Abstract:
A monitoring system for an air conditioning/refrigeration unit is disclosed that includes a microprocessor and a plurality of inputs operatively connected to the microprocessor for receiving data from a plurality of sensors operatively connected to the air conditioning/refrigeration unit. A memory operatively connected to the microprocessor stores data describing a pressure enthalpy diagram for at least one refrigerant and a nominal pressure enthalpy path for the ACR unit being monitored. The microprocessor uses data received at the inputs for calculating an actual pressure enthalpy path for the ACR unit and displaying the calculated pressure enthalpy path and the nominal system pressure enthalpy path on a display. A method for monitoring an air conditioning/refrigeration system is also disclosed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention is directed toward a method and apparatus for monitoring an air conditioning and refrigeration (ACR) unit, and more specifically, toward a method and apparatus for monitoring an ACR unit that includes a display of a pressure enthalpy path for the ACR unit to provide information on the operation of the unit.  
       BACKGROUND OF THE INVENTION  
       [0002]     Both air conditioning units and refrigeration units function by absorbing heat from a first location and transporting the heat to a second location where it is released. Air conditioning units are designed primarily to reduce the temperature of a living space to a comfortable temperature for its inhabitants. Refrigeration units are designed primarily to keep food and other items at an even lower temperature. However, both work according to the same basic principles and will be referred to jointly herein as air conditioning/refrigeration (ACR) units or ACR systems.  
         [0003]     ACR systems are often run until they break down or until an obvious problem develops. The consequences of a malfunction can be very costly. For example, a large commercial refrigerator may hold thousands of dollars of food that will be rendered worthless if it is not maintained at a proper temperature. Even the cost of replacing food in a home refrigerator that fails can be substantial. Drugs and cultures in a laboratory refrigerator could be difficult and expensive to replace if refrigeration is lost, and computer equipment may malfunction if it is not maintained at a controlled temperature. Homes and businesses also become difficult or nearly impossible to occupy if an air conditioner fails during a hot period.  
         [0004]     ACR systems generally work the hardest during the hottest times of the year. This also seems to be the time that such systems are likely to fail. The skilled technicians who generally service ACR systems are in high demand during such periods, and may not be able to service every ACR system that fails before expensive damage occurs. It may also be difficult to obtain the services of a skilled technician when a breakdown occurs outside of business hours or over the weekend.  
         [0005]     Efforts have been made to monitor the performance of ACR systems and detect malfunctions before they render a system inoperable. For example, U.S. Pat. No. 5,729,474 to Hildebrand discloses a method of monitoring the time required by an air conditioner to reduce the temperature of a space from a first level to a second level—each day when the system is started, for example. Increases in this time may suggest a problem with the air conditioner. Such a method provides little information about the overall operation of an air conditioner, and, when a failure occurs, may tell a technician little except that the air conditioner was working before and is not working now. It would therefore be desirable to provide a system and method for monitoring an ACR system that provides detailed information about the operating status of the system in a manner that allows for an analysis of system operation and a determination of corrective steps that can be taken to prevent the system from failing, before a failure occurs.  
       SUMMARY OF THE INVENTION  
       [0006]     The above problems and others are addressed by the present invention which comprises, in a first embodiment, a monitoring system for an ACR unit. The system includes a microprocessor having inputs operatively connected to a plurality of sensors that provide data about the ACR system to the microprocessor. A memory stores data describing a pressure enthalpy diagram for at least one refrigerant and a nominal pressure enthalpy path for the ACR unit. The microprocessor uses data from the sensors for calculating an actual pressure enthalpy path for the ACR unit and displays the actual, calculated, pressure enthalpy path for the ACR unit on a display.  
         [0007]     Another aspect of the invention comprises a method of measuring the performance of an ACR unit containing a refrigerant that involves the steps of measuring a plurality of characteristics of the ACR unit. These characteristics include refrigerant temperature at a first location and refrigerant pressure at a second location. A pressure enthalpy path for the ACR unit is calculated based on the measured characteristics. The pressure enthalpy curve for the refrigerant and the calculated pressure enthalpy path of the unit are then displayed.  
