Patent Publication Number: US-8117848-B2

Title: Temperature calibration device having reconfigurable heating/cooling modules to provide wide temperature range

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/453,713 filed Jun. 14, 2006, U.S. Pat. No. 7,607,309. This application is incorporated by reference herein in its entirety and for all purposes. 
    
    
     TECHNICAL FIELD 
     This invention relates to electrically powered devices, and, more particularly, to temperature calibration devices using Peltier cells to provide heating and cooling. 
     BACKGROUND OF THE INVENTION 
     A wide variety of electrically powered heating devices are in existence to provide a wide variety of functions. For example, temperature calibration devices, known as dry well calibrators, are commonly used in industry to calibrate precision temperature probes. 
     Conventional dry well calibrators use thermoelectric heating/cooling modules generally containing Peltier cells to heat or cool the calibration probes to temperatures that can be set by a user. Electrical power having one polarity is applied between the first and second substrates of the Peltier cells to cause the temperature of the first substrate to rise relative to the temperature of the second substrate, thereby heating the temperature probe being calibrated. Electrical power having the opposite polarity causes the temperature of the first substrate to fall relative to the temperature of the second substrate, thereby cooling the temperature probe being calibrated. 
     Peltier cells used in dry well calibrators are usually stacked on top of each other to provide heating and cooling over a range of temperatures that is wider than the temperature differential of each cell. The total temperature differential of a heating/cooling module is substantially equal to the sum of the temperature differentials that can be developed across all of the stacked Peltier cells. The temperature differential that can be developed between the substrates of each Peltier cell is limited to a specified maximum temperature. Therefore, the limiting factor in the operating range of a dry well calibrator is the maximum specified temperature differential of the Peltier cells used in the dry well calibrator. This limiting effect on the operating range of dry well calibrators is exacerbated by the unequal heating of the Peltier cells. Specifically, the temperature differential of Peltier cells in the outside of a stack tend to be greater than the temperature differential of cells that are located toward the inside of the stack. To limit the temperature differential of the cells at the outside of the stack to the specified maximum temperature differential, the other cells in the stack are usually well below the maximum specified temperature differential. Therefore, the maximum operating range of dry well calibrators is typically much smaller than the maximum range that would be possible if all of the Peltier cells in a stack had the same temperature differential. 
     The need for dry well calibrators to operate over wide temperature ranges frequently requires that the Peltier cells used in the calibrators be operated at or near their maximum specified temperature differential. Unfortunately, operation of the Peltier cells at or near their maximum specified temperature differentials can severely limit the useful life of the cells. Frequent replacement of the Peltier cells can be very expensive, not only because of the cost of the cells, but also because of the cost of labor required to disassemble dry well calibrators to replace the cells and the downtime cost during such replacement. As a result, there is an inevitable tradeoff between achieving a wide operating range for dry well calibrators and achieving reliable performance. 
     There is therefore a need for a dry well calibrator using Peltier cells that can operate over a wide range of temperatures without unduly limiting the useful life of the Peltier cells. 
     SUMMARY OF THE INVENTION 
     A temperature calibration device includes a block of thermally conductive material that is placed in thermal communication with a device to be calibrated. The block is in thermal contact with a plurality of Peltier cells that are connected to a configurable connection device, which may be a relay. The configurable connection device connects the Peltier cells to each other in a first configuration responsive to a first control signal, and it connects the Peltier cells to each other in a second configuration that is different from the first configuration responsive to a second control signal. A power supply applies to the Peltier cells a first voltage responsive to the first control signal and a second voltage responsive to the second control signal. The second voltage has a polarity that is different from the polarity of the first voltage, and it may also have a magnitude that is different from the magnitude of the first voltage. A control circuit is used to generate the first and second control signals. The control circuit generates the first control signal when the device to be calibrated is to be cooled, and it generates the second control signal when the device to be calibrated is to be heated. Therefore, the Peltier cells are connected to each other in different configurations for heating and cooling, and the cells may receive voltages having different magnitudes for heating and cooling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded isometric view of some of the internal components of a temperature calibration device according to one example of the invention. 
