Normal force transducer

Instrument compliance is another cause of concern in making mechanical measurements, since the forces generated by the fluid in response to a deformation will tend to twist, bend or compress the rheometer components that also experience these forces. These include shear and normal force transducers as well as the frame of the instrument. A sophisticated approach to dealing with this problem is the use of a Force Rebalance Transducer (product of Rheometrics/TA Instruments described, for example, by Vermant et al. [102]) in which a feedback loop provides torsional and axial motions to compensate for the corresponding compliances and thus minimize their effects on data. In the case of torsional motion, there remains some compliance due to the twisting of the shafts supporting the fixtures [103]. 

Another method, which makes use of a cone and a partitioned plate, is also based on Eq. 10.76, but instead of making a direct measurement of the radial normal stress gradient, the usual monolithic plate is replaced by one partitioned into two concentric parts, an inner disk with a radius Rj, and an outer ring whose outer diameter is Rq, the same as the cone [123]. The normal force transducer responds only to the thrust R on the inner disk. Data are collected for several samples having radii R, between R , and Rg and the measured thrust f j is related to Nj and Nj as shown by Eq. 10.77... 

Notice how normal viscous stress Xrr acts in the same coordinate direction as the pressure force. A force transducer implanted within the solid sphere cannot separate the effects due to each type of stress in the radial direction. 

In a rheometric dynamic spectrometer such as RDS7700 manufactured by Rheometrics, Inc., the torque and normal force generated in response to an imposed motion are measured by a transducer. A microcomputer determines stresses from these values with measured sample motion to calculate strains and viscoelastic functions such as G, G", and tan d [12]. 

Viscosity curve is shifted. There are a number of items that can cause the viscosity curve to shift up or down. Cheek to see that the proper calibration values have been entered for the pressure/force transducer. Check the die to ensure that it is not partially bloeked, eausing greater than normal pressure drops. Lastly, eheek for a shift in either the barrel or die temperature. Burned-out heater bands or maladjusted temperature eontrollers will eause large shifts in experimental results. 

During Rheometrics analysis, a torsional motion is imposed on the sample and the torque and normal forces resulting from the motion are measured by a transducer. From these values the storage modulus, G , the loss modulus, G" and the ratio of the latter to the former, the loss tangent (tan 6) can be calculated. 

Apparatus. The apparatus which is illustrated in (4) provides a linear sliding motfon to the lower specimen in contact with the upper specimen vdiich is prevented fi om moving by the force transducer.. The normal load is applied by dead weights and the fiiction force is measured by a piezoelectric force transducer. A DC servo actuator is connected to the lower specimen through a series of elastomeric isolators. The isolators permit the system stiffiiess to be varied independently of the force transducer stiffiiess. These isolators also provide system dancing. Four isolators in series had a combined... 

For cone and plate geometry, the primary normal stress difference thrust force F tending to separate the two surfaces, and this can be measured by a force transducer as in the Weissenberg rheogo-niometer the relation is... 

Figure 6.11 Cone-plate viscometer allowing measurement of the primary and secondary normal stress differences. Normal force (F) and pressure transducers recording hydrostatic pressures (p,) at different radial positions are shown.

Stiffer transducers have been made with strain gages (Macosko and Starita, 1971 Drislane et al., 1974). These devices can be very sensitive to temperature changes (Franck, 1985a). Very stiff and more thermally stable are piezoelectric transducers. Laun and Hirsch (1989) were able to measure stress overshoot and stress relaxation in less than 10 ms with a torque and normal force quartz load cell. However, it is difficult to get low torque levels with piezoelectric transducers. Furthermore, they are capacitance devices with a slowly decaying signal, making it difficult to accurately measure long relaxation processes. 

Nazem and Hansen (1976) analyzed the gap opening problem. Assuming that the gap opening is infinitesimally small and the liquid is Newtonian, the time for the transducer to reach 63% of the total travel when a step normal force is applied is... 

Franck, A. J. P., Quasi-Infinite Stiff Transducer for Measuring Torque and Normal Force, presented at conference on New Techniques in Experimental Rheology, University of Reading, U.K., September 1985a. Manuscript available from Rheometrics, Inc., Piscataway, NJ. 

Since pressure is, in fact, a normalized force (force/unit area), and since force cannot be measured without an accompanying displacement, we can expect that pressure transducers will look very much like displacement transducers. Since this is the case, it allows us to apply much of our knowledge regarding the behavior of displacement transducers to predicting the behavior of pressure transducers. 

Niemiec, J. M., Pesce, J.-J, McKenna, G. B. Anomalies in the normal force measurement when using a force rebalance transducer. /. Rheol. (1996) 40, pp. 323-334... 

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