         [0008]     A further aspect of the invention comprises a portable monitoring system for an ACR unit that includes a microprocessor having a plurality of data inputs and a plurality of sensors connected to the ACR unit and operatively connected to the plurality of inputs. A memory operatively connected to the microprocessor stores data describing a pressure enthalpy diagram for at least one refrigerant and a nominal pressure enthalpy path for the ACR. The portable monitoring system further includes an artificial neural network and a display. The microprocessor uses data received at the inputs to calculate an actual pressure enthalpy path for the ACR unit and display the nominal pressure enthalpy path for the ACR unit and the actual pressure enthalpy path for the ACR unit. Furthermore, using the data from the sensors, at least one other characteristic of the ACR unit is calculated, and this characteristic may be: refrigerant effect, amount of heat rejected to the condenser, compressor work performed, degree of superheating, degree of sub-cooling, percentage of flash gasses, coefficient of performance, and pressure drop between various points in the system.  
         [0009]     Another aspect of the invention comprises a monitoring system for an ACR unit that includes a microprocessor device having a memory and a mechanism for obtaining data describing the ACR unit. Data describing a pressure enthalpy diagram for a refrigerant and a nominal pressure enthalpy path for the ACR unit are stored in the memory. A mechanism for graphically displaying a pressure enthalpy path is also provided. In operation, the microprocessor device calculates from the obtained data a description of an actual pressure enthalpy path for the ACR unit and causes the nominal pressure enthalpy path and the actual pressure enthalpy path for the ACR unit to be displayed on the graphical display device. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The present invention will be better appreciated after a reading of the following detailed description thereof together with the following drawings of which:  
         [0011]      FIG. 1  is a side elevational view of a ACR monitoring device according to the present invention connected to an ACR system, which is shown schematically;  
         [0012]      FIG. 2  is a pressure enthalpy diagram for a first refrigerant;  
         [0013]      FIG. 3  is a pressure enthalpy path for an ACR unit superimposed on the pressure enthalpy diagram of  FIG. 2 ;  
         [0014]      FIG. 4  is a perspective view of the ACR monitoring device of  FIG. 1 ;  
         [0015]      FIG. 5  is a detail view of circle V in  FIG. 4  showing the display generated by the ACR monitoring device and ACR system of  FIG. 1 ; and  
         [0016]      FIG. 6  is a flow chart showing a method of monitoring an ACR system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Referring now to the drawings, wherein the showings are for the purpose of illustrating a preferred embodiment of the invention only and not for the purpose of limiting same,  FIG. 1  shows an ACR system  10  comprising a compressor  12 , a condenser  14 , a metering device  16 , an evaporator  18 , a condenser fan  20  and an evaporator fan  22  for moving air across the condenser  14  and evaporator  18  respectively. Refrigerant flows through the system  10  in the direction from compressor  12  toward condenser  14 , and the terms “upstream” and “downstream” as used herein are in relation to this direction of flow. System  10  further includes several ports at which refrigerant pressure can be measured, including a first port  24  immediately upstream of compressor  12 , a second port  26  immediately downstream of compressor  12 , a third port  28  upstream of metering device  16  and a fourth port  30  downstream of metering device  16 .  
         [0018]     System  10  operates as follows. High pressure liquid refrigerant in condenser  14  passes through a small opening in metering device  16  which causes the pressure of the liquid to drop. The refrigerant is selected so that, at the normal operating temperature of the unit, the boiling point of the liquid will fall below ambient temperature during the pressure drop through metering device  16 . Any liquid that does not immediately vaporize during the pressure drop does so in evaporator  18 . A significant amount of heat is absorbed from the air surrounding evaporator  18  when the refrigerant evaporates, and relatively warm, low pressure vapor exits evaporator  18 . This vapor moves to compressor  12  where it is compressed, thereby lowing its boiling point. The process of compression also adds heat to the refrigerant. The generally warm, high pressure gas passes through condenser  14  where heat is removed from it, by fan  20  blowing air over condenser  14 , for example. This cooling causes the vapor in condenser  14  to condense into a liquid, under pressure, which is fed to metering device  16  where the cycle begins again. The evaporator  18  is generally located in a first closed location separate from the condenser  14  so that heat absorbed from the air surrounding evaporator  18  is effectively transferred to a location outside the closed location surrounding evaporator  18 .  
