The problem awaiting the welder: how to apply the obtained parameters to real welding process? The contact tip diameter can be easily measured, electrode force can be determined from the machine's specifications, or from known air cylinder area and air pressure. Welding time is set with a controller and usually does not depend on the weld itself. However how would the welder set the required welding current? This is not a trivial task since the resistance of the welded parts is unknown. Three different methods can be used to solve this problem.
Figure 1. Circuit for measurement of resistance of the welded parts
The first way is to measure the unknown resistance that requires the True RMS Ampere/Volt Meters with a Hold feature. For measuring the resistance a few welds are needed to be made with the measurements of current flow through electrodes and voltage across the welded parts according to the circuit on Figure 1. Then the ratios of voltage over current has to be calculated and an arithmetic mean from these values be taken.
The Voltmeter must be connected to the welded parts, not to the electrodes! The Clamp Ammeter may be connected either to the primary or to the secondary circuit. Since the current range for such meters is limited to 2000 A, the primary circuit connection is only the case if the welding current is more than 2000 A. At that, the meter's readings must be multiplied by the transformer ratio:
Is = Ip x n
where Is is the secondary (welding) current,
Ip is the primary current and
n is the transformer ratio, which is equal to the ratio of the primary voltage over the secondary open circuit voltage.
The best, but more expensive way of measurements is to use Digital Storage Oscilloscope (DSO) with isolated channels Tektronix THS710A (the cheapest one), THS720A or THS720P (the best for power measurements). In this case, the Clamp Current Probe is connected to one channel of DSO and its other channel is connected in place of the Voltmeter as on the Figure 1. For a current range from 0 to 2000 A Tektronix A621 Clamp Current Probe is the best choice and it can be found for about $400. The Extech 380905 model works well for 50 - 2000 A range. This probe is much cheaper (about $100), but is noisy below 50 A. Furthermore other brand name models are available for the up to 10000 A.