The key element to understand the structure of a wound roll is the pressure inside it. This pressure cannot be too small or too large. If it is too small it may not withhold its structure. If it is too large it may damage the material. Researchers have spent decades on developing winging model to predict this inner pressure. Wound on tension, which is the stress occur in the outer layer of a roll during winding, have been proved to be one important parameter relates with the inner pressure of roll. Several methods are developed to measure this parameter. The accuracy of a common used interfering measurement method will be evaluated and a non-interfering measurement will be developed in this paper.