Abstract:
A method and apparatus for electrically heat welding a thermoplastic electrofusion fitting having an electrically resistive element disposed therein, whereby a regulated quantity of electric power is supplied to the resistive element during the welding process and the making of a high quality weld is ensured. In accordance with the method, electric power is supplied to the resistive element of the fitting. The resistance of the electrically resistive element is measured with low voltage electric power, and the size of the fitting to be welded is determined from a data table that can be loaded and modified by the user and sorted according to previously-fused fittings. The method includes the steps of generating a alternating current voltage from a direct current power supply and transmitting the alternating current voltage to the electrically resistive element. The method also includes a method of fitting identification based upon a comparison of the resistance of the resistive element with information stored in the data table.

Description:
This application claims priority from Provisional application Ser. No. 60/174,552, filed Jan. 5, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The present invention relates generally to a method and apparatus for electrically heat welding of thermoplastic fittings, and, more particularly, to an alternating current (AC) battery-powered apparatus for electrically heat welding of thermoplastic fittings, and method of using the same. 
     B. Description of the Related Art 
     Electric heat-weldable fittings (also known as electrofusion fittings) formed of thermoplastic materials have been developed and used heretofore. Such fittings generally include an electrically resistant heating coil or element positioned adjacent the inside surfaces of the fitting which are to be welded to one or more other thermoplastic members, such as plastic pipe sections. The electrically resistant heating element typically comprises a coil of resistance wire disposed in the thermoplastic material of the fitting, and connects to electric contacts attached to an outside surface of the fitting. Examples of such electric heat weldable thermoplastic fittings are described in U.S. Pat. Nos. 4,147,926 and 4,349,219. 
     Electrofusing is an effective method of installing branch connections or tapping into a main gas pipeline. Many vendors supply electrofusion fittings, wherein each of the fittings has a particular fusion voltage and fusion time. Many of the vendors have a proprietary method for identifying the fitting to be fused or for controlling the fusion process parameters such as time and voltage applied to the fitting, forcing a user to purchase a dedicated fusion system as well as the fittings from the vendor. This marketing method prohibits the user from purchasing electrofusion fittings competitively from all suppliers. 
     A method of identifying the electrofusion fitting and setting the correct fusion voltage and time is described in U.S. Pat. No. 4,837,424. This method allows the user to purchase electrofusion fittings competitively from all suppliers. A conventional electrofusion system  1  utilizing this method is shown in FIG.  1 . In the conventional system, an electrofusion fitting  2  contains electrically resistive elements  3  embedded near the surface of the fitting. Power is supplied from a gasoline engine-powered AC generator or a battery-powered inverter  4 . The system converts this power to a fixed AC voltage and supplies the fixed AC voltage to electrofusion fitting  2  through a pair of wires  5 . A barcode wand or magnetic card reader  6  is used to scan machine readable data regarding fitting  2 . This data is used to program the electrofusion system  1  with the proper fusion time and output voltage. When the user starts the fusion, an AC voltage is supplied to resistive element  3  in electrofusion fitting  2 . Resistive element  3  generates heat, and the molten plastic then fuses and permanently joins electrofusion fitting  2  to a pipe  7 , forming a leak tight joint. When the fusion time expires, electrofusion system  1  stops the AC voltage and terminates the fusion. Data from the fusion, including, an electrofusion system serial number, fusion number, fusion date and time, fitting manufacturer, fitting type, fitting size, ambient temperature, fusion voltage, nominal fusion time , adjusted fusion time (compensated for ambient temperature), actual time fused, fusion result, operator, location, and an information field, is stored in the internal memory of electrofusion system  1 , along with the fusion mode, i.e. barcode or manual mode. This data may be downloaded at a later time to provide complete fusion traceability. 
     Conventional electrofusion system  1  is also equipped with systems to monitor the performance of the fusion as it progresses, and to terminate the fusion if abnormalities are found. Abnormalities include, but are not limited to, shorts in the fitting, low or high output voltage, insufficient supply voltage, disconnection of the fitting during fusion, or stoppage of the fusion by the user. The absence of errors indicates that the fusion was done properly. This system and method suffers from several problems. For example, this fusion system must be equipped with a barcode or magnetic card reader, devices which are not reliable in the field and which inconvenience the user when they fail. 
