The BTM-400PLUS accurately measures the static tension of flat, V, and ribbed belts of rubber, leather, fabric and composite materials by measuring the natureal 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 BTM-400PLUS uses a pulse rubby-red light source to measure the natural frequency of the vibrating belt. Unlike competitive models, our unit is NOT affected by ambient noise, providing improved performance over accustic-sensor designs.

Link:
http://www.reliabilitydirect.com/tension_torqueproducts/ELE-BTM400PLUS.htm