Abstract:
An air conditioning service system includes an oil receptacle, a coupling port in fluid communication with the oil receptacle through an oil injection line, a solenoid valve configured to selectively allow the oil to flow from the oil receptacle into the oil injection line, a memory including program instructions stored therein, and a controller operably connected to the solenoid valve and the memory. The controller is configured to execute the program instructions to obtain at least one viscosity signal associated with a viscosity of the oil, obtain a volume signal indicative of an amount of oil to be charged, determine a time period based upon the obtained at least one viscosity signal and the obtained volume signal, control the solenoid valve to an open condition, and control the solenoid valve to a closed condition after the determined time period has passed since opening of the solenoid valve.

Description:
[0001]    This application claims the benefit of priority of U.S. provisional application Ser. No. 61/983,622, filed Apr. 24, 2014, entitled “System and Method for Injecting Oil into an Air Conditioning System,” the entire disclosure of which is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to refrigeration systems, and more particularly to refrigerant recovery systems for refrigeration systems. 
       BACKGROUND 
       [0003]    Air conditioning systems include a mechanical compressor that compresses refrigerant flowing through the air conditioning system. The compressor requires oil to function properly and efficiently. During normal operation of the air conditioning system, a portion of the compressor oil is entrained in the refrigerant and circulated through the air conditioning system. When the air conditioning system is serviced, the refrigerant, along with the oil entrained therein, is typically removed from the air conditioning system. Additionally, the air conditioning system may require replacement of parts within the circuit, which can also remove compressor oil within the replaced parts. As such, new compressor oil must be injected into the system to replace oil removed from the system during maintenance and service operations. For this reason, most air conditioning service (“ACS”) machines include a circuit for injecting oil into the air conditioning circuit prior to recharging refrigerant into the air conditioning system. 
         [0004]    Measuring the oil injected into the air conditioning circuit is important to ensure the proper quantity of compressor oil is in the air conditioning circuit. Excess or insufficient oil in the compressor reduces the overall operational efficiency of the air conditioning circuit. One commonly used method of measuring oil injected into the air conditioning circuit is visual identification. Some conventional ACS machines include a bottle of oil having graduated markings that indicate the amount of oil in the bottle. To measure the quantity of oil injected into the circuit, the user monitors the oil level in the bottle with reference to the graduated markings as the ACS machine injects the oil, and terminates the injection operation when the desired quantity of oil appears to have been injected. This method has the lowest cost, but relies entirely on the user to monitor the bottle and inject the correct amount. As a result, the visual identification method suffers from issues, including operator error and inaccuracy of the markings or in reading the markings that can cause deviation from the desired quantity of oil injected into the air conditioning circuit. 
         [0005]    Some conventional ACS machines include a load cell associated with the oil bottle to measure the weight of the oil bottle. The ACS system is configured with a controller that subtracts the weight of the bottle during the injection process from the initial weight of the bottle to determine the amount of oil injected. Once the controller determines that the desired quantity of oil has been injected into the circuit, the controller operates the oil injection valve to close. However, load cells are expensive and delicate, and, as a result, ACS machines having a load cell for the oil bottle are costly to manufacture and maintain, and may malfunction if handled incorrectly. 
         [0006]    Other typical ACS machines estimate the quantity of oil injected into the air conditioning system based on the time the oil injection solenoid valve is open. The oil flow rate is assumed, and a length of time that the oil injection solenoid valve needs to be open in order to inject the desired quantity of oil is estimated from the assumed oil flow rate. For example, in some ACS machines, the oil flow rate is assumed to be 2 ml per second. A user inputs the amount of oil for the system to inject, for example 10 ml. The ACS controller then calculates the time the system should be open, which, in this example, is 5 seconds. 
         [0007]    One issue with such an oil injection method is that the oil flow rate is not a constant. The flow rate varies depending on the oil viscosity and the temperature of the oil, which is typically approximately the ambient temperature of the ACS machine. Some ACS machines that include time-based oil injection also include a way for the user to input a correction factor to correct the injected quantity or the time the valve is open based on variations in the flow rate due to the current conditions. One problem with this is that the user may not have accurate information to determine the proper correction factor. Another issue is that the user may be required to perform baseline tests or calculations in order to determine the correction factor, and errors in these tests or calculations can result in an incorrect correction factor being input to the machine. As a result, the time injection method fails to provide adequate accuracy due to the required user intervention and system variables. 
         [0008]    For all of the above reasons, it would be desirable to provide an ACS machine that improves the precision of the quantity of oil injected into an air conditioning system at a low cost. Additionally, it would be desirable to provide an ACS machine that accurately injects a desired quantity of oil into the air conditioning system with minimal user intervention. 
