A control system is provided which can be retrofit to existing conventional blender devices, such as a blender trailer used for combining a pregelled liquid with a solid particulate additive at a well site. The control is provided either manually or automatically from a remote location where a display unit and a computer unit of the invention are located, such as in a frac van. The display unit and the computer unit are connected together by a multi-conductor cable, and the display unit is connected by a multi-conductor cable to a terminal unit mounted on the blender. Control signals are output from the display unit, under automatic control by the computer unit or under manual control by potentiometers attached to the display unit, to control electrohydraulic valves associated with screw conveyors mounted on the blender. Linear variable differential transformer signals from the electrohydraulic valves, rotational speed signals from the screw conveyors, and flow rate signals from the inlet pregelled liquid are communicated to the display unit for use by it and the computer unit in generating the valve control signals.