Checkline BTM-400Plus [BTM-400PLUS-TC] Belt Tension Meter, 10 - 800 Hz, with NIST Calibration Cert and Tuning Fork. The Checkline BTM-400PLUS Belt Tension Meter, Sonic Tension Meter or Belt Frequency Meter also called a Sonic Tension Meter Includes two (2) separate Sensors – a plug-in type for one-handed operation and a cable-connected type for measuring belts with limited access space.  The Checkline BTM-400PLUS Belt Tension Meter or Sonic Tension Meter 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.