Bolt tension measurement uses an ultrasonic pulse to detect the change in length of a bolt as it is tightened. The bolts elongation is directly proportional to the force being applied as expressed in Hooke's Law 1671

Bolt tension also known as Clamp force, is the axial stretching force created in a bolt when under tightening. Proof load is the optimized force applied and is usually around 60-80% depending on bolt type, optimized proof load will be determined by bolt manufacturer.

Ultrasonic measurement requires a signal to pass through the entire bolt. It is not impacted by certain critical factors such as frictional forces, strain variations from bolt geometry. As a result this method is used as a reliable method in many critical joint solutions.

The first number relates to the tensile strength, (the maximum stress before breaking) the second number relates to ratio of yield strength (point of deformation) to tensile strength.