A scanning system for monitoring the thickness, profile or other data from a moving product which includes a frame for mounting scanning transducers. The product is carried on a conveyor or the like through the frame and the transducers are mounted for transverse scanning motion across the top and bottom surfaces of the product at right angles to its direction of travel. Reference wires are connected between the side members of the frame and maintained at a constant tension independent of ambient temperatures. Reference wires are located between the transducers and both the top and bottom surfaces of the product. The wires provide a constant reference to detect distortion of the frame due to thermal or other stress and to provide correction data which is used to compensate for frame irregularities.