Flexural oscillator (device)

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A flexural oscillator is a device for determining the density of liquids and gases . The density is calculated from the measurement of the natural frequency of a flexural oscillator filled with the medium to be examined . The principle of measurement is based on a spring-mass oscillator , the mass of which is partly formed by the medium to be measured.

Digital density meter with built-in flexural oscillator; Anton pair
Principle of the digital density meter

The sample is filled into a vibrating U-shaped tube (usually made of glass, in the case of aggressive samples also made of special steel), the two legs of which form the spring elements of a flexible oscillator, similar to a tuning fork . The direction of oscillation is normal (perpendicular) to the plane of the two legs. The pipe is excited electronically to an undamped oscillation with the lowest possible amplitude .

The natural frequency of the oscillator is influenced by that part of the sample mass which actually takes part in the oscillation. The volume participating in the oscillation is limited by the stationary oscillation nodes at the clamping points of the oscillator.

If the oscillator is filled with the sample at least up to the clamping points (the suspension / mounting points of the oscillating U-tube), the same precisely defined volume always takes part in the oscillation, and the mass of the sample can therefore be assumed to be proportional to its density.

Overfilling the transducer beyond the clamping points is irrelevant for the measurement. For this reason, the transducer can also be used to measure the densities of media that flow through the transducer ( continuous measurement ).

Digital flexural vibration measuring devices

In the digital density meter, the mechanical vibration of the flexural oscillator z. B. electromagnetically converted into an alternating voltage of the same frequency . The period τ can be measured with high resolution and is related to the density ρ of the medium in the transducer as follows:

A and B are device constants of the respective transducer. Their values ​​can be determined by calibrating two substances with exactly known densities ρ 1 and ρ 2 . Modern devices calculate the constants A and B after the two calibration measurements , in which air and water are often used, and store them. By taking suitable measures, you can compensate for various parasitic influences on the measurement result, e.g. B. the influence of the viscosity of the sample and the non-linearity caused by the finite mass of the measuring device .

The first digital density meter for liquids and gases that measured the density using the oscillating flexure method by Otto Kratky et al. was presented at the Achema in 1967 by Anton Paar GmbH . In addition to the instruments for laboratories and for process measurement (e.g. for the beverage industry), there are also portable density measuring devices with a flexible oscillator (e.g. from Anton Paar or from Mettler-Toledo ).

literature

  • Otto Kratky et al., Device for Density Determination. US Patent 3523446, https://patents.google.com/patent/US3523446
  • Hans Stabinger: Density Measurement using modern oscillating transducers. South Yorkshire Trading Standards Unit, Sheffield 1994.
  • ISO 15212-1

See also