         [0019]     The enthalpy or heat content of a refrigerant varies with the pressure of the refrigerant. Different combinations of pressure (P) and enthalpy (h) will cause the refrigerant to exist in a liquid state, a vapor state, or a combination of the two. The relationship between the pressure and enthalpy of a material can be shown by using a pressure enthalpy, or P-h, diagram. A simplified pressure enthalpy diagram  32  for a refrigerant is shown in  FIG. 2 . On diagram  32 , pressure increases from the bottom to the top of the vertical axis of the diagram while enthalpy increases from left to right along the horizontal axis. The curve  34  separates the diagram into three regions. A first region  36  lies to the left of curve  34 . Under the combinations of temperatures and pressures represented by this portion of diagram  32 , the refrigerant will exist in a liquid state. In the second region  38  located under the curve  34 , the refrigerant will exist as a combination of liquid and vapor. The third region  40  lies to the right of curve  34 . Under the combinations of temperatures and pressures represented by this portion of the diagram, the refrigerant will exist in a vapor state. Curve  34  represents the limiting conditions under which the material can exist as entirely liquid (along the left side thereof as viewed in  FIG. 2 ) or entirely vapor (along its right side).  
         [0020]     Liquids in first region  36  are said to be sub-cooled. In other words, the liquid exists at a temperature below its boiling point for the pressure level specified on the diagram. The difference between the temperature of the liquid and its boiling point at a given pressure is referred to as its degree of sub-cooling. Water at 95° C., at 1 atmosphere, for example, has a degree of sub-cooling of 5°. Vapor in the third region  40  is said to be superheated. In other words, the vapor exists at a temperature above the temperature at which the vapor would begin to condense. Water vapor at 110° C. would therefore be described as having a degree of superheating of 10°. Different materials generally will have different temperature enthalpy diagrams.  
         [0021]     The physical state of the refrigerant traveling around an ACR system can be shown using a pressure enthalpy path superimposed on a pressure enthalpy diagram to allow the refrigeration cycle to be visualized. An idealized pressure enthalpy path  42  for an ACR system is shown in  FIG. 3  superimposed on the pressure enthalpy diagram  32  of  FIG. 2 . This path helps illustrate how the physical state of the refrigerant changes from liquid to vapor and back to liquid as the pressure and enthalpy of the system change.  
         [0022]     Starting at point a, the pressure of a refrigerant increases as it is compressed by a compressor. Its enthalpy also increases. The increases in pressure and enthalpy are shown by the line between points a and b in  FIG. 3 . Point b lies to the right of curve  34  indicating that the refrigerant at this point is superheated; heat must be removed from it before it will begin to condense. The superheated vapor looses heat between points b and c on path  42 , and the vapor begins to condense at point c on curve  34 . The pressure remains constant during the condensation process, and at point d, the refrigerant is fully in liquid form. The pressure under which the refrigerant is held is decreased between points d and e on path  42 . Heat is not added to or removed from the system during this pressure decrease, and the enthalpy of the system does not change; however, the temperature of the refrigerant drops. During the evaporation phase, shown between points e and a on the path, the pressure of the refrigerant remains constant while its enthalpy increases. When the refrigerant is fully evaporated at point a, it enters the compressor to begin the cycle again.  
         [0023]     The above description represents an idealized refrigeration cycle. Heat gains and losses between the system and the ambient air, for example, will cause the pressure enthalpy path for a given ACR to deviate somewhat from this ideal path.  
         [0024]      FIGS. 1 and 4  depict an ACR monitoring device  44  according to the present invention which comprises a housing  46  having a handle  48  for portability. As used herein, the term “monitoring” can refer both to ongoing monitoring of a system and short term monitoring of a system, for diagnostic purposes, for example. Device  44  further includes an on/off switch  50 , a power indicator light  52 , a hexadecimal keyboard  54 , a graphic display screen  56  and an LCD display  58 . Device  44  is also provided with a removable storage device  60  such as a floppy disk drive, and includes cables  62  for connecting the device to a power source (not shown). Twenty five socket inputs  64  are also provided for connecting sensors described herein. Sockets  64  are arranged in three rows labeled A, B and C and nine columns labeled  1 - 9 . Particular sockets  64  are identified herein by their location in these rows and columns by a pair of coordinates. For example, the upper, left most socket, as viewed in  FIG. 1 , is identified as socket  64 -A 1 .  
         [0025]     Device  44  also includes a microprocessor  66  having a memory  68 . Microprocessor  66  is operatively connected to display screen  56  and LCD display  58  and generates graphical and/or text output to the display screen  56  and LCD display  58 . Microprocessor  66  is also operatively connected to input sockets  64  for receiving input from sensors connected to the ACR unit.  
