A sensor apparatus to be used with extraction machinery (2), said extraction machinery (2) including a plurality extraction elements which when activated are adapted to deliver an exravted fluid from two or more extraction elements into a common collection line (3). The sensor apparatus including at least one sensor (4a/4b) positioned in the common collection line (3), and at least one controller (6) adapted to control the activation of said extraction elements, whereby activation of said extraction elements is controlled to prevent said at least one sensor (4a/4b) being exposed to extracted fluid supplied from all of said extraction elements at any one time.