     Another conventional electrofusion system and method, described in U.S. Pat. No. 5,951,902, is powered by a direct current (DC) power source and supplies a direct current voltage output to the resistive element. This system also suffers from a variety of problems. For example, the DC voltage does not vibrate the fitting like the magnetic fields generated by an AC voltage (as described in U.S. Pat. No. 4,684,789). Since all electrofusion fittings are tested for proper performance and certified with an AC voltage, the electrofusion fittings may have to be re-characterized and certified with a DC voltage if the system of U.S. Pat. No. 5,951,902 is used. Another problem with this system is that fusion can begin even when there is insufficient energy in the battery to complete the fusion. This reduces the economic viability of electrofusion because the incomplete fusion must be replaced, and increases the possibility that an operator may not replace the faulty joint, resulting in a potentially lethal situation of a natural gas leak. Still another problem with this system is the lack of data storage available for traceability. This prevents analysis of fusion data to determine whether all fusions were completed without errors. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an apparatus and method for creating an electrofusion between thermoplastic members that overcomes the problems set forth above with respect to the related art. 
     A further object of the invention is to provide an easy to use apparatus and method that ensures safe, effective electrofusion between thermoplastic members. 
     Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be learned from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the method including the steps of: generating an alternating current voltage from a direct current power supply; transmitting the alternating current voltage to the electrically resistive element; and regulating the alternating current voltage transmitted to the electrically resistive element at a predetermined alternating current voltage. 
     To further achieve the objects, the present invention comprises an apparatus for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the apparatus comprising: a direct current power supply; means for generating an alternating current voltage from the power supplied by the power supply; means for transmitting the alternating current voltage to the electrically resistive element; and means for regulating the alternating current voltage transmitted to the electrically resistive element at a predetermined alternating current voltage. 
     To achieve these objects, the present invention also comprises a method and an apparatus for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the apparatus comprising: means for identifying the thermoplastic member having the electrically resistive element embedded therein by measuring the resistance of the electrically resistive element; a programmable electronic data processing means for executing programmed arithmetic and logical processes and storing data, wherein the data processing means compares the resistance to a data table stored in the data processing means, and sorts duplicate fittings by the most frequently-used fitting; means for generating an alternating current voltage from a direct current power supply; and means for transmitting the alternating current voltage to the electrically resistive element. 
     To further achieve these objects, the present invention comprises a method and an apparatus for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the apparatus comprising: means for identifying the thermoplastic member having the electrically resistive element embedded therein by measuring the resistance of the electrically resistive element; a programmable electronic data processing means for executing programmed arithmetic and logical processes and storing data, wherein the data processing means compares the resistance to a data table stored in the data processing means, and sorts duplicate fittings by the most frequently-used fitting; means for generating a voltage from a power supply; and means for transmitting the voltage to the electrically resistive element. 
     Another feature of the present invention and further in accordance with the objects, the present invention comprises a method and apparatus for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the apparatus comprising: a programmable electronic data processing means for executing programmed arithmetic and logical processes and storing data, wherein the data processing means checks that sufficient energy is available in a direct current power supply to provide enough energy to the electrically resistive element to complete the electrofusion; means for generating an alternating current voltage from the direct current power supply; and means for transmitting the alternating current voltage to the electrically resistive element. 
     Still another feature of the present invention and further in accordance with the objects, the present invention comprises a method and an apparatus for creating an electrofusion between a plurality of thermoplastic members, wherein at least one of the members has an electrically resistive element embedded therein, the apparatus comprising: a programmable electronic data processing means for executing programmed arithmetic and logical processes and storing data, wherein the data processing means checks that sufficient energy is available in a power supply to provide enough energy to the electrically resistive element to complete the electrofusion; means for generating a voltage from the power supply; and means for transmitting the voltage to the electrically resistive element. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and together with the description, serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a block diagram illustrating a conventional electrofusion system; 
     FIGS. 2A,  2 B,  2 C, and  2 D is a block diagram illustrating the electrofusion system of a preferred embodiment of the present invention; 
     FIGS. 3A,  3 B,  3 C, and  3 D is an electrical schematic of a computer, readout, EPROM and flash memory of the electrofusion system shown in FIGS. 2A,  2 B,  2 C, and  2 D; 
     FIGS. 4A,  4 B,  4 C, and  4 D is an electrical schematic of a resistance measuring circuit, multiplexer and analog-to-digital converter of the electrofusion system shown in FIGS. 2A,  2 B,  2 C, and  2 D; 
     FIGS. 5A,  5 B,  5 C, and  5 D is an electrical schematic of a power supply and low voltage shutoff of the electrofusion system shown in FIGS. 2A,  2 B,  2 C, and  2 D; and 
     FIGS. 6A,  6 B,  6 C, and  6 D is an electrical schematic of an AC generator/voltage control, current sensor and voltage sensor of the electrofusion system shown in FIGS. 2A,  2 B,  2 C, and  2 D. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     A. General System Description 
     FIG. 2 illustrates an electrofusion system  8  of the preferred embodiment of the present invention. Electrofusion system  8  includes an AC generator and voltage control  9  that provides an AC voltage to a resistive element  10  embedded in a thermoplastic electrofusion fitting  11  that is to be fused to a thermoplastic pipe  12 . Electrofusion system  8  further comprises an LCD display  13  and pushbuttons  14  which the user reads and manipulates to control system  8 , and a microprocessor  15  which controls the operation of system  8 . The system  8  also includes a DC power source  16  which may comprise a primary battery located inside or outside system  8 , a secondary battery located inside or outside system  8 , a full cell located inside or outside system  8 , or any other DC power source known to one of ordinary skill in the art. As used herein, the term “primary batter” means a battery that cannot be charged, whereas the term “secondary battery” means a battery that can be charged. Examples of batteries capable of use with the present invention include twenty-four two-volt, sealed lead acid batteries, sixteen three-volt lithium batteries, forty nicad batteries, thirty-two D-size flashlight batteries, or a truck battery. 