         FIG. 2  is a cross-sectional view of the internal components of the temperature calibration device shown in  FIG. 1 . 
         FIG. 3  is an exploded isometric view of a case surrounding the internal components of the temperature calibration device shown in  FIG. 1 . 
         FIG. 4  is a front elevational view of the temperature calibration device of  FIG. 1 . 
         FIG. 5  is a block diagram of a system for driving Peltier cells in the temperature calibration device of  FIGS. 1-4  according to one example of the invention. 
         FIG. 6  is a block diagram showing the manner in which the system of  FIG. 5  connects the Peltier cells to each other when the temperature calibration device is to be used to cool a device to be calibrated. 
         FIG. 7  is a block diagram showing the manner in which the system of  FIG. 5  connects the Peltier cells to each other when the temperature calibration device is to be used to heat a device to be calibrated. 
         FIG. 8  is a block diagram showing the manner in which Peltier cells are connected to each in a prior art temperature calibration device. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention are directed to a system and method for allowing a dry well calibrator to operate over a wide range of temperatures without adversely affecting the service life of Peltier cells used in the calibration device. Certain details are set forth below to provide a sufficient understanding of the invention. However, it will be clear to one skilled in the art that the invention may be practiced without these particular details. In other instances, well-known circuits, control signals, and timing protocols have not been shown in detail in order to avoid unnecessarily obscuring the invention. 
     The internal components of a heating block assembly for a typical dry well calibrator  10  are shown in  FIG. 1 . The dry well calibrator  10  includes a cylindrical insert  14  having one or more cylindrical bores  16   a,b,c  sized to receive temperature probes “P” having corresponding dimensions. The insert  14  is typically manufactured from a thermally conductive metal. The insert  14  fits into a cylindrical bore  18  formed in a heated/cooled block  20  of a suitable material, such as a metal with good thermal conduction properties. The block  20  has a configuration that is rectangular in both vertical and horizontal cross-section. The inside diameter of the bore  18  is only slightly larger than the outside diameter of the insert  14  to ensure good heat conduction from the block  20  to the insert  14 . 
     With reference also to  FIG. 2 , a pair of upper thermoelectric heating/cooling modules  30 ,  32  and a pair of lower thermoelectric heating/cooling modules  36 ,  38  are bonded to opposite surfaces of the block  20 . Each of the thermoelectric heating/cooling modules  30 - 38  includes a first Peltier cell  40  having an inner substrate  42  ( FIG. 1 ) bonded to the block  20 . A second Peltier cell  44  has an inner substrate  46  ( FIG. 1 ) that is bonded to an outer substrate  48  ( FIG. 1 ) of the first cell  40 . Temperature conductive plates  50  are bonded to outer substrates  54  ( FIG. 1 ) of the second cells  44 . A pair of Peltier cells  60 ,  62  each having inner and outer substrates  66 ,  68 , respectively, ( FIG. 1 ) have their inner substrates  66  bonded to an outer surface of the plates  50 . The Peltier cells  60 ,  62  are positioned so that their abutting edges overlie the centers of the first and second Peltier cells  40 ,  44 . Finally, conductive leads (not shown) supply electrical power to the Peltier cells  40 ,  44 ,  60 ,  62 . As is well-known in the art, electrical power having one polarity causes the temperature of the inner substrates to rise relative to the temperature of the outer substrates thereby heating the block  20 . Electrical power having the opposite polarity causes the temperature of the inner substrates to fall relative to the temperature of the outer substrates, thereby cooling the block  20 . When the Peltier cells  40 ,  44 ,  60 ,  62  are used for either heating or cooling, the resulting temperature changes imparted to the outer surfaces  68  of the Peltier cells are moderated by heat sinks  74  abutting the outer substrates  68  ( FIG. 1 ) of the cells  60 ,  62 . 
     With reference also to  FIG. 3 , the above-described components of the dry well calibrator  10  are surrounded by an outer case  80  formed by case sections  80   a,b,c,d . The case section  80   d  contains control circuitry  82  that is connected to the Peltier cells  40 ,  44 ,  60 ,  62  for controlling the supply of power to the cells. Two fan assembly modules  84  containing a fan  86  are positioned inside the case section  80   a  so that the fan  86  is behind a grill  88 . The case  80  is separated from the heat sinks  74  by a space, and the fan  86  provides airflow through this space to remove heat from or supply heat to the heat sinks  74 . 
     As best shown in  FIG. 4 , a keypad  90  mounted on a panel  92  of the case section  80   a  is connected to the control circuitry  82  in the case section  80   d  ( FIG. 3 ) to control the operation of the dry well calibrator  10 . A display  94 , which is also connected to the control circuitry  82  in the case section  80   d  ( FIG. 3 ), provides information about the operation of the dry well calibrator  10 , such as the temperature of the block  20 . 
