lowering the installation&#39;s energy costs, and require no expensive countermeasures to maintain accuracy and performance of the apparatus and systems.  
         [0016]     It is another object of this invention to provide condensation control apparatus for a refrigerated display case having at least one door, a frame and at least one frame/door heater, the apparatus including a frame temperature sensing unit adapted for monitoring temperature of one of the display case door or display case frame and including a carrier having a temperature sensor thereat, a dew point sensing unit adapted for monitoring ambient air temperature and relative humidity outside of the display case and including a carrier having at least one dew point sensing component thereat, a control unit connected with the sensing units and including processing means connected for activating the display case frame/door heater when monitored display case door or frame temperature drops below a preselected set point above a dew point value derived from monitored ambient temperature and relative humidity, and housing means for locating the sensing units including structure establishing thermal isolation of the sensing units by minimizing heat transfer contacts between the sensing units and the housing means and locating the temperature sensor and the at least one dew point sensing component amid insulating air pockets formed by the structure of the housing means.  
         [0017]     It is still another object of this invention to provide a system for condensation control at an array of refrigerated display cases, the cases each having at least one door, a frame and at least one frame/door heater, the system including sensing and control apparatus configured for location at each display case in the array of display cases, the apparatus including a frame temperature sensing unit adapted for mounting at the display case for monitoring temperature of one of the display case door or display case frame, a dew point sensing unit adapted for mounting at the display case for monitoring ambient air temperature and relative humidity outside of the display case, an internal case temperature sensing unit positionable inside the display case, and processing means mountable at the display case and connected for activating the display case frame/door heater when monitored display case door or frame temperature drops below a first preselected set point above a dew point value derived from monitored ambient temperature and relative humidity, for deactivating the frame/door heater when monitored display case door or frame temperature rises above a second preselected set point above the dew point value, and for storing data related to readings at the sensing units and operation of the frame/door heater, and a communication control unit associated with the processing means for coordinating programming and data download access to the processing means of the apparatus.  
         [0018]     It is yet another object of this invention to provide a method for condensation control at a refrigerated display case having at least one door, a frame and at least one frame/door heater, the method including the steps of placing a temperature sensing unit in contact with the display case frame to monitor case frame temperature, mounting a dew point sensing unit on the display case to monitor ambient air temperature and relative humidity outside the display case, and utilizing monitored case frame temperature, ambient air temperature and relative humidity to anticipate formation of condensation at the display case and activating and deactivating the frame/door heater responsive thereto.  
         [0019]     With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, and arrangement of parts and method substantially as hereinafter described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiment of the herein disclosed invention are meant to be included as come within the scope of the claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The accompanying drawings illustrate a complete embodiment of the invention according to the best mode so far devised for the practical application of the principles thereof, and in which:  
         [0021]      FIG. 1  is a diagrammatic illustration of one embodiment of the condensation prevention apparatus and system of this invention;  
         [0022]      FIG. 2  is an exploded view of the local controller unit of the apparatus of this invention;  
         [0023]      FIG. 3  is a perspective view of the case temperature sensor and housing of the apparatus of this invention;  
         [0024]      FIG. 4  is an exploded view of the sensor/housing of  FIG. 3 ;  
         [0025]      FIG. 5  is a sectional view of the sensor/housing of  FIG. 3 ;  
         [0026]      FIG. 6  is an illustration showing positioning of the sensor/housing of  FIG. 3 ;  
         [0027]      FIG. 7  is an exploded view of the dew point sensor and housing of the apparatus of this invention;  
         [0028]      FIG. 8  is a reverse exploded view of the sensor/housing of  FIG. 7 ;  
         [0029]      FIG. 9  is an illustration showing positioning of the sensor/housing of  FIG. 7 ;  
         [0030]      FIG. 10  is an exploded view of an alternative locational, housing and mounting configuration for the sensors/housings illustrated in  FIGS. 3 and 7 ;  
         [0031]      FIG. 11  is a sectional illustration of the sensors/housing of  FIG. 10  in a first mounting;  
         [0032]      FIG. 12  is an illustration showing positioning of the sensors/housing of  FIG. 10  in an alternative mounting;  
         [0033]      FIG. 13  is an electronics block diagram of the apparatus of this invention;  
         [0034]      FIGS. 14   a  through  14   j  are flow diagrams illustrating program control of local controller components and communications modules of the apparatus of this invention; and  
         [0035]      FIGS. 15   a  through  15   q  are flow diagrams illustrating program control at centralized processing (an on- or off-site personal computer for example). 
