Regulate Tension (if required): In the event the tension is not inside of the required range, regulate the belt tensioner or change the belt as essential. Measuring basic principle: Frequency measurement by LED light though the belt is just not moving. The belt is toughed by hand to build vibration. https://jaspernswbe.digitollblog.com/32234124/the-definitive-guide-to-belt-tension-gauge-tool