Apparatus and method to measure tension in a moving web and to control properties of the web

A process and apparatus for determining the tension in a moving web is provided. The process includes the step of providing a web that is moving at a determined speed. The speed has a determined basis weight. A wave is created in the moving web. The speed of propagation of the wave is determined. Also, the tension on the moving web is determined through a mathematical relationship between the wave speed, the basis weight of the web, and the speed of the web. The instability index of the web is greater than or equal to 0.5. The apparatus includes an air pulse for applying a pulse of fluid to the web to create a wave in the web. At least two laser displacement transducers are present. The laser displacement transducers measure the displacement of the web as the wave moves through the web. A computer is present which obtains signals from the at least two laser displacement transducers. The computer calculates the instability index in the web based on the speed of the wave in the web and the speed of the web.