Rebalance transducer

For dynamic mechanical and steady shear measurements, the Rheometric Scientific RFSII rheometer was used equipped with the sensitive range force rebalance transducer and couette geometry or parallel plate tooling. 

Fuller and co-workers (1987 Mikkelsen et al., 1988) have attempted to make the opposed-nozzle device more quantitative by measuring the torque on the arm that holds the nozzle down into the beaker. Torque is detected by a rebalance transducer that maintains the gap constant. 

Schematic diagram of the Rheometrics RFSII. A dc motor C is controlled in steady rotation by a tachometer B and in sinusoidal or other angular position changes by a rotary variable capacitance transformer (RVCT) A. The sample is surrounded by a temperature ctmtrol bath D. The upper fixture is attached to a rebalance transducer E, described further in Figure 8.2.5. Adapted from Rheometrics (1 ).

Reduced torque versus time after steady shear of a 1 Pa s Newtonian oil at 10s" with a 25 mm diameter, 0.1 rad cone, (a) Strong spring response of a torsion bar transducer with a spring constant, AT = 10 N ln/rad and 250 rad/s resonant frequency, simulated using eq. 8.2.1. (b) Force rebalance transducer with resonant frequency of 960 rad/s. From Mackay et al. (1992). 

Force rebalance transducer. From Franck (1983a). 

Schematic of opposed-nozzle extensional rheometer. Stepper motor A moves a pair of syringes B, which suck (or blow) the test liquid from the beaker C through a matched pair of nozzles D. Screw E can translate arm F to change the gap between the nozzles. Arm G pivots around the inlet tube at H. Torque on the tube is measured by a rebalance transducer /. From Rheometrics (1991).

Method A ARES G1 (1 k force rebalance transducer (FRT) for torques ranging from 2 X 10 to 10 mNm) with external recording of the shear stress in the time domain for post-analysis using a custom-written MATLAB routine. The external recording was performed as described in [44]. 

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]. 

The measurement of normal stress differences in transient deformations is extremely sensitive to small variations in gap spacing, which can arise from instrument compliance or minute temperature variations. Venerus and Kahvand [43] have shown how to evaluate the effect of instrument compliance by measuring the response using several sets of cone-plate fixtures. If a Force Rebalance Transducer is used for a transient normal stress measurement to compensate continuously for compliance in order to keep the gap constant, the response time of the transducer may affect the data. Also, the thermal expansion that results from the power dissipated in the transducer can affect the gap spacing and is of particular concern when normal stresses are being measured [104]. 

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... 

Mackay, M. E., Halley, P. J. Angular comphance error in force rebalance torque transducers. /. Rheol. 

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