Several density measuring instruments are based on measuring an oscillating system’s resonant frequency, such as a tube filled with the fluid being tested or a cylinder completely immersed in the medium.
The sensing element consists of a single smooth bore tube through which the measuring fluid flows. The tube is fixed in heavy nodal masses at each end, isolated by bellows and ligaments from the outer case.
The electromagnetic drive and pick-up coil assemblies are located along the tube. The amplifier keeps the tube oscillating at its natural frequency in operation.
Since the tube’s natural oscillation frequency is a function of the mass per unit length, it must also be a function of the flowing fluid’s density. It also follows that the tube should be made from a material with a low and stable expansion coefficient.
If this is not possible due to corrosion or wear, it is important to measure the temperature and apply a suitable correction to the resonant frequency density value.
The tube typically vibrates at approximately 1.3 kHz (when filled with water) and at an amplitude of approximately 0.025 mm. With a precision of 0.2 kg / m3 and a repeatability of 0.02 kg / m3, densities up to 3000 kg / m3 can be measured.
The response is continuous throughout its operating range with no adjustments of span or zero. Recalibration is effected by adjustment of the constants in the associated readout or signal conditioning circuits.