CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application entitled “Method and Apparatus for Refrigerant Recovery Unit Filter Dryer Maintenance,” filed Jun. 30, 2006, having Ser. No. 11/477,585, now U.S. Pat. No. 7,726,137, the disclosures of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to refrigerant recovery units maintenance. More particularly, the present invention relates to filter dryer maintenance for refrigerant recovery units. 
     BACKGROUND OF THE INVENTION 
     Filter dryers in refrigerant recovery units are used to trap acid, moisture, or particulates as the refrigerant passes through the filter dryers. These filter dryers, typically, have a maximum capacity to trap these contaminants. In order for the refrigerant recovery unit to function properly, these filter dryers need to be replaced periodically to ensure the refrigerant is cleaned upon the recovery/recycle process. Otherwise, continued use of the saturated filter dryers can be damaging to the air conditioning system. 
     Currently, a user is prompted to change the filter dryer when a predetermined amount of refrigerant has passed through the filter dryer, such as one-hundred and fifty pounds. However, the user may simply ignore the notification by pushing a button indicating that he has changed the filter dryer and continues to use the expired filter dryer for the recovery/recycle process. Furthermore, the counter will reset to zero and the notification will not be activated until another one-hundred and fifty pounds of refrigerant has passed through the filter dryer. Thus, the refrigerant passing through the filter dryer will not be cleaned properly upon the recovery/recycle process. 
     Accordingly, it is desirable to provide a method and apparatus to ensure that a user performs filter dryer maintenance for a refrigerant recovery unit when the filter dryer needs to be replaced. 
     SUMMARY OF THE INVENTION 
     The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect an apparatus is provided that in some embodiments a filter dryer maintenance is performed for a refrigerant recovery unit when a certain amount of refrigerant has passed through the filter dryer, thereby the refrigerant recovery unit can be performed at its optimal condition at all times. 
     In accordance with one embodiment of the present invention is a method for a user to change a filter dryer of a refrigerant recovery unit which can determine whether the filter dryer needs to be replaced, such as by determining an amount of refrigerant that has passed through the filter, or determining a condition of the filter dryer, such as if the filter dryer is saturated, or determining the condition of the refrigerant that has passed through the filter dryer. The method can generate an alert to change the filter dryer in response to the determination that a filter dryer needs to be replaced, for example if a predetermined amount of refrigerant is set, i.e. at 150 pounds. The method can disable the refrigerant recovery unit, such that the refrigerant recovery unit is unable to further recover/recycle the refrigerant. The method can further prepare the filter dryer to be removed, wherein the filter dryer is ready to be removed when pressure in the filter dryer reaches a first predetermined vacuum level, wherein the first predetermined vacuum level is greater than 5 inches of Mercury (Hg). The method can further determine whether the filter dryer has been removed within a preset time, can generate the alert again if the filter dryer is not replaced within the preset time, wherein the preset time is between 3-7 minutes. The method can further determine a second vacuum level during the preset time, and can allow the user to continue if the second vacuum level is less than a second predetermined vacuum level, wherein the second predetermined vacuum level is less than the first predetermined vacuum level, wherein the second predetermined vacuum level is less than or equal to 1 inch of Hg. The method can further install a second filter dryer, and prepare the second filter dryer to be used by removing air with a compressor, such that the pressure reaches a third predetermined vacuum level, wherein the third predetermined vacuum level is greater than 13 inches of Hg. The method can further reset a filter dryer counter to zero upon disconnecting the old filter dryer or confirming that the new filter dryer has been replaced, and can activate the refrigerant recovery unit to recover/recycle the refrigerant. 
     In accordance with another embodiment of the present invention is an apparatus for a user to change a filter dryer of a refrigerant recovery unit which comprises a refrigerant determining module that is configured to determine if the filter dryer needs to be replaced, an alert device that can notify the user to change the filter dryer in response to a determination that the filter dryer needs to be replaced, wherein the alert device can display warnings or reminders in response to the amount of refrigerant passing through the filter dryer, a pressure module that can regulate the pressure of the refrigerant recovery unit, wherein the pressure module removes refrigerant from the filter dryer, and a pressure sensing device that is configured to detect a change of pressure in response to the filter dryer being disconnected from the system, wherein the pressure sensing device enables the refrigerant recovery unit after the filter dryer maintenance. The refrigerant recovery unit is disabled from recovering refrigerant when the refrigerant determining module determines to replace the filter dryer. The filter dryer is determined to be replaced in response to the filter dryer being saturated, or a predetermined amount of refrigerant has passed through the filter dryer, or when the refrigerant is not being cleaned properly. The refrigerant recovery unit is activated in response to the change of pressure detected by the pressure-sensing device or a confirmation that a new filter dryer has been installed. 