       SUMMARY 
       [0009]    In one embodiment, an air conditioning service system comprises an oil receptacle configured to store oil, a coupling port in fluid communication with the oil receptacle through an oil injection line, a first solenoid valve configured to selectively allow the oil to flow from the oil receptacle into the oil injection line, a memory including program instructions stored therein, and a controller operably connected to the first solenoid valve and the memory. The controller is configured to execute the program instructions to obtain at least one viscosity signal associated with a viscosity of the oil, obtain a volume signal indicative of an amount of oil to be charged, determine a first time period based upon the obtained at least one viscosity signal and the obtained volume signal, control the first solenoid valve to an open condition, and control the first solenoid valve to a closed condition after the determined first time period has passed since opening of the first solenoid valve. Determining the time period over which the oil is injected from the oil receptacle based upon the viscosity signal enables quick and accurate injection of the oil without the need for expensive equipment, such as a load cell. 
         [0010]    In an embodiment of the air conditioning service system, the at least one viscosity signal comprises a first temperature signal indicative of a temperature of one of the oil receptacle and ambient surroundings of the air conditioning service system. In a further embodiment, the air conditioning service system includes a temperature sensor configured to generate the first temperature signal. 
         [0011]    In another embodiment, the at least one viscosity signal comprises a first oil type signal associated with a type of the oil. 
         [0012]    In some embodiments, the air conditioning service system further comprises a vacuum pump configured to generate a vacuum in the oil injection line, and the controller is operably connected to the vacuum pump and further configured to execute the program instructions to control the vacuum pump to generate a vacuum in the oil injection line prior to controlling the first solenoid valve to the open condition. 
         [0013]    In one embodiment, the air conditioning service system further comprises a second solenoid valve configured to selectively isolate the oil receptacle from the oil injection line, a chamber having an inlet in fluid communication with the first solenoid valve and an outlet in fluid communication with the second solenoid valve, and a pressure sensor configured to generate pressure signals associated with pressures of the chamber. The controller is further operably connected to the second solenoid valve and the pressure sensor and configured to execute the program instructions to generate the at least one viscosity signal during a viscosity determining procedure using the generated pressure signals. 
         [0014]    In a further embodiment, the controller is configured to execute the program instructions to perform the viscosity determining procedure. The viscosity determining procedure comprises controlling the first solenoid valve to a closed position, generating a vacuum in the chamber, controlling the first solenoid valve to an open position thereby placing the chamber in fluid communication with the oil receptacle, obtaining a first of the generated pressure signals after controlling the first solenoid valve to the open position, and generating the at least one viscosity signal based upon the obtained first of the generated pressure signals and a second time period between controlling the first solenoid valve to the open position and obtaining the first of the generated pressure signals. 
         [0015]    In yet another embodiment, the viscosity determining procedure further comprises controlling the second solenoid valve to an open position before generating the vacuum in the chamber, generating the vacuum in the chamber through the second solenoid valve, and controlling the second solenoid valve to a closed position after the vacuum has been generated. 
         [0016]    In some embodiments, the viscosity determining procedure further comprises obtaining a second of the generated pressure signals after controlling the second solenoid valve to a closed position and prior to controlling the first solenoid valve to the open position, and generating the at least one viscosity signal based upon the obtained second of the generated pressure signals. 
         [0017]    Some embodiments of the air conditioning system include a vacuum pump configured to generate the vacuum in the chamber. The controller is operably connected to the vacuum pump and further configured to execute the program instructions to control the vacuum pump to generate the vacuum in the chamber. 
         [0018]    In another embodiment, a method of injecting oil into an oil injection line of an air conditioning service system, comprises obtaining with a controller at least one viscosity signal associated with a viscosity of the oil, obtaining with the controller a volume signal indicative of an amount of oil to be charged, and determining a first time period based upon the obtained at least one viscosity signal and the obtained volume signal by executing with the controller program instructions stored in a memory. The method further includes placing an oil injection line in fluid communication with an oil receptacle by controlling the first solenoid valve to an open condition with the controller, flowing oil from the oil receptacle into the oil injection line through the open first solenoid valve, and controlling the first solenoid valve to a closed condition with the controller after the determined first time period has passed since opening of the first solenoid valve. 
         [0019]    In another embodiment of the method, the obtaining of the at least one viscosity signal comprises obtaining a first temperature signal indicative of a temperature of one of the oil receptacle and ambient surroundings of the air conditioning service system. In some embodiments, the obtaining of the first temperature signal comprises obtaining the temperature signal with a temperature sensor. 
         [0020]    In a further embodiment, the obtaining of the at least one viscosity signal comprises obtaining a first oil type signal associated with a type of the oil. 
