Thermoplastic fitting electric heat welding method and apparatus

A method of electrically heat welding a thermoplastic fitting having an electric resistance heating element disposed therein whereby a predetermined regulated quantity of electric power is supplied to the heating element during the welding process and the making of a high quality weld is insured comprising the steps of:

• (a) sensing the initial temperature of the outside surface of said fitting;
• (b) connecting said heating element of said fitting to an electric power source;
• (c) supplying electric power to said heating element at a minimum voltage level for measuring the resistance of said element without significantly heating said element;
• (d) measuring the resistance of said heating element and thereby determining the initial temperature of said element;
• (e) supplying electric power to said heating element at a controlled voltage whereby said element is heated;
• (f) sensing the initial magnitude of the current flowing through said heating element and comparing such magnitude and said initial temperature of said element with predetermined current levels for heating elements of various sizes of fittings at various temperatures to thereby determine the size of said fitting being welded and the total time the controlled voltage electric power should be supplied to said heating element of said fitting to insure the making of a high quality weld;
• (g) continuing to sense the magnitude of the current flowing through said heating element over the time said controlled electric power is supplied thereto and comparing such magnitude at predetermined time intervals with predetermined current levels for the size of fitting determined in step (f) to thereby determine if the welding process is proceeding abnormally at such time intervals;
• (h) determining the increase or decrease in the total time the controlled voltage electric power should be supplied to said heating element to cause a proper quantity of thermoplastic material making up said fitting to melt based on the initial temperature of said element determined in step (d) and the initial outside surface temperature of said fitting determined in step (a);
• (i) adjusting the time the supply of electric power should be supplied to said heating element in accordance with said increase or decrease;
• (j) terminating the supply of electric power to said heating element of said fitting when it is determined that said welding process is proceeding abnormally in step (g) or otherwise at the end of the total time required for the making of a high quality weld as determined in accordance with step (i);
• (k) supplying electric power to said heating element at a minimum voltage level for measuring the resistance of said element without significantly heating said element;
• and
• (l) measuring the resistance of said heating element and thereby determining the final temperature of said element.

Apparatus for electrically heat welding a thermoplastic fitting having an electric resistance heating element disposed therein connected to a pair of electric contacts attached to said fitting whereby a regulated quantity of electric power is supplied to the heating element during the welding process and the making of a high quality weld is insured comprising:

• (a) first electric power generating means for producing controlled voltage electric power to heat said heating element of said fitting;
• (b) second electric power generating means for producing low voltage electric power for measuring the resistance of said element without significantly heating said element;
• (c) current sensor means for sensing the magnitude of the current flow from said first electric power generating means electrically connected thereto;
• (d) resistance sensor means for sensing the magnitude of the resistance of said heating element electrically connected to said second electric power generating means;
• (e) electric cable means connected to said first and second electric power generating means and adapted for removable connection to said electric contacts of said fitting for conducting electric power to said electric resistance heating element of said fitting;
• (f) switch means electrically connected between said first and second electric power generating means and said cable means;
• and
• (g) computer means operably connected to said current sensor means, said resistance sensor means and said switch means for receiving the resistance of said heating element and determining the initial and final temperatures of said element therefrom, for receiving and comparing the initial temperature of said heating element and the initial magnitude of the current flowing from said first electric power generating means through said heating element with predetermined temperatures and current levels for various sizes of fittings and thereby determining the size of fitting being welded and the total time the constant voltage power from said first power generating means should be supplied to said heating element for the making of a high quality weld, for comparing the magnitude of the current flowing through said heating element from said first power generating means at predetermined time intervals with predetermined current levels for the size of fitting being welded to thereby determine if the welding process is proceeding abnormally at such time intervals, for operating said switch means to start and terminate the supply of electric power to said heating element from said first and second electric power generating means, and for operating said switch means to terminate the supply of electric power to said heating element from said first electric power generating means when said welding process is proceeding abnormally or otherwise at the end of the total time determined to be required for the making of a high quality weld.