     Operation of electrofusion system  8  may be subdivided into three distinct parts: (1) the charging operation; (2) the fusing operation; and (3) the uploading/identification operation. 
     1. The Charging Operation 
     The charging operation includes a charger circuit  17  comprised of: a 120 VAC plug  18 ; a fuse  19 ; an isolation transformer  20 ; bridge rectifiers  130 ; a rectifier  21 ; a silicone controlled rectifier (SCR)  22  to control the charging voltage; a zero crossing detector  23 ; and a charging relay  24 . When 120 VAC plug  18  is plugged into an external 120 volt AC source, current flows to isolation transformer  20  and energizes relay coil  25 , closing relay contact  26  and providing a DC voltage to power supply  27 . Microprocessor  15  accordingly boots and looks for zero crossings from zero crossing detector  23 . When the zero crossings are detected, microprocessor  15  starts the battery charging code, which, in turn, starts a three-phase charging routine. The charging routine fires SCR  22  to maintain the proper charging current or voltage. A battery measurement circuit  28  and a current measurement circuit  29  are used in conjunction with a multiplexer  30  and an analog-to-digital converter  31  to measure battery  16  current and voltage and to provide feedback to the charging control algorithm. 
     The three-phase charging routine starts by closing charging relay  24 , then charges power source  16  at a high constant current until the voltage of power source  16  reaches a predetermined point. The charging routine then switches to a constant voltage charge until the current drops to a predetermined level. Next, the charging routine switches to a timed charge for a predetermined time. After the predetermined time has expired, charging relay  24  is opened to terminate the charge. When the charge is terminated, a battery capacity monitor  32  is loaded with the amount of charge that was returned to power source  16 . This information is used to determine if there is enough energy in power source  16  to complete a fusion. 
     2. The Fusing Operation 
     The fusing operation is started by first turning an on/off switch  33  to the on position. On/off switch  33  is used to either turn the system on or off in normal operation or to cut off power in an emergency. When on/off switch  33  is turned to the on position, direct current voltage goes to power supply  27  which, in turn, supplies the +5, −5, and +12 volt direct current necessary to operate all circuits. A low voltage detector  34  monitors the voltage of the power source  16  and shuts down power supply  27  if the system is inadvertently left on, protecting power source  16  from damage. A circuit breaker  35  is also provided to protect power source  16  from a short circuit. When on/off switch  33  is turned on, microprocessor  15  boots, conducts a pre-startup checkout to insure that it is operating properly, loads all measurement calibration data from internal memory, verifies that all calibration data is within acceptable tolerances, and instructs the operator to connect fitting  11  to output leads  36 . Microprocessor  15  then closes a switch  37  which, in turn, switches a low voltage source  38  through a resistance-current measurement circuit  39 , AC generator and voltage control  9  and output leads  36 . Microprocessor  15  also switches multiplexer  30  to read the current provided to fitting  11  with resistance-current measurement circuit  39 , and to read the voltage provided to fitting  11  with a voltage measurement circuit  40 . Microprocessor  15  then calculates the fitting resistance using Ohms Law (Resistance =Voltage/Current). 
     Microprocessor  15  then compares the fitting resistance to the data in a fitting resistance table  41 , stored in flash memory  42 , in order to find a unique match. If there are multiple fittings with the same resistance, then these fittings are sorted by the most frequently used fittings such that the most frequently used fitting is displayed first in order to eliminate user confusion. Fitting information is then displayed on LCD display  13  to allow the user to select the proper fitting to be fused by scrolling through the list with pushbuttons  14 . When the user selects the proper fitting, microprocessor  15  performs the following steps: 
     (1) re-measure the fitting resistance to verify that the resistance is within the specified tolerance; 
     (2) check to see if there is enough energy in the battery to fuse fitting  11  to completion; 
     (3) set the output voltage control to the correct voltage from a fitting data table  43 ; and 
     (4) measure an external temperature sensor  45  (or measure an internal temperature sensor  44  if external temperature sensor  45  is not attached), and calculate the appropriate fusion time with the fusion time and temperature compensation time from fitting data table  43 ; 
     (5) close a power contactor  46 , turn on AC generator and voltage control  9 ; 
     (6) measure the fitting voltage with fitting voltage measurement circuit  40 ; and 
     (7) modulate AC generator and voltage control  9  to force the voltage on fitting  11  to be equal to the voltage obtained from fitting data table  43 . 