         [0021]    In yet another embodiment, the method further comprises generating, with a vacuum pump operably connected to the controller, a vacuum in the oil injection line prior to controlling the first solenoid valve to the open condition. 
         [0022]    In some embodiments, the method further comprises obtaining with a pressure sensor pressure signals associated with pressures of a chamber having an inlet in fluid communication with the first solenoid valve and an outlet in fluid communication with a second solenoid valve that is configured to selectively isolate the oil receptacle from the oil injection line, and generating the at least one viscosity signal during a viscosity determining procedure using the generated pressure signals. 
         [0023]    In one embodiment of the method, the viscosity determining procedure comprises controlling the first solenoid valve to a closed position, generating a vacuum in the chamber, and controlling the first solenoid valve to an open position thereby placing the chamber in fluid communication with the oil receptacle. The viscosity determining procedure further includes obtaining a first of the obtained pressure signals after controlling the first solenoid valve to the open position and generating the at least one viscosity signal based upon the obtained first of the generated pressure signals and a second time period between controlling the first solenoid valve to the open position and obtaining the first of the generated pressure signals. 
         [0024]    In another embodiment, the viscosity determining procedure further comprises controlling the second solenoid valve to an open position before generating the vacuum in the chamber, generating the vacuum in the chamber through the second solenoid valve, and controlling the second solenoid valve to a closed position after the vacuum has been generated. 
         [0025]    In a further embodiment, the viscosity determining procedure further comprises obtaining a second of the generated pressure signals after controlling the second solenoid valve to a closed position and prior to controlling the first solenoid valve to the open position, and generating the at least one viscosity signal based upon the obtained second of the generated pressure signals. 
         [0026]    In another embodiment of the method, the generating of the vacuum in the chamber comprises operating a vacuum pump operably connected to the controller to generate the vacuum in the chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is a partial cutaway front view of a refrigerant service system. 
           [0028]      FIG. 2  is side perspective view of the refrigerant service system of  FIG. 1  connected to a vehicle. 
           [0029]      FIG. 3  is a schematic view of the refrigerant service system of  FIG. 1 . 
           [0030]      FIG. 4  is a schematic view of the control components of the refrigerant service system of  FIG. 1 . 
           [0031]      FIG. 5  is a process diagram of a method of operating a refrigerant service system to inject oil into an air conditioning system. 
           [0032]      FIG. 6  is a schematic view of an oil injection system for a refrigerant service system according to the disclosure. 
           [0033]      FIG. 7  is a schematic view of the control components of the oil injection system of  FIG. 6 . 
           [0034]      FIG. 8  is a process diagram of another method of operating a refrigerant service system to inject oil into an air conditioning system. 
           [0035]      FIG. 9  is a graph of oil absolute viscosity as a function of temperature for a variety of different oils. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]    For the purposes of promoting an understanding of the principles of the embodiments described herein, reference is now made to the drawings and descriptions in the following written specification. No limitation to the scope of the subject matter is intended by the references. This disclosure also includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the described embodiments as would normally occur to one skilled in the art to which this document pertains. 
         [0037]      FIG. 1  is an illustration of an air conditioning service (“ACS”) system  10  according to the disclosure. The ACS system  10  includes a refrigerant container or internal storage vessel (“ISV”)  14 , a manifold block  16 , a compressor  18 , a control module  20 , and a housing  22 . The exterior of the control module  20  includes an input/output unit or user input interface  26  for input of control commands by a user and output of information to the user. Hose connections  30 ,  32  (only one is shown in  FIG. 1 ), which are also referred to herein as coupling ports, protrude from the housing  22  to connect to service hoses that connect to an air conditioning (“A/C”) system (also referred to herein as an “air conditioning circuit”) and facilitate transfer of refrigerant between the ACS system  10  and the A/C system. 
         [0038]    The ISV  14  is configured to store refrigerant for the ACS system  10 . No limitations are placed on the kind of refrigerant that may be used in the ACS system  10 . As such, the ISV  14  is configured to accommodate any refrigerant that is desired to be charged to the A/C system. In some embodiments, the ISV  14  is particularly configured to accommodate one or more refrigerants that are commonly used in the A/C systems of vehicles (e.g., cars, trucks, boats, planes, etc.), for example R-134a, CO 2 , or R1234yf. In some embodiments, the ACS unit has multiple ISV tanks configured to store different refrigerants. 
         [0039]    The manifold block  16  is fluidly connected to the ISV  14 , the compressor  18 , and the hose connections  30 ,  32  through a series of valves, hoses, and tubes. The manifold block  16  includes valves and components configured to filter and purify refrigerant recovered from a vehicle during a refrigerant recovery operation prior to the refrigerant being stored in the ISV  14 , and to recharge the refrigerant back into the air conditioning circuit from the ISV  14 . 