     In accordance with yet another embodiment of the present invention is a system for a user to change a filter dryer of a refrigerant recovery unit which comprises a means which can determine if the filter dryer needs to be replaced, means which can generate an alert to change the filter dryer in response to a determination that the filter dryer needs to be replaced; means which can deactivate the refrigerant recovery unit; means which can prepare the filter dryer to be removed; means which can determine whether the filter dryer has been removed within a preset time, wherein the preset time is between 3-7 minutes; and means which can activate the refrigerant recovery machine in response to a confirmation that a second filter dryer being inserted. The system further comprises a means which can generate an alert to change the filter dryer if the filter dryer is not replaced within the preset time, means which can determine a vacuum level during the preset time, wherein the vacuum level is less than or equal to 1 inch of Mercury vacuum, means which can allow the user to continue if the vacuum level is less than a predetermined vacuum level, and means which can prepare the second filter dryer to be used. 
     There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a block diagram of a refrigerant recovery unit according to an embodiment of the invention. 
         FIG. 2  is a flow chart depicting a manner in which the refrigerant recovery unit illustrated in  FIG. 1  may be practiced. 
     
    
    
     DETAILED DESCRIPTION 
     An embodiment in accordance with the present invention provides an apparatus and method for a user to perform filter dryer maintenance of a refrigerant recovery unit when a filter dryer needs to be replaced. When the unit determines that the filter dryer needs to be replaced, the unit will be disabled from recovering/recycling more refrigerant. Upon being disabled, the unit would prompt the user through a filter dryer change procedure, which would remove refrigerant and create a vacuum in the filter dryer before it is disconnected from the unit. The vacuum in the filter dryer can be monitored by pressure sensing devices such as a pressure switch or transducer. When the user disconnects the filter dryer from the unit to replace it with another filter dryer, the pressure-sensing device would sense a pressure change, which would be a signal to reactivate the refrigerant recovery unit. 
     This patent will compare different vacuum levels using the terms ‘greater than” and “less than.” A “greater vacuum level” is intended to have the same meaning as “lesser absolute pressure.” Conversely, a “lesser vacuum level” has the same meaning as “greater absolute pressure.” All references to pressure use inch of Mercury (Hg) as the unit. 
     The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout.  FIG. 1  illustrates a block diagram of a refrigerant recovery unit  100  according to an embodiment of the invention. A microprocessor  110  having, at least, an internal memory  115  and a timer  150 . The microprocessor  110  is connected to a display  120 , a compressor  125 , a detector  130 , and a pressure-sensing device  135 . The compressor  125 , the detector  130 , and the pressure-sensing device  135  are further connected to a filter dryer connector  140 . The pressure sensing device  135  can be a pressure switch, a pressure transducer, or other pressure sensing module. 
     The filter dryer connector  140  is connected directly to a filter dryer  145 . The filter dryer  145  is used to trap acid, moisture, and particulates in the refrigerant. The detector  130  can determine the filter dryer  145  needs to be replaced. For example, the detector  130  can measure the amount of refrigerant that has passed through the filter dryer, or measure the weight of the filter dryer with the trapped contaminants, or determine whether the filter dryer is saturated, or determine the condition of the refrigerant that has passed through the filter dryer, or determine through other means and methods, whether the filter dryer  145  needs to be changed. 
     The detector  130  can send a signal to the microprocessor  110 , which then can display messages  155  on the display  120 . These can be messages that display the relative qualification of the remaining life of the filter dryer  145 , the current amount of refrigerant that the filter dryer has processed, reminders for the user to purchase a replacement filter dryer, warnings that filter dryer maintenance is approaching, or other messages. The display  120  can be a touch-screen display, which can serve as an input of the unit  100  and is in communication with the microprocessor  110 . In the alternative, the unit  100  can have separate input devices connecting to the microprocessor  110 . The abovementioned components are some of the main components that relate to the present invention. Other components of the refrigerant recovery units are known in the art. 