         [0026]     A plurality of pressure sensors and temperature sensors are connected to various parts of ACR system  10 . Each of these sensors is connected to one of a plurality of data acquisition cards  70 , which cards convert temperature and pressure information into digital signals usable by a microprocessor. Preferably, data acquisition card  70  include inputs having over voltage protection up to +/−35V and conversion rate ranges from 0.5 Hz to 250 kHz with selectable time intervals At that can be software controlled. Triggering and sampling is controlled by the hardware on the card. The card used might also have the following sampling modes: 1) individual sampling of various channels, 2) alternate sampling of all or a subset of channels, 3) the ability to accommodate signal edge or threshold value triggering, 4) adjustable pre-triggering with a recording depth of about 16,000 values, and 5) sampling at software controlled intervals.  
         [0027]     Additionally, the data acquisition card is capable of 250 kS/s multifunction data acquisition and has a 32 bit VXD driver and a kernel driver for Microsoft Windows NT and Windows 2000 or Windows XP operating systems, 12 bit ADC with 16 single-ended or  8  differential analogue input channels with trigger and pre-trigger functions, a built in anti-aliasing filter and unipolar and bipolar input ranges. The data acquisition card is capable of single ended or differential operation and includes software controllable ranges and operation modes. The card  70  further includes digital I/O ports, is TTL compatible and has a maximum current of 4 mA. Suitable data acquisition cards are available, for example, from Keithley Instruments, Inc. of Cleveland, Ohio and from Agilent Technologies of Palo Alto, Calif. The use and operation of these cards is well known and will not be described further herein.  
         [0028]     Data from the data acquisition cards  70  is fed to microprocessor  66  running MATLAB software that includes the MATLAB Data Acquisition Toolbox. These programs are available from The Mathworks of Natick, Mass. These utilities allow data to be stored, manipulated and graphed in a user-defined manner.  
         [0029]     Referring again to  FIG. 5 , a first pressure sensor  72  is connected to first port  24  and connected to a first data acquisition card  74  via a cable  76 . First data acquisition card  74  is connected to socket input  64 -A 1  of device  44  by a second cable  78  connected to a bus  79 . Bus  79  is preferably an IEEE 488 (GPIB) bus. A second pressure sensor  80  is connected to second port  26  and connected to a second data acquisition card  82  via cable  84 . Second data acquisition card  82  is connected to socket input  64 -A 2  via a cable  86  connected to bus  79 . A third pressure sensor  88  is connected to third port  28  and connected to a third data acquisition card  90  via cable  92 . Third data acquisition card  90  is connected to socket input  64 -A 3  via a cable  94  connected to bus  79 . A fourth pressure sensor  96  is connected to fourth port  30  and connected to a fourth data acquisition card  98  via cable  100 . Fourth data acquisition card  98  is connected to socket input  64 -A 4  via a cable  102  connected to bus  79 .  
         [0030]     Temperature sensors are also connected to socket inputs of device  44  as follows. A first temperature sensor  104  is connected to the ACR system immediately upstream of compressor  12  and to a fifth data acquisition card  106  via a cable  108 . Fifth data acquisition card  106  is connected to input socket  64 -B 1  via cable  110  connected to bus  79 . A second temperature sensor  112  is connected to ACR system  10  just downstream of compressor  12  and to a sixth data acquisition card  114  via a cable  116 . Sixth data acquisition card  114  is connected to input socket  64 -B 2  via cable  117  connected to bus  79 . A third temperature sensor  118  is connected to the ACR system at the upstream side of condenser  14  and to a seventh data acquisition card  120  via a cable  122 . Seventh data acquisition card  120  is connected to input socket  64 -B 3  via cable  124  connected to bus  79 . A fourth temperature sensor  126  is connected to the ACR system at the downstream side of condenser  42  and to a eighth data acquisition card  128  via a cable  130 . Eighth data acquisition card  128  is connected to input socket  64 -B 4  via cable  132  connected to bus  79 . A fifth temperature sensor  134  is connected to the ACR system at the upstream side of evaporator  18  and to an eighth data acquisition card  136  via a cable  138 . Eighth data acquisition card  136  is connected to input socket  64 -B 5  via cable  140  connected to bus  79 . A sixth temperature sensor  142  is connected to the ACR system at the downstream side of evaporator  18  and to a ninth data acquisition card  144  via a cable  146 . Ninth data acquisition card  144  is connected to input socket  64 -B 6  via cable  148  connected to bus  79 .  
