Bogus Torque Specifications

Evaluation of single head, or combined motor and transducer rheometers requires clarification of how torque specifications are determined.  There are several reasons for investigating the true meaning of the torque specification numbers printed on a brochure. 

First, the torque range will depend on one of four control modes: controlled stress in oscillation; controlled strain in oscillation; controlled stress in steady shear; and controlled rate in steady shear.  Many manufactures will only specify the one or two of these ranges that present their instrument in the best light.  A published torque specification that is not obtainable in all modes is misleading and does not accurately present the true performance of the rheometer. 

Second, stress rheometers generate and apply torque to a sample.  In the ideal case, no bearing friction and inertia effects would be present, and the instrument applied motor torque (M_m) and the torque received by sample (M_s) would be equal. But in real world, the bearing friction and instrument moment of inertia (See High Inertia Rheometer Design for more details) are always present and need to be corrected.  Thus:

M_motor  = M_sample + M_friction + M_inertia

So the sample torque is calculated (or corrected) by subtracting friction torque and inertia torque from the applied motor torque.  Sample torque is the value that is normalized to geometry and used to calculate material parameters (G’, G”, Viscosity, etc.).  For a Newtonian material, the sample torque will always be smaller than applied motor torque, and the magnitude of the difference will depend on several factors.   For example, the torque contribution due to inertia will depend on the mass of all moving parts of the rheometer, the sample’s viscosity, and in oscillation, the frequency.  Figure 1 shows how M_m and M_s are different for a dynamic strain sweep on 0.1 Pa.s silicone oil standard at 1 Hertz (6.28 rad/s).  In this experiment, the frequency, mass of moving rheometer parts, and the sample’s viscosity are constant, therefore the inertia effect is almost equal over testing range.  Figure 2 shows how M_m and M_s for the same oil differ in a frequency sweep experiment.  Because the inertia of the moving parts of the rheometer are more significant at higher frequencies, larger corrections are required at higher frequencies, and the difference between M_m and M_s gets larger with increasing frequencies. 

Why is this important to understand?  Because some rheometer manufactures specify the low-end torque as M_s (not M_m) in an attempt to make the low-end torque specification look better.  This value is corrected for contributions from the instrument as opposed to an instrument input torque specification.  In other words, that value of torque could not be programmed as an instrument parameter.  However, the same manufacturer will specify the torque applied by the motor, M_m (NOT M_s) as the maximum torque specification, where the specification will look better without removing the inertia contribution.  This is inconsistent and misleading to the customer.

Specifying M_s can also make a design disadvantage, such as a high inertia motor design, appear to be an advantage.  Why is this?  Simply consider taking two rheometers; one with relatively low inertia (such as TA Instruments design), and one of high inertia (such as an ECM motor design).  If you apply the same M_m to a sample under identical testing conditions, once you subtract the M_i (inertia torque) from the M_m, the high inertia design will appear to have a lower minimum torque.   Under the same conditions, a low-inertia design will be working more efficiently to apply the programmed torque to the sample, yet the high inertia motor will appear to have a better specification. 

Rheometer torque specifications should not be defined as matter of convenience to hide design weaknesses.  Ask questions as to how torque specifications are defined.  Some simple experiments on silicone oil standards can quickly reveal true torque performance.  You can count on TA Instruments to deliver consistent torque specifications, all of which are defined as the applied motor torque as the minimum measurable torque depends on the sample.  In addition, TA Rheometers minimize all torque corrections with enhanced designs that reduce the bearing friction and motor inertia for superior data.