Method of detecting residual amount of web roll

A method of measuring and expressing the unwound amount or residual amount of a web roll wound on a winding core, such as carbon film roll or printing paper roll, in terms of a unit length, said method comprising the steps of:

• starting a count of pulses to be produced by a rotational displacement detecting sensor for each predetermined angle of rotational displacement of said web roll, said count being initiated when the diameter of said web roll has been measured to a certain preset factor of the diameter of said winding core, said diameter of said web roll being correlated to a predetermined residual amount of the web roll;
• and warning when said count reaches a predetermined count number (initially set on the basis of the average value of the amounts of unwinding rotation of said web roll per unit length) during the time when said web roll is unwound and fed by a unit length from the count start time.