         [0031]     While a separate data acquisition card has been shown for connecting each sensor to device  10 , data acquisition cards capable of handling several channels of input may be used to reduce the number of system components. Likewise, while the various connections described above are made with cables, other types of connectors, including radio frequency transmitters and receivers could also be employed for transferring data from sensors to data acquisition cards and/or from data acquisition cards to device  10 .  
         [0032]     In use, a pressure enthalpy curve  150  for the refrigerant used in ACR system  10  is plotted on screen  56  (shown enlarged in  FIG. 5 ). Pressure enthalpy curves for more than one refrigerant may be stored in memory  68 , and the appropriate refrigerant selected at the start of monitoring. Pressure enthalpy curve  150  defines three regions, a first region  152  which represents combinations of pressure and enthalpy under which the refrigerant is entirely in a liquid state, a second region  154  which represents combinations of pressures and enthalpies under which the refrigerant is a mixture of liquid and vapor, and a third region  156  which represents combinations of pressures and enthalpies under which the refrigerant is entirely in a vapor state. Also on screen  56  is plotted a nominal pressure enthalpy path  158 , shown in dashed lines, which shows the pressures and enthalpies the refrigerant should ideally have as it circulates through ACR system  10 . This information is obtained from the manufacturer of the ACR system or, optionally, could be derived from measurements taken on the ACR system when it began operation or was otherwise known to be operating normally.  
         [0033]     Nominal pressure enthalpy path  158  includes point a 1  representing the pressure and enthalpy the refrigerant should have as it enters compressor  12 , point b 1  representing the pressure and enthalpy the refrigerant should have as it leaves the compressor, point c 1  representing the pressure and enthalpy the refrigerant should have at the point it begins to condense, point d 1  showing the pressure and enthalpy the refrigerant should have as it enters metering device  16  and point e 1  showing the pressure and enthalpy the refrigerant should have as it exits the metering device.  
         [0034]     An actual pressure enthalpy path  160  is also displayed on screen  56 . Actual pressure enthalpy path  160  is plotted from the data provided by the pressure and temperature sensors connected to the ACR system. Actual pressure enthalpy path  160  includes a point a 2  representing the actual pressure and enthalpy of the refrigerant as it enters the compressor, point b 2  representing the actual pressure and enthalpy of the refrigerant as it leaves the compressor, point c 2  representing the actual pressure and enthalpy of the refrigerant when it begins to condense, point d 2 , representing the actual pressure and enthalpy of the refrigerant when it arrives at the metering device  16  and point e 2  representing the actual condition of the refrigerant when it leaves the metering device  16 .  
         [0035]     As can be seen in  FIG. 5 , the actual pressure enthalpy path  160  is not identical to the nominal pressure enthalpy path  158 . The differences between paths  158  and  160  helps a technician diagnose various problems with the ACR system. For example, the enthalpy at point e 2  is greater than the nominal enthalpy e 1 . This shows that refrigerant is arriving at compressor  12  at a higher than normal temperature and may suggest, for example, that evaporator fan  22  is not removing sufficient heat from the refrigerant in evaporator  18 . Likewise, the vertical separation between points a 2  and a 1  suggests that insufficient pressure is being created in the compressor, possibly indicative of a failing compressor. Many other problems and potential problems with an ACR system can be recognized by a technician using such a system.  
         [0036]     Other useful quantities describing the system can also be calculated and displayed on display  56  once the data for generating the pressure enthalpy path of  FIG. 5  has been acquired. These quantities can also be compared to nominal values stored in the system to detect changes indicative of abnormal system operation. These characteristics can be used along with the pressure enthalpy path for the ACR device  10  to analyze the performance of the system. These characteristics include:  
         [0037]     Refrigeration effect, or the amount of heat absorbed by the evaporator, is shown as the difference in enthalpy at points d and e.  
         [0038]     Heat of rejection, or the amount of heat given up by the condenser, is shown by the difference in enthalpies at points b and d.  
         [0039]     Heat of Compression, or the increase in enthalpy caused compressing a vapor in the compressor, is shown by the horizontal separation between points a and b on the path.  