     While the fusion is in progress, microprocessor  15  measures fitting voltage using fitting voltage measurement circuit  40  and further measures the fitting current using current measurement circuit  39  to assure that the fusion is progressing properly. If the fitting voltage drifts out of its control limits of ± two percent, or the current rises outside of its limits, the fusion is terminated and the proper error is displayed to the user on LCD display  13 . If no errors occur then the fusion is terminated at a proper time calculated earlier. 
     When the fusion is terminated, normally or otherwise, fusion data is written to flash memory  42  for later downloading. The fusion data written to memory  42  includes, but is not limited to: electrofusion system serial number, electrofusion system model number, fusion number, fusion date and time, fitting manufacturer, fitting type, fitting size, ambient temperature, fusion voltage, nominal fusion time, adjusted fusion time (compensated for ambient temperature), actual time fused, measured fitting resistance, highest and lowest measured output voltage, highest and lowest measured power supply voltage, highest and lowest calculated duty cycle, fusion result, operator, and location. 
     3. The Uploading/ldentification Operation 
     The system for uploading fitting data and identifying a thermoplastic electrofusion fitting includes electrofusion system  8 , a personal computer  47 , and a serial interface cable  48 . Preferably, personal computer  47  comprises a typical microprocessor-based computing device such as an IBM-compatible personal computer. Software running on personal computer  47  is used to generate custom lists of thermoplastic electrofusion fittings  11 . These lists are saved as fitting data files (one fitting data file for each list) on some type of storage media compatible with the hardware available to personal computer  47 , such as, for example, a hard disk drive, floppy disk drive, CD ROM, etc. Each fitting data file will include a version number, a version date, a version title as well as the number of thermoplastic electrofusion fittings included in the list. In addition, each fitting data file includes the following information for each thermoplastic electrofusion fitting in the list: (1) manufacturer of the thermoplastic electrofusion fitting; (2) type of the thermoplastic electrofusion fitting; (3) size of the thermoplastic electrofusion fitting; (4) resistance of the thermoplastic electrofusion fitting; (5) resistance tolerance of the thermoplastic electrofusion fitting; (6) fusion voltage; (7) fusion time (in seconds); (8) time/temperature compensation; and (9) cooling time. 
     Each fusion data file also includes a checksum character and an end of file character used by electrofusion system  8  to determine when the file was received and to verify that the contents are correct. Electrofusion system  8  connects to personal computer  47  using serial interface cable  48 , and software running on computer  47  transfers the requested fitting data file to microprocessor  15 . Using the checksum and end of file characters, microprocessor  15  verifies that the fitting data file has been received without error. The data is then stored in flash memory chip  42  in 3 separate parts: 
     (1) fitting data table  43 , as described above; 
     (2) fitting resistance table  41  containing the minimum and maximum allowable values for each thermoplastic electrofusion fitting; the values in fitting resistance table  41  are calculated using the resistance and resistance tolerance specified in fitting data table  43 ; and 
     (3) a fitting use table  49  containing the number of fusions that have been performed using the respective thermoplastic electrofusion fitting; when electrofusion system  8  is reprogrammed, the values in fitting use table  49  are reset to zero and are incremented by one each time that particular fitting is fused. 
     As described in Section A.2, when a thermoplastic fitting is connected to output leads  36 , microprocessor  15  calculates the resistance of thermoplastic fitting  11 . This measured resistance is then compared to the maximum and minimum resistance values stored in fitting resistance table  41  and a list of possible fittings is compiled. If there are no fittings in fitting resistance table  41  that match the measured resistance, an appropriate error message is displayed on LCD  13 . If one fitting in fitting resistance table  41  matches the measured resistance, then microprocessor  15  obtains a detailed fitting description from fitting data table  43  and displays that data on LCD  13 . If more than one fitting in fitting resistance table  41  matches the measured resistance, then microprocessor  15  will first prioritize the list based on information contained in fitting use table  49  so that the fitting used the most in the past will be displayed first. Microprocessor  15  then obtains a detailed fitting description from fitting data table  43  and displays that data on LCD  13 . The operator may now use pushbuttons  14  to scroll through the list of possible fittings. When the operator has selected the correct fitting description, electrofusion system  8  applies the correct voltage for the time calculated using the data from fitting data table  43  as described in Section A.2 above. This data is stored in flash memory  42  for future downloading. 
