Weight-on-bit and torque measuring apparatus

Abstract From the Canadian patent

A drill String sub is disclosed for measuring the torque and axial compression in the drill string. The drill string sub includes an outer tubular housing and an inner sleeve type apparatus mounted thereto for amplifying the strain the sensors measure. The sub further includes a section for compensating for the axial stresses due to the local pressure differential between the drill string bore and the well bore annulus and for the thermal gradients occurring during operation. This section includes a balance tube for isolating the internal bore pressure from acting on the strain amplifier and for creating an upward axial force on the tubular housing and strain amplifier which is responsive …