One application required element control because it was heating a petroleum product to within a few degrees of it's flash point. The element surface temperature could easily rise above the flash point while controlling the process. Without element control, an explosion would occur. It also required a phase angle SCR since the requirement was to control wattage to the heater, not temperature. A "zero cross" controller with burst of power would not produce a stable wattage indication. The Watlow "cascade" control could not be used since the second input was taken up by a need to establish the wattage set-point remotely. The process signal from the two controls (element and oil temperature) were fed to a "less of two" transmitter which only allowed the safe temperature to be in command of the heater.