     The automatic fitting identification process can be used with any electrofusion system, processor and power supply currently available. For example, the automatic fitting identification process is not limited to use with battery-powered DC power source  16 , but may be used with any DC or AC power supply. 
     B. Detailed Circuit Description 
     FIG. 3 illustrates the preferred embodiment of microprocessor  15  and associated support circuits. Power for microprocessor  15  and associated support circuits  60  is supplied from power supply  27 . Operator interface is handled by twenty-character-by-four-line liquid crystal display  13  and pushbuttons  14 . A display select circuit  58  is used to select the display during specific memory writes. A low voltage detector  53  monitors the five-volt supply and resets microprocessor  15  if the five-volt supply drops below  4 . 5  volts. An EPROM  54  stores the program, a static RAM  55  stores temporary variables, and flash memory  42  stores fitting data, fusion data and charging history. A serial port driver chip  57  provides data communication between computer  47  and microprocessor  15 . Time is kept by a real time clock chip  59 , wherein power supply  27  allows real time clock chip  59  to run when microprocessor  15  is turned off. 
     FIG.4 illustrates the preferred embodiment of the analog measurement circuits. All analog measurements are handled by analog-to-digital (A/D) converter  31 . Digital data is communicated serially to microprocessor  15  by ADC-CS, SCK, MOSI, and MISO lines  63 . All analog information is compared to a voltage reference  64  and converted to digital data by A/D converter  31 . Multiplexer  30  is used to switch the various analog signals to A/D converter  31  for measurement. Multiplexer  30  is controlled by microprocessor  15  through data lines SEL 0 , SEL 1  and SEL 2  on A/D converter  31 . Actual values such as temperature and fitting resistance use one or more measured values and require calculations by microprocessor  15 . 
     Input INO connects to ground and is used to measure the ground potential and compensate for voltage offsets in A/D converter  31 . Input IN 2  measures battery voltage to be used in the battery charging control. Input IN 3  measures the output of a fitting resistance current amplifier  67 . Input IN 4  measures the output of a fitting resistance voltage amplifier  68 . Input IN 5  measures the output from a fitting fusion voltage RMS-to-DC converter  69 . Input IN 6  measures the output of a fitting current RMS-to-DC converter  70 . Input IN 7  measures the voltage from temperature sensors  71 . A temperature selector  80  selects either the internal or external temperature sensor  71  and is controlled by microprocessor  15  through data line SEL 3  in A/D converter  31 . A fitting voltage amplifier circuit  40  measures the fitting voltage, and comprises voltage dividers  73 ,  74  which lower the voltage into the common mode range of the rest of the circuit, unity gain amplifiers  75 ,  76 , and a differential amplifier  77 . A current amplifier  78  amplifies the voltage drop across a current shunt  79  (shown in FIG.  6 ). 
     1. Battery Voltage Measurement 
     As shown in FIGS. 3-6, during normal operation, on/off switch  33  is closed and current flows through a wire  109 , a diode  86 , a voltage divider  84  to lower the voltage to an appropriate level for the rest of the circuits, a wire  110 , multiplexer  30 , input IN 2 , and to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31 , and multiplies the analog voltage by the ratio provided by voltage divider  84  and by a calibration gain factor stored in flash memory  42 , to measure the actual battery voltage. During the charging cycle when on/off switch  33  is open, the battery charge voltage is measured through a diode  85 , voltage divider  84 , wire  110 , multiplexer  30 , input IN 2 , and A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31  and multiplies the analog voltage by the ratio provided by voltage divider  84  and by the calibration gain factor stored in flash memory  42 , to measure the actual battery charge voltage. 
     2. Fitting Resistance Measurement 
     During normal operation, fitting  11  is connected to output cable  36 . The fitting resistance is measured by energizing fitting  11  with a low voltage, measuring the current through fitting  11  and the voltage drop across fitting  11  and calculating the resistance using Ohms Law. Microprocessor  15  sets an output FIRE 1  to high which causes an AC Driver  116  to turn on MOSFETs  117 ,  120 . Microprocessor  15  also sets an output FIRE 2 /MEAS-RES to low which turns on a fitting resistance current source  82 , causing current to flow through a current limiting resistor  122 , a current sense resistor  123 , a diode  83 , a wire  124 , MOSFET  117 , the left wire of output cable  36 , the right wire of output cable  36 , MOSFET  120 , and current shunt  79 , to ground. A fitting resistance current amplifier  67  amplifies the voltage drop across a current sense resistor  123 , and sends the amplified voltage through multiplexer  30  to the A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31  and multiplies the analog voltage by the calibration gain factor stored in flash memory  42  to measure the actual current flowing through fitting  11 . The voltage drop across fitting  11  is sensed by wires V-SENSE-H, V-SENSE-L. This voltage is amplified by fitting voltage amplifier  40 , is further amplified by a fitting resistance-voltage amplifier  68 , and is sent through multiplexer  30  to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31 , and multiplies the analog voltage by the calibration gain factor stored flash memory  42  to measure the actual voltage across fitting  11 . Microprocessor  15  performs this calculation to calculate the actual fitting resistance. 
