The present invention relates to viscometers. The present application claims priority of Swiss applications 2220/87, filed June 12, 1987, and 730/88-0, filed February 26, 1988, incorporated herein by reference.
A viscometer is described in an article by John L. Schrag and Robert M. Johnson in "The Review of Scientific Instruments", Vol. 42 Nr.2, February 1971, Page 224. In the realization described in the article a resonator consisting of a rotationally symmetric body with several inertial masses and torsional springs arranged in series along its axis is excited by an oscillator in the neighborhood of one of its resonant frequencies. This frequency is varied in a certain range about the resonance, and for each frequency, the phase angle between the oscillator output and the torsional motion of the resonator is measured. The damping and therefore also the viscosity of the fluid can be determined from the resulting curve. In spite of the large investment in apparatus which this device requires, it offers only modest accuracy in a relatively narrow range of viscosities. Although it is suited for laboratory measurements, it is not useable for continuous measurements in industrial production processes.
A further measurement device for measurement of viscosity is described in an article by William H. Robinson in the "Journal of Applied Physics", Vol. 49, March 1978, page 1070 ff. A resonator in the form of a cylindrical quartz rod is excited at one of its torsional resonant frequencies by means of a piezoelectric transducer. A second transducer measures the changes in vibrational amplitude and frequency due to the fluid. The resonator rod is immersed to half its length in the fluid. It is sealed into the wall of the vessel containing the fluid by means of an O-ring seal at one of its vibrational nodal points. This device, too, is mainly useful for measurements under laboratory conditions. The evaluation of the measurement is demanding, and requires calibration measurements with fluids with known viscosities.
Further viscometers are known from U.S. Pat. Nos. 4,005,599 and 3,986,388. A rotationally symmetric rotor with domed ends is suspended coaxially in a cylindrical housing by means of four flexural springs. A rod-shaped magnet is mounted in the rotor perpendicular to its axis of rotation. The rotor can be caused to vibrate at a frequency near to its resonant frequency by means of electromagnetic coils mounted in the housing. A sensor to measure the vibration is mounted on the springs. The signal from this sensor is fed back to the coil driver in such a manner that the oscillation of the system becomes self-exciting. A supplementary phase shift can be introduced into the feedback loop by means of a phase shifter which can be switched into and out of the circuit. The viscosity of the fluid in the gap between the housing and the rotor can be determined by comparing the difference in the oscillatory frequencies with the phase shifter switched in and out of the circuit to that obtained with calibration fluids of known viscosity. This viscometer has strong coupling effects in both the excitation system and in the mechanical resonator, so that an unknown and unpredictable damping is superimposed upon the damping caused by the fluid. Reliable measurements are therefore only possible when the apparatus is calibrated. Since the apparatus is meant for batch operation, it is primarily useful for laboratory measurements. A further viscometer with a phaseshifter in the feedback loop of a resonator has been described in "The Review of Scientific Instruments", Vol. 46, Nr. 11, Nov. 1975, p. 1560 ff. Here, the resonator is a vibrating string. This arrangement is primarily useful for laboratory measurements of fluids with known density in a relatively narrow range of viscosities.