Abstract: A ball valve assembly of the quarter-turn type for use in isolating an electronic level transmitter comprising a valve body having an inner chamber, an inlet or tank-side port, an outlet or transmitter side port, a valve body axis defined by the inlet and outlet ports, and a substantially spherical ball valve member having an inner surface forming a through passage mounted in the chamber for selective rotation between an open position and a closed position. The present invention allows for flushing the diaphragm face of a level transmitter with the ball valve member in an open position via a purge port positioned on the valve body and a purge passage connecting the through passage and a seating face portion of the outer surface of the ball valve member. The present invention is provided with a full port opening through the ball valve assembly for enhancing the accuracy of level transmitter readings and valve backflush capabilities.