     3. Fitting Voltage Measurement 
     An accurate fitting voltage measurement is needed in order to control the AC voltage output. The fitting voltage is measured by transmitting the actual fitting voltage through the sense wires located in output cable  36 . Current travels through wires V-SENSE-H, V-SENSE-L, and voltage dividers  73 ,  74  to bring the voltage level down to the common mode range. The signals traveling through wires V-SENSE-H, V-SENSE-L are then buffered through unity gain amplifiers  75 ,  76  before passing through differential amplifier  77  to isolate the voltage drop across fitting  11 . The output of differential amplifier  77  is then passed through a RMS-to-DC converter in order to convert the AC signal to a DC value required by A/D converter  31 . This signal is then sent through multiplexer  30  to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31 , and multiplies the analog voltage by the calibration gain factor stored in flash memory  42  to measure the actual voltage across the fitting. 
     4. Fitting Current Measurement 
     An accurate fitting current measurement is needed in order to make battery capacity calculations as well as to detect a variety of errors including a rapid rise in fitting current, a rapid decrease in fitting current, or a fitting disconnection. Fitting current is calculated using the fact that current through a resistor is directly proportional to the voltage drop across it, wherein the resistor is current shunt  79 , and the voltage drop is the voltage drop across shunt  79 . Voltages from the load side and the ground side of shunt  79  are transmitted through wires I-SENSE-H, I-SENSE-L to current amplifier  78 . The output of current amplifier  78  is then passed through a RMS-to-DC converter in order to convert the AC signal to a DC value required by A/D converter  31 . This signal is then sent through multiplexer  30  to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31 , and multiplies the analog voltage by the calibration gain factor stored in flash memory  42  to measure the actual current through fitting  11 . This method of using the voltage drop across current shunt  79  is also used to calculate current passing to batteries  113  during the charging operation (described below). 
     5. Temperature Measurement 
     Accurate temperature measurements are required in order to accurately compensate fusion times based on compensation factors specified by fitting manufacturers as well as to adjust the capacity of the batteries as they becomes less efficient at lower temperatures. The preferred embodiment of the invention includes circuitry to perform measurements from either internal temperature sensor  71  located within electrofusion system  8 , or external temperature sensor  71  located near fitting  11 . Microprocessor  15  selects either the internal or the external temperature measuring circuitry by setting data line SEL 3  of A/D converter  31  to high or low. When data line SEL 3  of A/D converter  31  is set to high, a temperature selector  80  enables the external temperature measuring circuitry, and when data line SEL 3  of A/D converter  31  is set to low, temperature selector  80  enables the internal temperature measuring circuitry. Temperature is measured by calculating the resistance of thermistor  71 , and using this resistance in The Steinhart and Hart equation to calculate a temperature. When microprocessor  15  selects the internal temperature sensor, microprocessor  15  measures the voltage from a voltage divider formed by a resistor  81  and the internal temperature sensor. This signal is then sent through multiplexer  30  to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31  and converts that value to a voltage based on VREF of A/D converter  31 . When microprocessor  15  selects the external temperature sensor, microprocessor  15  measures the voltage from a voltage divider formed by resistor  81  and the external temperature sensor. This signal is then sent through multiplexer  30  to A/D converter  31 . Microprocessor  15  receives the data from A/D converter  31  and converts that value to a voltage based on VREF of A/D converter  31 . 
     Once microprocessor  15  receives the data from A/D converter  31  and calculates the voltage drop across thermistor  71 , the thermistor resistance (internal or external) is calculated as follows: 
     Internal Thermistor Resistance =(Voltage drop across the internal thermistor×reference resistor  81 )/(VREF−Voltage drop across the internal thermistor) 
     External Thermistor Resistance =(Voltage drop across the external thermistor×reference resistor  81 )/(VREF−Voltage drop across the external thermistor) 
     Microprocessor  15  multiplies the thermistor resistance by the calibration gain factor stored in flash memory  42  to measure the actual thermistor resistance. This value is then used in The Steinhart and Hart equation to calculate a temperature. 
