Abstract: A rheometer incorporates a blade (1) mounted for rotation about an axis (2). The blade is of twisted form such that it has first and second regions. A first region of the blade (1) substantially at the axis of rotation (2) has a first angle formed by its surface with respect to a plane perpendicular to the axis of rotation such that the surface of the blade in the first region extends substantially parallel to the axis of rotation. A second region of the blade spaced from the axis of rotation has a second angle, different to the first angle, formed by its surface with respect to the plane perpendicular to the axis of rotation.