By AUGUST MANZ Fellow of the American Welding Society

edited by KIMBERLEY GILLES, associate editor

In arc welding, the number of amperes (amps) indicates the size and strength of direct current arcs that are electrode negative (DCEN) or electrode positive (DCEP). They also are a measure of balanced wave alternating current (BWAC), conventional unbalanced waves (AC plus a DC component), and pulsed current waveforms, among others.

Current can be measured with analog meters and with hot-wire ammeters, which are based on a resistance heating mechanism that causes a pointer to move. Ammeters — sometimes called ampmeters — also can be based on magnetic devices, including hall-effect devices that do not have moving parts. Lastly, computer systems can measure current by calculating the area of the shape of a current's wave and presenting the results in digital form. An ampere is defined by an agreed upon electrical engineering definition based on the heating effect of a direct current passing through a resistance. In practical terms, one ampere is the amount of current that must pass through a 1 ohm resistor to create 1 watt of heat.

One-amp arcs have low heat and melt metal foil, 10-amp arcs melt thin metal and weld small objects, and 100-amp arcs are suitable for the practical welding of sheet metal. Arcs of 1,000 amps or more are used for welding the thickest of metals requires, but are too hot for manual welding.

The normal AC ampere is based on a sinusoidal wave shape and is the amount of alternating current to produce the same heat effect in the same resistance as a DC ampere. One AC ampere is the root-mean square (rms) of the sinusoid (sine wave). Represented mathematically,

I_rms = 0.707I_max,

where I_max is the peak amplitude of the sine wave.

However, not every wave shape is plain, ordinary, direct current or a smooth sinusoidal alternating current. Consequently, measuring a true root-mean-square value requires special measuring devices that can perform the necessary calculations. Comparing the results of welding done using simple direct current with the results of welding performed using other wave shapes is not easy. This means that the measurements taken by ammeters of the welding results produced by different waves cannot be compared directly. Knowledge of the waveform and its true root mean square value are required to make accurate comparisons.

For example, a comparison of the results produced by different gas metal arc (GMAW) power sources and wave shapes can be made by using the same wire electrode feed rate at about the same visual arc length (even voltage measurements have similar wave shape problems) for each power source. The arc heat produced by the different power sources and wave shapes will be about the same when the feed rate and arc length are about the same. Do not be surprised if the ammeter readings of the different power sources are not the same. The differences may be the result of the meter's design or the shapes of the waves. Ammeter readings should be treated as a simple guide to compare welding conditions between power sources.