Abstract: The RF output of a voltage controlled oscillator (VCO) 11 is periodically gated by gate 15 to a transducer 18 which produces acoustic waves in bolt 19. The reflected acoustic waves are converted to electrical signals by transducer 18 and gated by a gate 20 to mixer 31. Mixer 31 also receives the output from VCO 11 and produces an output which is filtered by low pass filter 32. The output of filter 32 is a DC signal proportional to the phase difference change from a fixed phase difference between the two input signals to mixer 32. This DC signal is sampled at an instant and held by circuit 33 in response to the "P" signal. The output of circuit 33 is integrated by integrator 34 and then applied to VCO 11 to change the frequency of VCO 11 such that the phase difference between the two inputs to mixer 31 remains at said fixed phase difference. The frequency of VCO 11 is a measure of the change in strain of bolt 19.