         [0040]      FIG. 2  is an illustration of a portion of the ACS system  10  illustrated in  FIG. 1  connected to a vehicle  50 . Service hoses  34 ,  36  include coupling connectors  38 ,  40  ( FIG. 3 ) configured to connect an inlet and/or outlet port of the air conditioning circuit of the vehicle  50  to the hose connections  30  (shown in  FIGS. 1 and 3 ) of the ACS unit  10 . 
         [0041]      FIG. 3  illustrates a schematic diagram of the ACS system  10 . The ACS system  10  includes a bulkhead manifold  104 , a vacuum pump  108 , a recovery manifold  112 , the ISV  14 , and a controller  120 . In some embodiments, one or both of the bulkhead manifold  104  and the recovery manifold  112  are at least partially integrated within the manifold block  16 , while in other embodiments the bulkhead manifold  104  and the recovery manifold are separate from the manifold block  16 . 
         [0042]    The high-side service hose  34  and the low-side service hose  36  connect to the coupling ports  30 ,  32  of the bulkhead manifold  104  at one end, and the hose couplers  38 ,  40  at the other end of the service hoses  34 ,  36  are configured to attach to the high-side and low-side, respectively, of the air conditioning circuit of the vehicle  50 . The bulkhead manifold  104  includes a high-side line  140  and a low-side line  144  fluidly connecting the coupling ports  30 ,  32 , respectively, to a vacuum line  148 , a recovery line  152 , and an ISV charge line  156  through a high-side solenoid valve  160  and a low-side solenoid valve  164 , respectively. 
         [0043]    The vacuum pump  108  and a vacuum solenoid valve  168  are disposed in the vacuum line  148 . A recovery solenoid valve  172  is located in the recovery line  152 , which fluidly connects the recovery manifold  112  to the high-side and low-side lines  140 ,  144 . The recovery manifold  112  includes components, for example a compressor, oil separators, a heat exchanger, and filters and dryer units, configured to remove oil entrained in refrigerant and purify the refrigerant when the refrigerant is recovered from an air conditioning circuit. The purified refrigerant is then stored in the ISV  14 . The ISV charge line  156  connects the ISV  14  to the high-side and low-side lines  140 ,  144  through a charge solenoid valve  176  to enable recharging refrigerant from the ISV  14  to the air conditioning circuit. 
         [0044]    A first oil receptacle  180 , a second oil receptacle  184 , and a dye receptacle  188  are fluidly connected to a first oil supply line  192 , a second oil supply line  196 , and a dye supply line  200 , respectively. A first oil injection check valve  204  and a first oil injection solenoid valve  208  are fluidly connected to the first oil supply line  192 , a second oil injection check valve  212  and a second oil injection solenoid valve  216  are fluidly connected to the second oil supply line  196 , and a dye injection check valve  220  and a dye injection solenoid valve  224  are fluidly connected to the dye supply line  200 . The solenoid valves  208 ,  216 ,  224  are fluidly connected to the high-side line  140  via a first oil injection line  226 , a second oil injection line  228 , and a dye injection line  230 , respectively. In some embodiments, the solenoid valves  208 ,  216 ,  224  are directly connected to the high-side line  140  such that the high-side line  140  is the oil injection line. 
         [0045]    Each of the first and second oil receptacles  180 ,  184  is configured to store a type of oil. In some embodiments, the oil stored in the first oil receptacle  180  has a different viscosity and different thermal properties than the oil stored in the second oil receptacle  184  to enable use of the ACS system  10  with a wider variety of air conditioning circuits. The dye receptacle  188  stores dye, which can be injected into the air conditioning circuit to aid a user in diagnostic operations, for example locating a leak in the air conditioning circuit. In some embodiments, one or both of the oil receptacles  180 ,  184  are connected to the recovery manifold  112  by a system oil return line (not shown) to transfer oil separated from recovered refrigerant back into the oil receptacle  180 ,  184  for subsequent reuse. 
         [0046]    The injection check valves  204 ,  212 ,  220  and solenoid valves  208 ,  216 ,  224  are all disposed in the bulkhead manifold  104  in the embodiment of  FIG. 3 , though in other embodiments the valves  204 ,  208 ,  212 ,  216 ,  220 ,  224  may be in another manifold or installed individually within the ACS machine  10 . In the embodiment of  FIG. 3 , the ACS machine  10  includes two oil receptacles  180 ,  184  and one dye bottle  188 . In some embodiments, the ACS machine includes only one oil receptacle or more than two oil receptacles. In other embodiments, the ACS machine does not include a dye receptacle or the associated valves and lines, or the ACS machine may include more than one dye bottle to store different types of dye. 
