The run-out ratio is a simple method of monitoring heat input levels without the need to measure arc voltage, current, or travel speed directly. The run-out ratio is based on the relationship between welding current and the rate at which an electrode is consumed. It has been shown that the rate at which an electrode is consumed and the welding current are proportional. Therefore, for a given electrode diameter, the ratio of the length of electrode consumed to the length of the weld deposited is proportional to the heat input. The inverse of this ratio (i.e., the length of the weld deposited to the length of electrode consumed) is defined as the run-out ratio. The run-out ratio is used to control welding heat input by either specifying a run-out ratio that corresponds to the required heat input level or by specifying a run-out length for an entire electrode. Run-out ratios for most EXX18-type electrodes for various electrode diameters are shown in Table below.
To control the risk of burnthrough, the minimum required run-out ratio that corresponds to the maximum-allowable heat input should be specified. Alternatively, minimum required run-out length for an entire electrode could be specified. The values in Table end to provide a good lower-bound estimate of heat input, so they should be used with caution to control the risk of burnthrough. Heat input is directly proportional to welding current and voltage and inversely proportional to travel speed. The level of welding current required for a given electrode tends to increase proportionally with electrode diameter. A general rule of thumb is that the approximate current level required for a given electrode diameter is the diameter in inches times 1000 (e.g., 125 amps for a 3.2 mm [0.125-in.] diameter electrode.
Reference: ESSENTIAL ELEMENTS OF AN EFFECTIVE IN-SERVICE WELDING PROGRAM, Bruce W A and Etheridge B
Keep reading, happy welding