         [0040]     Degree of Superheating, or the amount by which the temperature of the vapor is increased beyond the vapor saturation point, is shown by the spacing between point b and the pressure enthalpy curve.  
         [0041]     Degree of Sub-cooling, or the amount by which the temperature of the refrigerant is decreased below its liquid saturation point, is shown by the distance between point d and the pressure enthalpy curve.  
         [0042]     Percentage of Flash gases. Immediately downstream of a metering device, high pressure liquid refrigerant will change into a flash gas, that is, a supercooled gas. As the flash gas absorbs heat, it will turn into vapor.  
         [0043]     Coefficient of Performance, or the ratio of the Refrigeration Effect to work input to the system. The work input to the system is the difference between the heat rejected by the condenser and the heat supplied to the evaporator.  
         [0044]      FIG. 6  illustrates the operation of the monitoring device  44 . Monitoring of the system begins at step  164 , and at step  166  sensors are connected to various parts of ACR system  10 . At step  168 , information concerning ACR system  10  is input into device  44  along with relevant environmental data such as ambient temperature. Monitoring time periods Δt are next established at step  169 . Two such periods are relevant. First, Δt a , a monitoring interval for immediately after system startup is set. Next, Δt b , or a monitoring interval for all times after startup is set. Current drawn by the compressor and other variables may be monitored by system  44 , and these variables, as well as temperature and pressure variables may change relatively quickly immediately after startup. Setting a relatively short Δt a  allows for changes occurring at startup to be observed, while a longer Δt b  after startup prevents overloading the system with data that is changing at a slower rate, if at all, after the system has reached an equilibrium operating state.  
         [0045]     At step  170 , readings are taken from sensors at the established time period, and the data is stored in memory  68 . At step  172 , a nominal pressure enthalpy path and actual pressure enthalpy path are displayed on display  56 , and the two paths are compared at step  174 . At step  176 , an operator must determine whether to stop monitoring system  10  or continue monitoring using different Δt values in order to look for different types of system faults. If additional monitoring is needed, Δt values are reset at step  178 , and the system returns to step  170  to take additional readings using the new time intervals Δt.  
         [0046]     If, at step  176  the operator decides to stop monitoring the system, monitoring is stopped at step  180  and troubleshooting begins at step  182 . The operator is prompted for additional data if necessary to diagnose certain types of problems. For example, the operator may be prompted at steps  184  to enter information concerning the age of the ACR system or the conditions under which it is normally operated or other information that is not generally obtainable from a sensor. This information helps the device  44  assess the likelihood of various system problems and suggest appropriate repairs using artificial intelligence or artificial neural network software, for example. A knowledge base of faults, their symptoms and appropriate repairs may be stored locally or device  44  may be connectable to the Internet to allow a larger, centralized knowledge base to be consulted. A fault list is established at step  186  which comprises a list of potential problems with the system, and each of the potential faults are checked, either by a technician or by analyzing additional system data, at step  188 . If the first fault is found to exist at step  190 , the technician proceeds to order or perform the necessary repair. If the fault is not present, the next fault on the fault list is checked at step  192 . Troubleshooting ends at step  196 .  
         [0047]     The present system can beneficially be used with the Neural Network Toolbox available from the Mathworks as a compliment to the MATLAB software running on microprocessor  66 . This software collects and analyzes data and learns to diagnose future ACR system problems based on the data input and faults determined at initial stages by technicians. For example, a certain problem indicated by the nominal and actual pressure enthalpy paths may suggest one of two fault conditions. However, one of the fault conditions may be much more likely for systems having compressors that are more than ten years old. This type of information would allow the device  44  to more quickly determine the problem with a later-analyzed system. In addition, such neural networks may help a technician become aware of other sets of conditions that indicate a system problem based on the amount of data collected from numerous systems.  
         [0048]     The present invention has been described in terms of a preferred embodiment. However, various additions and modifications to the preferred embodiment will become apparent to those skilled in the relevant arts upon a reading and understanding of the foregoing detailed description. For example, while the present invention relies primarily upon refrigerant temperature and pressure measurements, device  10  could also obtain additional system data from additional sensors. For example, current drawn by the compressor could be monitored as could compressor and evaporator fan speed. Making these other variables available to the system, especially when the neural network software is run, will lead to improved diagnostic accuracy. It is intended that all such obvious modifications and additions to the system comprise a part of this invention to the extent they fall within the scope of the several claims appended hereto.