         [0047]      FIG. 4  illustrates a schematic diagram of a control system  236  for the ACS machine  10 . The control system  236  includes the controller  120 , which is operably connected to a user input interface  26 . The controller  120  is configured to receive inputs from the user input interface  26 , and, in some embodiments, display information for a user on the user input interface  26 . The controller  120  is also operably connected to an ambient temperature sensor  244 , which is configured to sense the ambient temperature of the ACS unit  10  and generate electronic signals corresponding to the ambient temperature. In some embodiments, an oil receptacle temperature sensor, which senses the temperature of the oil in the oil receptacle, is used in place of the ambient temperature  244 . 
         [0048]    The controller  120  is operably connected to a memory  252  to store data received from the user input interface  26  and the temperature sensor  244 . The controller  120  and the memory  252  may be integrated in the control module  20  of the ACS system  10 . In some embodiments, in addition to or as an alternative to storing the data in the memory  252 , the data is stored outside the ACS machine  10 . In one embodiment, the data is transmitted via a wired or wireless internet connection to a “cloud” storage location. In another embodiment, the data is transmitted to a memory device such as a hard disk drive, a USB drive, a solid state drive, a network attached storage (NAS) device, or the like. The controller  120  is also operably connected to the solenoid valves  160 ,  164 ,  168 ,  172 ,  176 ,  208 ,  216 ,  224  and to the vacuum pump  108 . The controller  120  is configured to transmit electronic signals to operate the solenoid valves  160 ,  164 ,  168 ,  172 ,  176 ,  208 ,  216 ,  224  to an open or closed condition and to operate the vacuum pump  108  to activate and deactivate. 
         [0049]    Operation and control of the various components and functions of the ACS machine  10  are performed with the aid of the controller  120 . The controller  120  is implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions are stored in the memory unit  252  associated with the controller  120 , or in a memory unit (not shown) integrated in the controller  120 . The processors, memory, and interface circuitry configure the controller  120  to perform the functions described above and the processes described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
         [0050]    In operation, the high-side and low-side hose couplers  38 ,  40  are connected to the high-side and low-side connection ports of an air conditioning circuit, for example an air conditioning system of vehicle  50  of  FIG. 2 . To perform a recovery operation, the recovery solenoid  172  and one or both of the high-side and low-side solenoids  160 ,  164  are opened. Compressed refrigerant within the air conditioning system flows to the recovery manifold  112 , where system oil entrained in the refrigerant is separated from the refrigerant and the refrigerant is purified for storage in the ISV  14 . 
         [0051]    It is often necessary during normal maintenance of an air conditioning circuit to replace the system oil entrained in the refrigerant removed from the air conditioning system so that the air conditioning system continues to perform optimally. As such, an oil injection operation is performed after the refrigerant recovery operation, during which the refrigerant is recovered from the air conditioning circuit, and prior to a refrigerant recharge operation, during which refrigerant is recharged into the air conditioning circuit. Once the recovery operation is complete, the recovery solenoid  172  is closed, the vacuum solenoid  168  is opened, and the vacuum pump  108  is activated. The vacuum pump  108  produces a negative pressure in the high-side and low-side lines  140 ,  144 , pulling any remaining refrigerant from the air conditioning circuit and reducing the pressure in the air conditioning circuit below atmospheric pressure. The high-side and low-side solenoid valves  160 ,  164  are then closed and the vacuum pump  108  is deactivated to close off the air conditioning system and retain the air conditioning system at the vacuum pressure. 
         [0052]    The controller  120  then controls one of the oil injection solenoids  208 ,  216  to open. In some embodiments, the controller  120  is programmed to automatically select the appropriate solenoid valve  208 ,  216  to open, while in other embodiments a user instructs the controller  120  which oil injection solenoid valve  208 ,  216  to open via the user input interface  26 , an external electronic device operably connected to the controller  120 , or a combination thereof. Opening one of the oil injection solenoids  208 ,  216  fluidly connects the associated oil receptacle  180 ,  184  to the respective check valve  204 ,  212 , which opens due to the negative pressure in the air conditioning system and the high-side line  140 . As such, the respective oil receptacle  180 ,  184  is fluidly connected to the air conditioning circuit via the high-side line  140 . 
