Sometimes there are disturbances that appear unexpectedly that will cause a missing weld. In this case, the weld control does not regulate current to be constant (red line). The control quickly determines that the resistance curve will not acheive its qualified profile so automatic adjustments are made during the production of the weld to recover the process. You can see here that at approximately 40mS, the current is increased drastically to try to make the dynamic resistance peak at a point to assure the development of a nugget. Then when it detects that this occurs, the control eventually decreases current to obtain a desired weld penetration.

A tool such as SoftQ will annunciate a good weld because the dynamic resistance signature has become acceptable.