     
    
     DESCRIPTION OF THE INVENTION  
       [0036]     The system of this invention is illustrated in  FIG. 1  in association with a common commercial freezer/cooler display unit  21  including a display container  23  having a frame (not shown in this FIGURE) and doors  25  in frame  27  (hereinafter, refrigeration case(s)). Up to fifteen such cases  21  can be served by the system as currently configured (though greater or fewer cases could be accommodated). Each case  21  is provided with a local controller unit  31  connected with an array of case sensors including internal case temperature sensor  33 , external case frame temperature sensor  35  and dew point sensor  37 . The array of sensors may be hard wired (as shown herein) to unit  31  at each case  21  through frame members, or may be configured to function wirelessly utilizing known technologies. While separate frame temperature and dew point sensors  35  and  37  are shown in this FIGURE, an alternate configuration that combines both sensors into one housing is shown hereinafter. Sensor mounting positions may be selected as desired, though the configuration illustrated in  FIG. 1  is preferred where the sensors are separately housed.  
         [0037]     Controller unit  31  is illustrated in  FIG. 2 . AC power connector  41 , switching relay  43 , transformer  45 , internal case temperature sensor connection (an RJ-22 connector for example)  47 , frame temperature sensor and dew point sensor connectors (RJ-11 type connectors, for example)  49  are located on circuit board  51 . Microprocessor  53  is located on board  51 , as are in-coming and out-going connectors  55  (RJ-45 type connectors, for example) utilized so that a number of serially connected control units  31  are connectable with a single communication board/transmitter/receiver  57  in communication with receiver/transmitter/centralized processing  59  ( FIG. 1 ). LED&#39;s  65  and  67  (preferably green and red, respectively) on board  51  flash to indicate a status update from central processing  59  to microprocessor  53  (LED  65 ) and to indicate that relay  43  has switched power on to frame/door heaters (LED  67 ). Reset/programming interface  71  is mounted between LED&#39;s  65 / 67  at board  51 .  
         [0038]     Board  51  is maintained in housing  75  including first and second housing portion  77  and  79 , respectively, with all connectors, LED&#39;s and user interfaces provided with access openings therethrough. Housing  75  is configured to be mounted between freezer door frame mullions where case lighting is typically located (thus providing AC power routing access). The housing is designed to be less than the height and width of a typical heretofore known and utilized mullion lighting lens cover adapted for such cases  21 .  
         [0039]     Controller unit microprocessor  53  can be calibrated to initiate operations anywhere above or below calculated dew point. Adjustments can be made from central processing  59  with wireless communication via the communications board/transmitter/receiver  57  connected to controller unit  31 . Central processing  59  (a PC for example) receives data from units  31  via communications board/transmitter/receiver  57 , including data enabling tracking, storing and making available reports of the history of ambient air temperature and humidity from dew point sensor  37 , case frame and internal case temperatures from sensors  35  and  33 , respectively, and power consumption of door heaters (all as shown herein in  FIG. 15 ). This information can be used to improve energy efficiency by recalibrating operating ranges of the apparatus and system of this invention, as well as for improved operational cost accounting.  
         [0040]     The internal case temperature must remain below FDA approved levels or food will spoil, such being both a health concern as well as an expense to the installation for potential loss of goods. It has heretofore been difficult to determine if temperature are within guidelines 24 hours a day. Utilization of the apparatus and system of this invention, however, and particularly utilization of internal case temperature monitoring by sensor  33  through controller unit  31 , allows such detailed tracking and reporting. Moreover, the apparatus and system are configured to automatically send an alarm (one or both locally and by autodialer to a responsible technician) if maximum allowable internal case temperature as programmed is exceeded. If central processing  59  receives an alarm message from any of the local control units  31  in the system, an alarm or auto dial via a modem is initiated and a message is sent with regard thereto, including the exact case in the system wherein temperature has deviated (see  FIGS. 14 and 15 ).  
         [0041]     Temperature sensors  33  and  35  are similar and are housed as illustrated in  FIGS. 3 through 6  (for purposes of description herein, only the housing of sensor  35  will be described since sensor  33  in much simpler to implement and maintain). Temperature sensor  35  housing  83  includes cover  85 , sensor locating base  87  and circuit board/carrier  89  locating temperature sensor  35  including temperature sensing chip  91  at one surface thereof. This assembly is mounted to case frame unit or door frame unit  93  of display case  21  utilizing screws or adhesive, with base  87  adjacent frame unit  93  as shown in  FIG. 5 . Cabling from controller unit  31  is routed through cable protector  95  at base  87 .  
