Densitometer

A densitometer comprising:

• a probe having an input lead and an output lead;
• a loop circuit having an input lead connected from said probe output lead, a first output lead connected to said probe input lead, and a second output lead, said probe and said loop circuit forming an electromechanical oscillator, said probe including a vane, means connected from said probe input lead to vibrate said vane, a transducer, means connecting said transducer to said probe output lead to produce a first A.C. signal thereon of the same frequency as that at which said vane vibrates, said first A.C. signal having a frequency which is a known invariant function of the density of the fluid in which said vane is immersed, a second A.C. signal being impressed upon said loop circuit second output lead of the same frequency as that of said first A.C. signal;
• utilization means;
• output means connecting said loop circuit second output lead to said utilization means;
• a phase detector for adjusting the phase of said second signal so that it remains the same relative to said first signal independent of changes in said probe, said loop circuit or said fluid, said phase detector having first and second input leads and an output lead, said loop circuit including an input circuit having an input lead connected from said probe output lead, and an output lead connected to said phase detector first input lead, a tracking filter having first and second input leads and an output lead, means connecting said tracking filter output lead to said loop circuit first output lead, said loop circuit second output lead being connected to said phase detector second input lead, a low pass filter having an input lead and an output lead, means connecting said input circuit output lead to said first input lead of said tracking filter, said low pass filter output lead being connected to said second tracking filter input lead, said phase detector output lead being connected to said low pass filter input lead.