         [0053]    The flow rate of the oil through the solenoid valve  208 ,  216  into the air conditioning circuit is dependent on the absolute viscosity of the oil, which is a function of the temperature and viscosity rating, or oil type, of the oil. As discussed in further detail below, the controller  120  is therefore configured to determine the absolute viscosity of the oil based on the current ambient temperature and the viscosity rating or oil type of the oil, and to calculate the flow rate based upon the absolute viscosity of the oil. The controller  120  is further configured to calculate the length of time the respective solenoid valve  208 ,  216  is to be open based on the quantity of oil desired to be injected into the air conditioning system and the calculated flow rate. The controller  120  then controls the respective oil injection solenoid valve  208 ,  216  to open for the calculated length of time to inject the desired amount of oil into the high-side line  140  and the air conditioning system. 
         [0054]    Once the oil has been injected, the controller  120  controls the respective oil injection solenoid valve  208 ,  216  to close and performs a recharge operation. During the recharge operation, the charge solenoid valve  176  and the high-side solenoid valve  160  are opened. Refrigerant in the ISV  14  flows from the ISV  14  through the high-side line  140  into the air conditioning circuit. Any residual oil remaining in the high-side line  140  from the oil injection operation is entrained in the refrigerant and transferred to the air conditioning circuit. In some embodiments, the low-side solenoid valve  164  is also opened during the recharge operation such that refrigerant flows from the ISV  14  through both the high and low side lines  140 ,  144  into the air conditioning circuit. 
         [0055]      FIG. 5  is a process diagram of a method  300  of injecting oil into an air conditioning circuit. The controller  120  of the refrigerant service system  10  includes a processor configured to execute programmed instructions stored in a memory associated with the controller to implement the method  300 . 
         [0056]    The method  300  begins with the controller  120  obtaining the rated viscosity of the oil and the ambient temperature (block  304 ). In one embodiment, the rated viscosity of the oil, or the type of oil, and the ambient temperature are input by the user via a user input interface, such as the user input interface  26  of the embodiment of  FIGS. 3 and 4 . In another embodiment, the user inputs the rated viscosity of the oil or the oil type, and the controller obtains the ambient temperature from the temperature sensor  244 . In a further embodiment, the rated viscosity or the oil types of the oils stored in the oil receptacles are stored in the memory  252  when one of the oil receptacles is filled or changed or the rated viscosity is programmed into the memory  252  during manufacture of the ACS machine  10 , and the rated viscosity is obtained by the controller  120  from the memory. 
         [0057]    The controller  120  then determines the absolute viscosity of the oil and the corresponding flow rate of the oil at the ambient temperature (block  308 ). In one embodiment, tables or charts of oil absolute viscosity for various different oil types or rated viscosities at various temperatures are stored in the memory  252 . One example of a chart of oil absolute viscosity as a function of temperature for a variety of oil grades is shown in  FIG. 9 . The controller  120  recalls the absolute viscosity of the oil from the table or chart based on the ambient temperature and the rated viscosity of the oil. 
         [0058]    The flow rate of the oil is dependent primarily on the size of the tubes and openings connecting the oil receptacle and the air conditioning system (for example oil supply and injection lines  192 ,  196 ,  226 ,  228  check valves  204 ,  212 , and solenoid valves  208 ,  216 ), the length of the lines, the pressure difference between the oil receptacle and the air conditioning circuit, and the absolute viscosity of the oil. The size and length of the oil path are constants, and may be programmed into the memory  252  of the ACS system  10 . In some embodiments, the pressure difference is calculated based on a pressure signal received from a pressure transducer configured to determine the pressure in the air conditioning circuit. In other embodiments the pressure is assumed based upon a known vacuum condition in the air conditioning circuit and the ambient pressure, which is either measured or assumed. The controller is then configured to determine the flow rate of the oil at the determined absolute viscosity. In another embodiment, the system includes a viscosity sensor configured to sense the viscosity of the oil, which the controller uses to calculate the flow rate of the oil. 
         [0059]    Next, the controller  120  calculates the length of time to open the oil injection solenoid valve (block  312 ). For simplicity of description, the remainder of the method  300  will be described with reference to oil being transferred from the oil receptacle  180  through solenoid valve  208  and oil injection line  226 , through the reader should appreciate that the oil may be transferred from the second oil receptacle  184  in a similar manner. In one embodiment, the user inputs an amount of oil to inject into the air conditioning system into the user input interface  24 , which transmits a volume signal to the controller  120 . In another embodiment, the user inputs a vehicle type, air conditioning circuit model, or air conditioning circuit refrigerant capacity to the user input  24  of the ACS machine  10  and the controller  120  recalls a volume signal representing the amount of oil to inject from the memory  252  based upon the value input by the user. In a further embodiment, the ACS machine  10  determines the quantity of oil removed from the air conditioning circuit during the recovery operation and stores the quantity of oil removed in the memory  252 , and the controller  120  recalls a volume signal corresponding to the removed quantity to determine the amount of oil to inject into the air conditioning circuit. The controller  120  then determines the length of time the oil injection solenoid valve  208  is open based on the amount of oil to inject and the oil flow rate. 