         [0042]     Temperature sensing chip  91  is located on board/carrier  89  so that when received in housing  83  it is located through sensor opening  99  at base  87 . With proper materials sizing, chip  91  will be in direct contact with the surface of unit  93  being monitored when base  87  is mounted on frame unit  93 . Housing  83  should be located at the coldest spot on the exterior of the freezer. That spot is usually in the center of the case between the bottom of a door  25  and door or case frame unit  93  (see  FIG. 6 ). Door and frame heaters  103  are located inside doors and/or frames of cases  21 .  
         [0043]     Housing  83  is configured at cover  85  and base  87  to thermally isolate sensing chip  91  from the warming effects of ambient air temperature in the vicinity of the chip and transfer thereof by the housing to the chip. Air space pockets  105  and  107  above and around chip  91 , respectively, and limited housing contact (see  FIG. 5  wherein only a single locating post  109  maintains board/carrier  89  at it position in housing  83 ) isolate chip  91  from the housing. By thus configuring board/carrier  89  (made typically of FIBERGLASS material) and housing  83  (preferably made of plastic such as acrylonitrile butadiene styrene polycarbonate) little transfer of heat to sensor chip  91  occurs, assuring accurate measurement of surface temperature of unit  93  unaffected by inaccuracies introduced as artifact into the measurement by ambient air temperature conditions in the vicinity of housing  83  (accuracy of frame temperature measurements has been documented by testing to be within 1 or 2 degrees Centigrade).  
         [0044]     Air space pocket  105  is created by the depth of cover  85  adjacent board/carrier  89 , the cover also insulating the sensor chip from the ambient. Air space pocket  107  is created by opening  99  having an opening area greater than surface area of chip  91 . These improvements allow accurate tracking and monitoring of case frame temperature, used not only for precisely timed heater activation, but also for better timing of heater deactivation given the fast rise in frame temperature after frame/door heaters  103  are activated to prevent condensation.  
         [0045]     Turning now to  FIGS. 7 through 9 , dew point sensor  37  is housed in housing  115  including cover  117  and mounting base  119 . Circuit board/carrier  121  maintains sensor  37  circuitry including dew point sensing chip  123 . Dew point sensing chip  123  is located, when assembled, in opening  129  in the face of cover  117 , opening  129  including protective shield  131  having gaps  133  along the sides thereof to allow airflow over sensing chip  123  for measuring humidity and ambient air temperature. A membrane  141  covers opening  143  at sensing chip  123  to protect internal humidity and temperature sensing components from foreign matter such as cleansers and the like. The design of cover  117  protects membrane  141  without preventing airflow to sensing chip  123 .  
         [0046]     Housing base  119  includes cabling routing protector  145  for introducing and stabilizing cabling to sensor  37 . Spacer ribs  147  at base  119  assure an insulating air gap between housing  115  and its adjacent mounting surface. Housing  115  is preferably mounted to door or case frame unit  151  located at the top of case  21 , as shown in  FIG. 9 , and between the frame unit and door  25  so that the cold air exiting the case when a door is opened will not affect the sensor readings.  
         [0047]     Housing  115  is configured to thermally isolate dew point sensor  37  so that accuracy of the dew point sensor is maintained within to 1 to 3 degrees Centigrade. First, by assuring that sensor  37  is held away from the unnaturally heated frame (either cooled by the freezer or heated by frame heaters) with spacer ribs  147 , the frame temperature will not influence the relevant dew point readings (primarily ambient air temperature). The air pocket ( 155  in  FIG. 9 ) behind board/carrier  121  and extending all the way to the mounting surface at unit  151  prevents thermal transfer through housing  115  from the mounting surface of frame unit  151 . The cover includes openings  157  at opposite ends thereof to provide room temperature airflow at air pocket  155  of room temperature air. Housing mounting surface  159  is perpendicular to the dew point sensor board/carrier  121 , and may be mounted by screws or (preferably) adhesive.  
         [0048]     By thus configuring board/carrier  121  (made typically of FIBERGLASS material) and housing  115  (preferably made of plastic such as acrylonitrile butadiene styrene polycarbonate) little transfer of heat to sensor chip  123  occurs, assuring accurate measurement of ambient air temperature and humidity unaffected by inaccuracies introduced as artifact into the measurement by freezer case  21  frame temperature conditions in the vicinity of housing  115 . Housing  115  is configured utilizing board support protrusions  163  defining retainer pockets  165  having minimal contact surface along the outer edges of board/carrier  121  to maintain board positioning, thus further reducing thermal transfer. Depth of housing  115  behind pockets  165  establishes air pocket  155 .  
