What we know about a weld heat source either comes from experimental observation or more detailed models of the welding process. Experimentally, currents, voltages, frequency, wire feed rates, welding speeds, etc. can be measured. Also welds can be sectioned and cross-sections of a weld measured optically. Various forms of video cameras, some with laser illumination, can be used to measure the weld pool surface and to visualize droplet transfer into the weld pool. Thermocouples and infra-red cameras can be used to measure temperatures in and near the weld pool. The distribution of power density in arcs, lasers and electron beams can be measured. Spectroscopy of the welding arc plasma provides useful information. A great deal of experimental data can be obtained for any given weld and a great deal of data of this type has been published.

The other source of knowledge and data about welding heat sources comes from mathematical models of the weld heat source. Such models can include in addition to the energy equation, surface tension of the weld pool surface, hydrostatic forces, Lorentz and Marangoni forces in the weld pool, pressure and shear forces from the arc or plasma, and droplet transfer to the weld pool. Once we are able to make models that predict the behavior we observe, we tend to say that we understand the welding process. It is equally important to note that to the extent that we cannot make models that predict the behavior of a welding process, we should say that we do not understand the welding process.