         [0060]    The controller  120  controls the vacuum pump  108  to generate a vacuum pressure in the air conditioning circuit, the high-side line  140 , and the oil injection line  226  (block  316 ). The reader should appreciate that the generation of the vacuum pressure may be performed before, during, or after the determination of the oil viscosity and the valve open time. In one embodiment, the controller controls the high-side solenoid valve  160 , the vacuum solenoid valve  168 , and the vacuum pump  108  to generate the vacuum, closing the valves  160 ,  168  to retain the air conditioning circuit, the high-side line  140 , and the oil injection line  226  at the vacuum pressure. 
         [0061]    Once the air conditioning circuit is at vacuum pressure and the opening time for the oil injection solenoid valve  208  has been calculated, the controller controls the oil injection solenoid valves  208  to open for the calculated time (block  320 ), such that the amount of oil flows through the solenoid valve  208  into the oil injection line  226 , the high-side line  140 , and the air conditioning circuit. After the valve  208  is open for the calculated length of time, the controller  120  controls the valve  208  to close. Refrigerant is then charged into the air conditioning circuit by, for example, opening the charge solenoid valve  176  and the high-side solenoid valve  160  to open a path from the ISV  14  to the air conditioning circuit (block  324 ). Refrigerant flows from the ISV  14  into the air conditioning circuit, capturing any oil remaining in the oil injection line  226  and the high-side line  140  and transferring the oil into the air conditioning circuit. 
         [0062]      FIG. 6  illustrates another oil injection system  400  for a refrigerant service system, which can be used in place of the oil injection system of the refrigerant service system  10  depicted in  FIG. 3 . The oil injection system  400  is disposed in a bulkhead manifold  404 , and includes an oil supply line  408 , a check valve  412 , a first solenoid valve  416 , a chamber  420 , a second solenoid valve  424 , an oil supply line  428 , and a pressure transducer  432 . 
         [0063]    The oil injection line  408  fluidly connects the oil receptacle  180  to the check valve  412 , which is fluidly connected to the first solenoid valve  416 . The chamber  420  includes an inlet  434 , which is fluidly connected to the first solenoid valve  416 , and an outlet  436 , which is fluidly connected to the second solenoid valve  424 . In some embodiments, the inlet  434  and the outlet  436  are combined into a single combination inlet/outlet line or port used to both receive and discharge oil. In some embodiments, the chamber  424  is connected to a separate line that is connected to the line between the first and second solenoid valves  416 ,  424 , instead of being directly positioned between the first and second valves  416 ,  424 . 
         [0064]    The second solenoid valve  424  is fluidly connected to the oil injection line  428 , which discharges oil into the high-side line  140 . As described above with regard to  FIG. 1 , the high-side line  140  is fluidly connected to a high-side hose  34  and high-side hose coupler  38  via the coupling port  30 . 
         [0065]    The chamber  420  is configured to hold a predetermined volume of oil, which, in one embodiment, is approximately 5 mL. The pressure transducer  432  is configured to sense the pressure within the predefined volume of the chamber  420  and generate an electronic signal corresponding to the pressure within the chamber  420 . 
         [0066]      FIG. 7  illustrates the control system  438  of the oil injection system  400  of  FIG. 6 . A controller  440  is operably connected to the user input interface  26  and a memory  252 , both of which are configured substantially the same as described above with regard to the embodiment of  FIGS. 3 and 4  and may be integrated within the control module  20  of the ACS system  10 . The controller  440  is also operably connected to the chamber pressure transducer  432  to receive the signal corresponding to the pressure in the chamber  420 . The controller  120  is operably connected to the first and second oil injection valves  416 ,  424  and configured to transmit electronic signals to control the solenoid valves  416 ,  424  to open and close. 
         [0067]    Operation and control of the various components and functions of the oil injection system  400  are performed with the aid of the controller  440 . The controller  440  is implemented with general or specialized programmable processors that execute programmed instructions. The instructions and data required to perform the programmed functions are stored in the memory unit  252  associated with the controller  440 . The processors, memory, and interface circuitry configure the controller  440  to perform the functions described above and the processes described below. These components can be provided on a printed circuit card or provided as a circuit in an application specific integrated circuit (ASIC). Each of the circuits can be implemented with a separate processor or multiple circuits can be implemented on the same processor. Alternatively, the circuits can be implemented with discrete components or circuits provided in VLSI circuits. Also, the circuits described herein can be implemented with a combination of processors, ASICs, discrete components, or VLSI circuits. 