     C. AC Output 
     This section describes a method for generating an AC voltage and current to be supplied to electrofusion fitting  11  in order to complete a thermoplastic weld. The example described herein uses a square wave output, however any output known to one of ordinary skill in the art may be used, such as, for example, a sine wave, a pseudo-sine wave, or a similar AC waveform. 
     The output of the electrofusion system  8  is an AC voltage because a conventional DC voltage fails to vibrate fitting  11  like the magnetic fields present in an AC voltage vibrate fitting  11 . Furthermore, since all electrofusion fittings are tested for proper performance and certified with an AC voltage, the electrofusion fittings may have to be re-characterized and certified if a DC voltage were used. The AC output voltage of the present invention is provided as follows. While fusing, microprocessor  15  controls two output lines FIRE-MOSFET, FIRE 1 /MEAS-RES in order to establish the frequency and duty cycle of the AC output. Output lines FIRE-MOSFET, FIRE 1 /MEAS-RES enable an AC output driver  116  to control AC output MOSFETs  121 , enabling current to flow in alternating directions through fitting  11 . 
     When the FIRE-MOSFET output is high, AC output driver  116  energizes MOSFETs  117 ,  120 . This permits current to flow from the positive terminal of battery pack  113 , through a circuit breaker  114 , on-off switch  33 , MOSFET  117 , the fourth terminal of output terminal  112 , the left side of output cable  36 , fitting  11 , the right side of output cable  36 , the first terminal of output terminal  112 , MOSFET  120 , current shunt  79 , and back to the negative terminal of battery pack  113 . 
     When the FIRE 1 /MEAS-RES output is high, AC output driver  116  energizes MOSFETs  118 ,  119 . This permits current to flow from the positive terminal of battery pack  113 , through circuit breaker  114 , on-off switch  33 , MOSFET  119 , the first terminal of output terminal  112 , the right side of output cable  36 , fitting  11 , the left side of output cable  36 , the fourth terminal of output terminal  112 , MOSFET  118 , current shunt  79  and back to the negative terminal of battery pack  113 . 
     By using the voltage measurement circuit described above as a feedback, microprocessor  15  times when outputs FIRE-MOSFET, FIRE 1 /MEAS-RES are to be turned on and off in order maintain a pre-programmed, AC voltage across fitting  11 . Preferably, a 60 Hertz AC voltage is maintained across fitting  11 . 
     D. Power Supply 
     While in operation, regulated DC power is required by all active components, and there are several types of regulated DC voltages generated. This section explains how these voltages are generated under normal conditions and when electrofusion system  8  is charging. The systems used for generating the various supply voltages are collectively referred to as power supply  27 . 
     1 . +5 Volt Supply 
     While in normal operation, the +5 volt supply is generated as follows. When on-off switch  33  is closed, current flows from the positive terminal of battery pack  113 , through circuit breaker  114 , on-off switch  33 , the normally closed contacts of a relay  26 , a diode  158  and into a +5-volt regulator  100 . The output from +5-volt regulator  100  is used to power microprocessor  15 , as well as other analog and digital components shown in FIGS. 3,  4  and  6 . 
     When electrofusion system  8  is charging, the +5 volt supply is generated as follows. When the 120-volt AC source is connected, current flows through a fuse  127 , and powers a transformer  92 . Power is taken from a secondary winding  129  of transformer  92  and is passed through a bridge rectifier  91 . The unfiltered, rectified output from bridge rectifier  91 , passes through a diode  126 , and energizes the coil of relay  99 . When the coil energizes, the unfiltered, rectified output from bridge rectifier  91  is allowed to pass through a diode  126  into +5-volt regulator  100 . The output from +5-volt regulator  100  is used to power microprocessor  15 , as well as other analog and digital components shown in FIGS. 3,  4  and  6 , as described above during normal operation. As soon as this +5 volt source is generated, microprocessor  15  will “boot up” and begin processing instructions. 
     2. −5 Volt Supply 
     Certain devices require a −5 volt supply both when electrofusion system  8  is operating normally and when it is charging. This −5 volt supply is generated by passing a +5 volt supply through a −5-volt regulator  102 . This −5 volt supply is then used by various components shown in FIGS. 3,  4  and  6 . The +5 volt supply is generated while operating normally and while charging, as described above. 
     3. +12 Volt Supply 
     AC output driver  116  requires a +12 volt supply both when electrofusion system  8  is operating normally and when it is charging. This +12 volt supply is generated by passing a +5 volt supply through a +12-volt regulator  104 . This +12 volt supply is then used by AC output driver  116 . The +12 volt supply is generated while operating normally and while charging, as described above. 