     In this embodiment, when the microprocessor  110 , through the detector  130 , determines that the filter dryer  145  needs to be replaced. One condition for the filter dryer  145  to be replace is if a predetermined amount of refrigerant has passed through the filter dryer  145 , for example one-hundred and fifty pounds. Other conditions can also be used to determine when the filter dryer  145  needs to be replaced, such as the saturation of the filter dryer or the condition of the refrigerant after passing through the recycling process. Once determined that the filter dryer needs to be replaced, the microprocessor  110  will notify the user of mandatory filter dryer maintenance and will disable the unit  100  by displaying a message  155  on the display  120 . The compressor  125  will remove refrigerant and air, thereby creating a vacuum on the filter dryer  145 , until the pressure sensing device  135  detects that the vacuum level is greater than five inches of Mercury (Hg). At this point, the timer  150  will begin and the user can change the filter dryer  145  within a preset time, for example 5 minutes. The unit  100  can also have other desired preset times, such as between 3-7 minutes. 
     If the user disconnects the filter dryer  145  during the preset time, the pressure-sensing device  135  will sense the change of pressure (i.e. the absolute pressure rises to above one inch of Hg vacuum or the vacuum level decreases to less than one inch of Hg) and that will be a signal for the microprocessor  110  to activate the unit  100 . If the user does not change the filter dryer  145  or if the pressure-sensing device  135  does not detect a rise in pressure within the preset time, the microprocessor  110  will signal the display  120  to display a mandatory filter dryer maintenance notice, and the process will continue until the user changes the filter dryer  145 . 
     Upon changing the filter dryer  145 , the compressor  125  will prepare the new filter dryer  145  by removing air from the new filter dryer  145  until the pressure sensing device  135  senses the vacuum level is greater than 13 inches of Hg. In addition, the detector  130  will reset to zero and the unit  10  is ready to recover/recycle refrigerant. 
     If the detector  130  detects that the predetermined amount of refrigerant has passed through the filter dryer  145  in the middle of a recovery/recycle cycle, the unit  100  will allow the user to finish the cycle before prompting the user to change the filter dryer. However, upon the end of this cycle, the microprocessor  110  will disable the unit  100  and force the user to change the expired filter dryer  145 . 
     Furthermore, the microprocessor  110  can also signal the display  120  to display warning messages  155  regarding filter dryer maintenance when twenty-five pounds, or any other desired increment, of refrigerant has passed through the filter dryer  145 . Therefore, the user will be notified well before the filter dryer  145  reaches the mandatory replacement weight. Hence, the user will not be surprised when the unit  100  is disabled due to the mandatory maintenance. 
       FIG. 2  is a flow chart  200  depicting a manner in which the refrigerant recovery unit  100  illustrated in  FIG. 1  may be practiced. The process begins at step  205  where the display  120  displays a main menu. Next, in step  210 , the detector  130  detects the amount of refrigerant (by weight) that has passed through the filter dryer  145 . If the amount is over one-hundred and fifty pounds, or another amount determined by the user or the manufacturer, the process continues to step  215 . At this point, the unit  100  is disabled from recovering/recycling additional refrigerant. 
     In step  215 , the filter weight is displayed on the display  120  and the user is prompted to change the filter dryer  145 . Next, in step  220 , the display  120  displays a message  155  giving the user an option to change the filter dryer  145 . If the user decides to change the filter dryer  145 , the process continues to step  225 . If the user decides to not change the filter dryer  145 , the process returns to step  215 . At this point, the unit  100  remains locked until the user changes the filter dryer  145 . 
     In step  225 , the compressor  125  clears refrigerant from the filter dryer  145 , thereby creating a vacuum in the filter dryer  145 . Next, in step  230 , the pressure-sensing device  135  detects the vacuum level in the filter dryer  145 , the compressor  125  continues to operate until the vacuum level is greater than five inches of Hg. This process continues to step  235 . 
