It is hard work, but it has also been a timely reminder of everything that goes into making sure our instruments are working properly and delivering the correct results.
I have been calibrating and installing Kinexus rheometers (and servicing the previous generation of Bohlin rheometers) for the most part, and each of these visits can take several hours to run through the various calibrations and verifications required.
I readily confess that I am no rheologist, but I am aware that there are three main things which need to be well controlled in order to get good measurement quality and the correct results;
1) Geometry Gap
Accurate measurement and control of the working gap between upper and lower geometries is critical for rheological measurements since shear strain and shear rate are linearly dependent on measurement gap. For the cone type geometry the specific gap used (which is fixed) is dependent on the cone angle; the cone is truncated at the center. For the parallel plate geometry the gap is variable which can be better for accommodating samples that change volume during measurement or contain large particles.
Geometry gap is accurately controlled using a precision stepper motor which has a resolution of 0.1 μm. This resolution is important since for a 1° cone angle the gap is just 30 μm. Furthermore the Kinexus uses an intelligent geometry recognition system to ensure that the correct gap is being used and the strain constant (which converts angular displacement and angular velocity to shear strain and shear rate respectively) is correct for the inserted geometry.
Accurate temperature control is critically important since temperature has a significant influence on the rheology of a material, particularly for viscous materials like Asphalt, where viscosity can vary by more than 15% per degree Celsius. Furthermore, due to thermal expansion of the measuring systems it is necessary to account for temperature dependent gap changes also as part of the calibration procedure.
3) Motor Torque (and all that goes with it…)
If the rheometer is not applying the stresses and strains that you’re requesting (but thinks it is) then it is inevitable that the results will not be correct! Since shear stress is determined from motor torque (multiplied by the stress constant for the inserted geometry) then an accurate calibration of motor torque is essential. Torque is calibrated and verified in at least four different ways to ensure that the torque response over the entire torque range and at each bearing position is well understood.
Although it is hard going out in to the field, the work is very rewarding as I am getting to meet customers face to face (some of whom I have been in regular contact with in my Helpdesk role) and the knowledge I am picking up should also allow me to offer improved support to our customers and partners around the world.
If your Kinexus is reminding you that it’s time for a calibration, please get in touch – it’s very important!
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