Abstract: Improvement for measuring fluid flow with an orifice meter. The device to measure flow in a conduit includes an orifice of variable section defined by two moveable plates in contact with each other, and which can be positioned with the orifice coaxial with the conduit. The plates are movable by equal translations in opposite directions. Each plate has an opening, and edges of the repective openings which face each other have a cutout in the shape of a right angle isosceles triangle. The bisector of the right angle cutout of each plate is parallel to the direction of translation of the plates and intersects the axis of the conduit. The device permits treating with the same precision, the measurement of flow having variations of a large range.