The Checkline BTM-400PLUS Sonic Tension Meter / Belt Tension Meter is a precision instrument that accurately measures the static tension of flat, V and ribbed belts of rubber, leather, fabric and composite materials by measuring the natural frequency (Hz) of a segment of the belt that has been tapped and then converting it into tension units of newtons (N) or pounds force (lbf). The instrument uses a pulsed ruby red light source to measure the natural frequency of the vibrating belt. Unlike competitive models, the BTM-400PLUS is NOT affected by ambient noise, providing quick reliable results in all production and field applications.