In a mold for the superplastic forming of a piece from a blank, the distortion of the blank is measured by a hollow tube located within the mold and having one end positioned in contact with the blank. The hollow tube extends through a sleeve fixed to the mold and includes an axial array of holes. A bore in the sleeve is connected to a source of low pressure gas via a delivery tube so that the low pressure gas can selectively communicate with the interior of the hollow tube as the movement of the tube causes the selective registry of the holes therein with the delivery tube. Such selective registry causes a pressure drop in the delivery tube which is measured by a pressure drop detector and accompanying recorder. Electrical leads are provided on the inner surface of the mold for evaluating when the distortion of the blank causes the face of the blank to reach selected portions of the mold inner surface.