Abstract: A sensing device for sensing the position of a movable element in a fluid-operable actuator including a relatively movable piston and cylinder. The sensing device includes a pair of co-operating pressure tappings in the cylinder wall, a pair of peripheral and axially spaced seals on the piston for sealing between the piston and the cylinder wall and a detector responsive to the pressure at the pressure tappings by producing an output indicating piston position. The spacing of the pressure tappings is greater than the axial width of each of the seals and less than the axial distance between the seals.

Abstract: A slide valve in which the valve member has an apertured interior partition extending in the direction of sliding movement of the valve member in a position intermediate a ported wall with which the valve member is engageable and a further wall of the housing containing a cavity, the surface areas of the partition on each side thereof being substantially equal, whereby fluid pressure inside the valve member is transmitted to the cavity through the apertured internal partition and acts on each side of the latter, thereby to hold the valve member in substantial fluid balance in a direction perpendicular to the ported wall and hence to reduce the force required to slide the valve member into and out of its operative positions.