     In operation, the keypad  90  ( FIG. 4 ) is used to set the temperature of the block  20  as well as the rate at which the temperature of the block  20  is changed to reach the set temperature. If the temperature set by the keypad  90  is for a temperature above ambient temperature, power having a first polarity is applied to wires that are connected to the Peltier cells  40 ,  44 ,  60 ,  62 , thereby causing the cells to cool the block  20 . If the temperature set by the keypad  90  is for a temperature below ambient temperature, power having a first polarity is applied to wires that are connected to the Peltier cells  40 ,  44 ,  60 ,  62  to cause the cells to cool the block  20 . Once the temperature of the block  20  has stabilized, the temperature probe P ( FIG. 1 ) is inserted into a corresponding sized bore  16  of the insert  14 . The probe P is then calibrated by ensuring that a readout device (not shown) connected to the probe P indicates the temperature of the probe P is equal to the set temperature of the dry well calibrator  10 . 
     As explained above, the operating range of the dry well calibrator  10  is limited by the maximum specified temperature differentials of the Peltier cells  40 ,  44 ,  60 ,  62  and the unequal heating of the Peltier cells  40 ,  44 ,  60 ,  62 . Balancing the temperature differentials of the Peltier cells  40 ,  44 ,  60 ,  62  allows the dry well calibrator  10  to operate over a wide temperature range without the temperature differential of any of the cells  40 ,  44 ,  60 ,  62  approaching the maximum specified temperature differential. It has been discovered that the temperature differentials of the Peltier cells  40 ,  44 ,  60 ,  62  can be equalized by driving the cells  40 ,  44 ,  60 ,  62  differently for cooling purposes than they are driven for heating purposes. In particular, the excessive temperature differential of the center Peltier cells  44  compared to the temperature differential of the other cells  40 ,  60 ,  62  is more of a problem when the Peltier cells  40 ,  44 ,  60 ,  62  are used for heating the block  20  than it is when they are used for cooling the block  20 . 
     One embodiment of a system  100  for driving the Peltier cells  40 ,  44 ,  60 ,  62  in the upper heating/cooling modules  30 ,  32  in a more balanced manner is shown in  FIG. 5 . A second system that is identical to the system  100  is used for driving the Peltier cells  40 ,  44 ,  60 ,  62  in the bottom heating/cooling modules  36 ,  38 . The system  100  includes a relay driver  110  that receives a control signal H/C* signal from the control circuitry  82  ( FIG. 3 ). Also included in the system are a relay  120  for reconfiguring the connections between the Peltier cells  44 ,  60 ,  62 , and a relay  130  for applying voltages from a power supply  140  to the Peltier cells  40 ,  44 ,  60 ,  62 . More specifically, the relay  130  applies a cooling voltage of +48 volts to the Peltier cells  44 ,  60 ,  62  and a cooling voltage of +6 volts to the Peltier cells  40 . The relay  130  applies a heating voltage of −24 volts to the Peltier cells  44 ,  60 ,  62  and a heating voltage of −12 volts to the Peltier cells  40 . The relays  120 ,  130  are both driven by a signal from the relay driver  110 . 
     When the control circuitry  82  applies a low H/C* signal to the relay driver  110  to cool the block  20 , the relay  120  connects the Peltier cells  44 ,  60 ,  62  as shown in  FIG. 6 . In this configuration, the middle Peltier cells  44  are connected in parallel with each other, and this parallel combination is connected in series with the Peltier cells  60 ,  62 . When the H/C* signal is low, the relay  130  applies +48 volts to this combination of the Peltier cells  44 ,  60 ,  62 , and it applies +6 volts to the series combination of the inner Peltier cells  40 . 
     When the control circuitry  82  applies a high H/C* signal to the relay driver  100  to heat the block, the relay connects the Peltier cells  44 ,  60 ,  62  as shown in  FIG. 7 . In this configuration, the middle Peltier cells  44  are connected in series with the Peltier cells  60 ,  62 . When the H/C* signal is high, the relay  130  applies −24 volts to this combination of the Peltier cells  44 ,  60 ,  62 , and it applies −6 volts to the series combination of the inner Peltier cells  40 . 
     Assuming each of the Peltier cells  40 ,  44 ,  60 ,  62  have a resistance of R, the total current drawn by the Peltier cells  44 ,  60 ,  62  when they are configured for cooling as shown in  FIG. 6  is +48/4.5R, which is equal to +10.67/R. Therefore, the current drawn by the Peltier cells  44  is half that current, or +5.33/R. The total current drawn by the Peltier cells  40  is +6/2R, which is equal to +3/R. In this cooling configuration, the current through and the power dissipated by each of the Peltier cells  40 ,  44 ,  60 ,  62  is as shown in Table 1, below. The current drawn by the Peltier cells  44 ,  60 ,  62  when they are configured for heating as shown in  FIG. 7  is −24/6R, which is equal to −4/R. The current drawn by the Peltier cells  40  is −12/2R, which is equal to −6/R. The current through and the power dissipated by each of the Peltier cells  40 ,  44 ,  60 ,  62  is also shown in Table 1, below. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Heating 
                   