         [0068]    In operation, the controller  440  is configured to initiate an oil injection operation after a refrigerant recovery operation and before a refrigerant recharge operation. The oil injection operation begins with execution of a viscosity determining procedure. The viscosity determining procedure beings with the controller activating a vacuum pump (not shown in  FIG. 6 or 7 ) to produce a vacuum in the high-side line  140 , the high-side hose  34 , the oil injection line  428 , and the air conditioning circuit. The controller  440  then controls the second oil injection solenoid  424  to open, such that the vacuum pressure is transferred to the chamber  420 . The controller  440  controls the second oil injection solenoid  424  to close, deactivates the vacuum pump, and opens the first oil injection solenoid  416 . The negative pressure in the chamber  420  opens the check valve  412 , drawing oil from the oil receptacle  180  through the oil supply line  408  and into the chamber  420 . 
         [0069]    The controller  440  monitors the signal produced by the pressure transducer  432  as the oil travels into the chamber  420 . Once the controller  440  identifies that the pressure in the chamber  420  is equal to or greater than a predetermined threshold, which, in one embodiment, is atmospheric pressure, the controller  440  calculates the flow rate of the oil from the oil receptacle  180 . The amount of time required for the chamber  420  and the known volume of the chamber  420  is representative of the viscosity of the oil. The controller  120  then generates a viscosity signal that enables the controller  440  to calculate the flow rate of the oil being transferred from the oil receptacle  180 , and the viscosity determining procedure is completed. 
         [0070]    Once the oil injection flow rate is calculated, the controller calculates the amount of time the valves  416 ,  420  are opened such that the desired quantity of oil is injected into the air conditioning circuit. The controller  440  then controls the second solenoid valve  424  to open, so that both valves  416 ,  424  are open. Oil travels from the oil receptacle  180  through the oil supply line  408  and the chamber  420 , through the oil injection line  428 , the high-side line  140 , the high-side hose  34 , and into the air conditioning circuit of the vehicle. After the calculated amount time has elapsed, the solenoid valves  416 ,  424  are closed and the air conditioning circuit is charged with refrigerant, capturing any remaining oil in the lines  428 ,  140 , and the hose  34  and transporting the oil into the air conditioning circuit of the vehicle. 
         [0071]      FIG. 8  illustrates a process diagram of a method  500  of injecting oil into an air conditioning circuit. The controller  440  of the refrigerant service system  10  includes a processor configured to execute programmed instructions stored in a memory associated with the controller to implement the method  500 . 
         [0072]    The method  500  begins with the controller  440  opening the outer solenoid valve  424  (block  504 ) to fluidly connect the chamber  420  to the oil injection line  428 , the high-side line  140 , and the air conditioning circuit. The controller  440  then controls the vacuum pump to generate a vacuum in the air conditioning circuit, the high-side line  140 , the oil injection line  428 , and the chamber  420  (block  508 ). The outer valve  424  is closed (block  512 ), isolating the chamber  420  from the air conditioning circuit, and an inner solenoid valve  416  is opened (block  516 ), connecting the oil receptacle  180  to the chamber  420 . The controller  440  obtains the pressure signal produced by the pressure transducer  432  (block  520 ) and compares the pressure in the chamber  420  with a predetermined pressure threshold (block  524 ). 
         [0073]    If the pressure is less than the threshold, the process continues at block  520  by obtaining the signal corresponding to the pressure inside the chamber  420  again. If the pressure is equal to or greater than the threshold, the controller  440  determines the average oil flow rate into the chamber  420 , which is equal to the chamber volume divided by the amount of time required after opening the inner solenoid valve  416  for the pressure to reach the threshold, indicating that the chamber  420  is full (block  528 ). In some embodiments, the controller  440  corrects the determined average flow rate by a correction factor based upon temperature, ACS machine specifications, or other environmental or system variables. The execution of blocks  504  through  528  are referred to collectively as the viscosity determining procedure. 
         [0074]    Based on the determined average flow rate, the controller calculates the solenoid open time (block  532 ), which is equal to the amount of oil desired to be injected into the air conditioning circuit divided by the determined average oil flow rate. The outer solenoid valve  424  is then opened (block  536 ), fluidly connecting the oil receptacle  180  to the air conditioning circuit of the vehicle. The controller  440  waits for the calculated period of time to elapse (block  540 ), and then controls both the inner and outer solenoid valves  416 ,  424  to close (block  544 ). The controller  440  then controls the components in the ACS system  10  to charge the air conditioning circuit with refrigerant (block  548 ). 
         [0075]    It will be appreciated that variants of the above-described and other features and functions, or alternatives thereof, may be desirably combined into many other different systems, applications or methods. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements may be subsequently made by those skilled in the art that are also intended to be encompassed by the foregoing disclosure.