     E. Low Voltage Cutoff Circuit 
     In order to prevent damage to batteries  113  if on-off switch  33  is left in the on position for long period of time, a low voltage protection circuit is used to shut off power supply  27 , thereby, automatically turning electrofusion system  8  off if the voltage from batteries  113  falls below a certain level. 
     When on-off switch  33  is closed, current flows from the positive terminal of battery pack  113  through circuit breaker  114 , on-off switch  33 , and into low voltage cutoff circuit  34 . The current then passes through a voltage divider in order to bring the voltage down to a level compatible with low voltage detector  104 . 
     If the voltage on the second pin of low voltage detector  104  is above a cutoff point, then the second pin of transistor  107  is pulled to +5 volts. This causes current to flow through transistor  107 , bringing the fifth pin of +5-volt regulator  100  to ground. This action turns on +5-volt regulator  100 , providing power to microprocessor  15 , as well as other analog and digital components shown in FIGS. 3,  4  and  6 . 
     When the voltage on the second pin of low voltage detector  104  falls below the cutoff point, the second pin of transistor  107  is pulled to ground. This causes current to stop flowing through transistor  107 , bringing the fifth pin of +5-volt regulator  100  to +5 volts. This action turns off +5 volt regulator  100 , removing power to microprocessor  15  and other analog and digital components shown in FIGS. 3,  4  and  6 . This, in turn, causes electrofusion system  8  to turn off. 
     While the processor is charging, low voltage detector  104  is bypassed. Diode  105  causes the second pin of transistor  107  to be pulled to +5 volts, causing current to flow through transistor  107  and bringing the fifth pin of +5-volt regulator  100  to ground. This action turns on +5-volt regulator  100 , providing power to microprocessor  15 , as well as other analog and digital components shown in FIGS. 3,  4  and  6 . 
     F. Charging Circuit 
     In the preferred embodiment of the invention, microprocessor  15  controls a three-phase charging cycle to fully charge batteries  113 . As described above, when the 120 VAC source is connected, power supply  27  is energized, causing microprocessor  15  to “boot up” and begin executing instruction cycles. At this time a signal from a secondary winding  128  of transformer  92  is passed through a zero crossing detector  93  and a series of buffers, into microprocessor  15 . Microprocessor  15  detects these signals upon starting and automatically executes the charging algorithm. Microprocessor  15  calculates the frequency of the  120  volt supply by measuring the time between zero crossing pulses, and then energizes a CHARGE-BATTERY line. The CHARGE-BATTERY signal passes through an output driver  95 , energizing the coil of a charging relay  87 . When the contacts of charging relay  87  close, the charging system is connected to batteries  113 . 
     Power from secondary windings  128 ,  129  of transformer  92  is passed through bridge rectifiers  90 ,  91 , respectively. The resulting, combined, rectified, unfiltered output from bridge rectifiers  90 ,  91  provides the power for charging batteries  113 . 
     During the charging cycle, microprocessor  15  uses the FIRE 1  output, combined with the CHARGE-BATTERY output. When the CHARGE-BATTERY output is passed through an AND gate  96  from the SCR-ON signal, which when passed through output driver  95 , forms the FIRE-SCR signal to fire SCR  88 . By firing SCR  88 , power is transmitted from bridge rectifiers  90 , 91 , through a diode  21  the closed contacts of charging relay  87 , the CHARGER-OUTPUT line, circuit breaker  114 , to the positive terminal of battery pack  113 . The circuit is completed when the current is transmitted through current shunt  79  back through the CHARGER-GROUND line. 
     Microprocessor  15  measures battery voltage and current, using the methods described above, as feedback variables in order to control the charging cycle. The charging cycle is completed as follows: 
     Phase I (Constant Current) 
     Microprocessor  15  uses a phase angle firing technique to vary the CHARGER-OUTPUT signal in order to maintain a constant current through batteries  113 . When the CHARGER-OUTPUT voltage reaches a pre-defined level, microprocessor  15  switches to Phase II; 
     Phase II (Constant Voltage) 
     Microprocessor  15  uses a phase angle firing technique in order to maintain a constant CHARGER-OUTPUT voltage. When the measured current through batteries  113  fall below a pre-determined level, microprocessor  15  switches to Phase III; and 
     Phase III (Timed Charge) 
     Microprocessor  15  uses a phase angle firing technique in order to maintain a constant CHARGER-OUTPUT voltage for a predetermined amount of time. When the pre-determined time expires, the charge is complete. 
     After the charge, microprocessor  15  de-energizes the CHARGE-BATTERY line. This, in turn, forces output driver  95  to de-energize the coil of charging relay  87 . When the contacts of charging relay  87  open, the charging system is physically disconnected from batteries  113 . Also, when the CHARGE-BATTERY line is de-energized, AND gate  96  will not permit SCR  88  to be fired. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.