     In step  235 , the compressor  125  stops, the timer  150  starts and a message  155  is displayed telling the user to replace the filter dryer  145 . Next, in step  240 , the user has five minutes, or another predetermined time set by the user or manufacturer, to replace the filter dryer  145 . If the user does not change the filter dryer  145  within five minutes, or the allotted time, the process returns to step  215 . Next, in step  245 , the pressure sensing device  135  detects the vacuum level in the filter dryer  145 , if the vacuum level is greater than one inch of Hg, the process returns to step  240 . If the vacuum level is less than one inch of Hg, the process continues to step  250 . At this point, the unit  100  is reactivated to recover/recycle refrigerant. 
     In step  250 , the user is prompted to press a button to confirm that a new filter dryer has been installed. If the user does not push the button, the process returns to step  240  and if the user pushes the button, the process continues to step  255 . The button communicates to the microprocessor  110  that the process may continue. 
     In step  255 , the detector  130  is reset to zero, the compressor  125  prepares the new filter dryer  145  by removing air from the new filter dryer  145 . Next, in step  260 , the compressor  125  continues to remove air until the pressure-sensing device  135  detects that the vacuum level is greater than thirteen inches of Hg. The process continues to step  265 . 
     In step  265 , the compressor  124  stops. Next, in step  305 , the display  120  returns to the main menu. 
     Returning to step  210 , if the detector  130  detects that the amount of refrigerant is less than one-hundred and fifty pounds, the process continues to step  270 . In step  270 , if the filter dryer weight is between seventy-five and eighty pounds, the process continues to step  275 . 
     In step  275 , the display  120  displays the amount of refrigerant that has passed through the filter dryer  145  and displays a message reminding the user to order a replacement filter dryer  145 . Next, in step  280 , the user is prompted to press a button. If the user does not push the button, the process remains at step  280 . If the user presses the button, then next, in step  310 , the display  120  returns to the main menu. 
     Returning to step  270 , if the detector  130  detects that the amount of refrigerant is not between seventy-five and eighty pounds, the process continues to step  285 . In step  285 , if the filter dryer weight is between one-hundred and one-hundred and five pounds, the process continues to step  275  as stated above. 
     Returning to step  285 , if the detector  130  detects that the amount of refrigerant is not between one-hundred and one-hundred and five pounds, the process continues to step  290 . In step  290 , if the detector  130  detects that the amount of refrigerant is between one-hundred and twenty-five pounds and one hundred and fifty pounds, the process continues to step  295 . In step  295 , the display  120  displays the amount of refrigerant that has passed through the filer dryer  145  and displays a message  155  warning the user about the mandatory filter dryer change at one-hundred and fifty pounds. Next, in step  300 , the user is prompted to press a button. If the user does not push the button, the process remains at step  300 . Upon pushing the button, in step  315 , the display  120  returns to the main menu. 
     Returning to step  290 , if the detector  130  detects that the amount of refrigerant is not between one hundred and twenty-five and one-hundred and fifty pounds, the process continues to step  320 . In step  320 , the display  120  returns to the main menu. 
     The filter dryer weights in the above embodiment are used as an example to demonstrate the warning capability of the system prior to the mandatory filter dryer change. These filter dryer weights are not meant to be limiting and can be modified based on the user&#39;s preference. 
     The various pressure and time in the above embodiment are used as an example to demonstrate the capability of the system to ensure the user to proceed with the mandatory filter dryer change. These pressure and time are not meant to be limiting and can be modified based on the user&#39;s preference. Although this specification discloses the use of negative pressure to detect pressure changes, the disclosed invention can also be practiced using positive pressure. 
     The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.
A method and apparatus for ensuring a user to change a filter dryer of a refrigerant recovery unit having a refrigerant determining module that is configured to determine a condition of the refrigerant or an amount of refrigerant that has passed through the filter dryer, or a condition of the filter dryer, an alert device that notifies the user to change the filter dryer in response to the condition, a pressure module that regulates the pressure in the filter dryer, and a pressure-sensing device that is configured to detect a change of pressure in response to filter dryer maintenance. The refrigerant recovery unit is disabled from recovering refrigerant when the refrigerant determining module determines that the filter dryer needs to be replaced, and it is activated in response to the change of pressure detected by the pressure-sensing device.