Patent GB-910881-A (GB)
Title: Improvements in or relating to viscometers
Inventor: BRADFIELD GEOFFREY
IPC: G01N11/16
Filed: 1957-09-02
Abstract: 910,881. Viscometers. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Dec. 2, 1958 [Sept. 2, 1957; May 14, 1958], Nos. 27627/57 and 15565/58. Addition to 830,463. Class 40 (1). [Also in Groups XXXV and XXXIX] The vibrating-stem viscometer of the parent Specification is modified for high-temperature use by exciting the vibrations with an electromagnetic or magnetostrictive transducer in which the magnetic members have a Curie temperature higher than that of iron and the insulation of the windings is able to withstand 300‹ C. The vibrations are usually torsional as in the parent Specification, but the device may be modified to use axial vibrations. Suitable materials are described for the magnetic members and the winding-insulation of the transducer. In the case of a magnetostrictive transducer, the magnetostrictive element may be part of the resonating stem, and with either type of transducer the oscillating probe (other than any magnetostrictive part) preferably has low damping for acoustic waves and a low temperature-coefficient of change of velocity of such waves. Fig. 1 shows an electromagnetic transducer comprising a stem 11 with a conductive or conductively-coated end 13 located between slotted polepieces 14 which carry a copper shading-ring 17 and an exciting winding 18. If the resonant frequency is 25 kc./s., application of current at 100 kc./s. pulsed at 20-25 kc./s. will generate torsional vibrations in the stem 11. The intensity of the vibrations is measured by pick-up coils on a separate yoke (Figs. 2 and 3, not shown) in which a constant radial magnetic field is produced by a D.C. winding or a permanent magnet. This yoke may be separate from the energizing windings, or may be set in the gap between the polepieces 14. Alternatively (Fig. 5, not shown), a single yoke may be used, with separate windings on four poles. When estimating viscosity by comparing the decay of vibrations in the liquid under test and in air, no pick-up coils are necessary. The result obtained is then the product of viscosity and density. In Fig. 8, a horseshoe magnet 36 has limbs bent inwards towards one another but in opposite directions away from the mid-plane of the axis of the stem. The energizing windings are carried by bobbins 37 and the whole system is polarized by a permanent magnet 38. Alternatively, two balanced opposed ring-shaped permanent magnets could be fitted at 32, or a D.C. winding could be used. The end of the resonator should be of non-magnetic material. Fig. 10 shows a magnetostrictive transducer comprising a magnetostrictive stem 39 in a tubular yoke 41 having two inwardly-projecting limbs 42 and 43, of which the former is radial and the other sweeps away from the radial limb towards the end 46 of the tube. This limb is dished at 45 to reduce leakage of torsional energy. Flux linkage is provided by conducting loops 51 about an annular core 52. These loops may be of one or more turns each, and there may be two or more around the circumference of the stem, located in slots in the limb 43. Longitudinal polarizing flux is provided by a toroidal winding 55 or alternatively the limb 42 may be a disc-shaped permanent magnet. The intensity of the torsional vibrations in the stem 39 is measured by a pick-up winding on the core 52. Temperature compensation for change in velocity of acoustical waves in the stem is achieved by making it of two parts of opposite sign of coefficient. Suitable dimensions are given for the parts of the transducer. In many cases the stem has sufficient coercive force (supplemented if necessary by a pair of longitudinal bar magnets) for it to be set into vibration by an energized axial coil (Fig. 11, not shown). In all the above types of transducer; the resonating stem is mounted at a velocity node, the supporting diaphragm being tubular except for a small portion immediately adjacent to the junction with the stem (Fig. 4, not shown). To maintain a high Q value for measuring high viscosities, the part of the stem immersed in the material under test can be a solid rod of a heavy material such as tungsten. After the point of junction with the diaphragm, this rod is secured to a tapering hollow member to the other end of which the transducer is attached (Fig. 12, not shown). The resonating system is arranged to be “ wavelength in each direction from the junction. It is also desirable to provide an extra diaphragm support at a velocity node before the point where the transducer is attached (Fig. 13, not shown). A short probe for use at low frequencies (or harmonics thereof) may be partly solid and partly hollow (Fig. 14, not shown). The space within the tubular diaphragm, and the stem if hollow, can be pressurized by gas or oil for added strength. Fig. 15 shows a magnetostrictive transducer employing axial vibrations, and comprising a stem 71 clamped at its centre by means of lugs 72 to a support 73. Coils 74 are energized with A.C. and are also polarized by D.C. Measurements can be made using two separate transducers, one for exciting the vibrations and the other for measuring them, and if these are of different types (as described above) there will be little leakage interaction between them. The bridge circuit shown in the parent Specification may be used, with suitable modifications, either for one or two transducers. The vibrations are preferably set off by pulses or by a short train of waves of decreasing amplitude. A magnetostrictive transducer may have a stem of separate parts, matched for acoustic impedance, and resonating at frequencies in the ratio of 1: 3: 5. According to the first Provisional Specification, a simultaneous measurement of temperature can be made by means of a thermocouple attached to the top of the resonating member.