         [0049]     When door and frame heaters are activated it is important that they do not affect the reading from sensor  37  of actual room air temperature. This design sufficiently isolates sensor  37  from door and frame temperature swings, thereby preserving accuracy of readings of ambient air temperature and thus accuracy of dew point calculation. Absent such thermal isolation, it is possible for shut-off of heaters to be delayed indefinitely. For example, under the influence of heaters, if temperature readings were to rise at an insufficiently isolated dew point sensor then calculated dew point would increase. In such case, the doors would have to be heated further as the dew point is chased. Heating the doors further would raise the dew point still more in this scenario, thereby beginning an upwardly spiraling cycle until the heaters reach their maximum temperature and finally power off.  
         [0050]     Turning now to  FIGS. 10 through 12 , an alternative embodiment for sensors  35  and  37  housing is illustrated wherein housing  115  is utilized to house both sensors. Thus all elements of the housing as heretofore described are the same unless otherwise specified hereinbelow. Sensor  35  is maintained at cabling routing protector  145  at retainer pockets  175  (one on each side, though only one side is shown in the drawings) defining ledges having minimal contact surface with board/carrier  89  to maintain the board, thus reducing thermal transfer through the housing to the board. A single locating post  177  may be utilized to further stabilize the board if necessary. As shown in  FIG. 11 , sensor  91  and board/carrier  89  are completely surrounded by an insulating dead air pocket established by the depth of protector  145 . This housing may be mounted either above ( FIG. 11 ) or below ( FIG. 12 ) door  25  of a case  21 , and retains all the thermal isolation advantages discussed for the individually housed units as discussed above.  
         [0051]      FIG. 13  illustrates the electronics utilized by the apparatus of this invention. Incoming AC power is provided to relay  43  and is transformed at input/transformer  41 / 45  (12 volt DC output) for use by controller unit  31  and sensors  33 ,  35  and  37 . Sensors  33  and  35  inputs are A/D converted and input to the central processing unit microprocessor  53  embodying the various comparitors and look up tables utilized (Sensor  37  is A/D converted at the sensor). LED&#39;s  65 / 67  are controlled in response to microprocessor functions, and microprocessor programming can be conducted on-site through connector/interface  71 . Communications flow between microprocessor  53  and central processing  59  is monitored and controlled by communications board  57  in cooperation with microprocessor  53  and central processing  59 . Relay activity is monitored and controlled by microprocessor  53  in response to set point achievement as specified in software and as sensed by the sensor array.  
         [0052]     Thermal isolation of sensors  35  and  37  thus insure the accuracy of the sensors and thus the efficacy of the apparatus both in terms of its ability to anticipate condensation formation as well as its ability to save on energy expenses due to overheating of doors and frames or improper cycling of heaters. Overall functioning of the apparatus and system of this invention are as illustrated in  FIGS. 14   a  through  14   j  and  15   a  through  15   q . Heaters are activated just above dew point and thus just before moisture appears on freezer surfaces. Controller unit  31  may utilize any known sensor(s)  37  for monitoring ambient air temperature (for example, thermistors may be utilized for the temperature sensors herein, including by sensors  33  and  35 ) and relative humidity (capacitive polymer or thin film type sensor, for example, may be utilized wherein capacitance is proportional to the relative humidity of surrounding air). Combination sensors providing relatively accurate dew point sensing are known and commercially available. Microprocessor  53  looks up the dew point value from a stored table. Input from monitored dew point sensor  37  and frame temperature sensor  35  are utilized in conjunction therewith to determine when to relay on the frame/door heaters as the frame temperature approaches the dew point (once the monitored frame temperature drops below a preselected set point above the dew point). Once the monitored frame temperature rises to a preselected set point above the dew point, the heaters are relayed off. On-board software builds a table of over 600 values into the dew point lookup table, set points, as well as other values, being updatable in real time from central processing. The look up table can be reprogrammed through interface  71 .  
         [0053]     The entire system preferably includes wireless communication for receiving data from multiple control units  31  at different cases  21 . Controller unit  31  and communications board/transmitter/receiver  59  transfer data to central processing  59  for data viewing, tracking and report preparation. Central processing  59  is capable of printing efficiency reports, remote recalibration of heater operations and dial up warning programs should any freezer have a failure. Each controller unit  31  tracks internal case temperatures and will warn of unsafe temperatures or freezer failure. Unit/system calibration or recalibration is typically only required when sensors might be influenced by freezer temperatures. Freezer temperatures primarily influence sensor readings dependent on where sensors are mounted at installation. The case frame is not as cold at the top of the freezer as the bottom, so the operating range of the sensors and unit  31  in the combined sensor housing configuration of  FIG. 10  are calibrated for the alternative locations.