Size Gauge

a measuring transducer that converts the dimensions of an object into a signal suitable for reading or for further use. Thus, in operating monitoring systems the signal may indicate that a prescribed dimension is being exceeded. Various types of size gauges exist—for example, thickness gauges and length-measuring transducers. Depending on their design, size gauges are classified as contact or noncontact.

In contact size gauges, the sensing element comes into contact with the object being measured and converts the size variation into a mechanical displacement, which is then transformed into an electrical, mechanical, pneumatic, or hydraulic signal. The electromechanical, inductive, capacitive, resistive, and electron-parametric (mechanotron) types are the most common. Ultrasonic size gauges constitute a special group; here, instead of a movable sensing element, an ultrasonic vibrator is placed on the surface of the object being measured. Contact size gauges are simple and convenient to use; no additional amplification is needed for their output signals, except in the case of ultrasonic size gauges. Their principal drawback is that deformation and wear of the sensing element at the point of contact cause the measurement error to increase.

Noncontact size gauges are used when no mechanical contact is permissible between the sensing element and the object—for example, when the thickness of polymer films and coatings is measured while the films are being produced. Radioisotope thickness gauges are often used. This type is based on the dependence of the intensity of the radioactive radiation passing through the object on the object’s thickness. Pneumatic, photoelectric, induction, and capacitive noncontact size gauges are also often used. Noncontact size gauges are characterized by a low-power output signal, which makes their operation more complicated and increases their cost.