                   
                   
               
               
                 Cell 
                 Current 
                 Cooling Current 
                 Heating Power 
                 Cooling Power 
               
               
                   
               
             
            
               
                 40 
                 −6/R 
                    +3/R 
                 36/R 
                    9/R 
               
               
                 44 
                 −4/R 
                  +5.33/R 
                 16/R 
                  28.44/R 
               
               
                 60, 62 
                 −4/R 
                 +10.67/R 
                 16/R 
                 113.77/R 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 1, the Peltier cells  40  dissipate more power for heating than they do for cooling, but the Peltier cells  44  dissipate less power for heating than they do for cooling, and the Peltier cells  60 ,  62  dissipate much less power for heating than they do for cooling. Further, for cooling, the power dissipated by the Peltier cells  40 ,  44 ,  60 ,  62  increases from the inner Peltier cell  40  to the outer Peltier cells  60 ,  62 , but, for heating, decreases from the inner Peltier cell  40  to the outer Peltier cells  60 ,  62 . Therefore, in both heating and cooling, the power dissipated by the Peltier cells  40 ,  44 ,  60 ,  62  increases from the cooled surface to the heated surface. 
     In contrast, prior art dry well calibrators use Peltier cells that are connected to each other is shown in  FIG. 8 , in which the Peltier cells  40 ′,  44 ′,  60 ′,  62 ′ correspond to the Peltier cells  40 ,  44 ,  60 ,  62  shown in  FIGS. 1-7  for both the upper thermoelectric heating/cooling modules  30 ,  32  and the lower thermoelectric heating/cooling modules  36 ,  38 . In the prior art configuration of the Peltier cells  40 ′,  44 ′,  60 ′,  62 ′, the configuration of the Peltier cells  40 ′,  44 ′,  60 ′,  62 ′ is the same for both heating and cooling. Also, the ±96 volt power applied to the Peltier cells  40 ′,  44 ′,  60 ′,  62 ′ has the same magnitude for both heating and cooling so that the current through and power dissipated by the cells is the same for both heating and cooling. The current through the Peltier cells  40 ′,  44 ′, and  60 ′,  62 ′ is 2.74/R, 5.49/R and 10.07/R, respectively. Therefore, the current through and power dissipated by the Peltier cells  40 ′,  44 ′,  60 ′,  62 ′ in one prior art dry well calibrator is similar to the current through and power dissipated by the Peltier cells  40 ,  44 ,  60 ,  62  for cooling but quite different from the current through and power dissipated by the Peltier cells  40 ,  44 ,  60 ,  62  for heating. By allowing the Peltier cells  40 ,  44 ,  60 ,  62  to be reconfigured for heating and cooling and/or by applying voltage having different magnitudes to the Peltier cells  40 ,  44 ,  60 ,  62  for heating and cooling, the operating range of the dry well calibrator  10  can be maximized without operating the Peltier cells  40 ,  44 ,  60 ,  62  at or beyond their maximum specified temperature differential. As a result, the dry well calibrator  10  can operate over a large range of temperatures without sacrificing reliability. 
     Although the present invention has been described with reference to the disclosed embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. Such modifications are well within the skill of those ordinarily skilled in the art. Accordingly, the invention is not limited except as by the appended claims.