I am using AR-G2 to test some DNA solution, and the geometry I am using is a pair of 8 mm disposable aluminum plates. On the "Instrument Status" window, torque and shear rate cannot be zero simultaneously, which is weird. If I try to set the torque to zero, shear rate will have a value around -0.2 (1/s), and fluctuate around this value. If I set shear rate to zero, torque will have a value of about 40 nN.m (0.4 Pa). If I set the shear rate to be zero before a test, after the test, the torque is zero and shear rate is -0.2 again.
When I do a creep test by using a stress of 0.3 Pa, the shear rate is a negative constant.
Does anyone have such experience with ARG2 in the past, or know what is going on here? Is it a problem of the instrument itself, like misalignment of the plates, or something else?
Hi Yanfei,
We have also have an ARG2 instrument in our lab. As you might know, the instrument has a bearing which combines magnetism with pressurised air. It is possible that the inlet of the air is blowing slightly agains the central axis and causing the geometry to turn, even when no stress applied.
The instrument software contains the option to do "mappings". This is a way in which the instrument checks friction dependent on the orientation of the central axis. As a consequence this allows the system to recognize the position of the pressurised air inlet and remember to counteract this effect. So as a first solution I would advise you to do a "rotational mapping" before starting your experiment. Choose the option "precission" and 2 iterations should be sufficient. Since this is to increase sensitivity, make sure there is no airflow in the close environment of the device or people passing by.
To test the abilities of the instrument, I would advise you to do a simple rotational test of the instrument without geometry and draw rod at 0,1 rad/s for a few minutes. Before this experiment you should do everything possible to get accurate values: determination of the new instrument inertia, geometry inertia (should be 0 since no geometry is attached), bearing friction correction and rotational mapping. If the instrument is still in good shape, your torque values for this test should be at least below 10nNm.
If you have any more questions about the instrument or test procedures feel free to contact me on my email address: [email protected]
Kind regards,
Eline
I don't have at disposal an ARG2 but such a problem appears also in other instruments and can be reasonably attributed to air flow within the bearing.
You should always perform both rotational and ocisllatory mappings on a new geometry. Furthermore, make sure you also redo the inertia for the geometry when you put it on. YOu need to map the insturment and do the geometry calibrations to make sure the results are accurate.
If you still have a problem you should take the bearing calibration and see what the peak to peak values obtained are. They should be around 10^-1.
good orning. I work in TA instruments as an aplication specialist. The problem may came from the bearing friction. becouse the instrument has an air bearing for the horizontal position, if the value of the bearing friction is not well calibrated there will be an offset. Please, do the bearing friction crrection once you have made the zero gap after the instalation of the plates. perform a rotational mapping of 2 iterations in precision mode. be sure that the bearing friction correction is activated. If you want I can give you my mail (I could check with an experimental file if the bearing friction correction is activated). after that the offset should disappear. Let me know.
PD. the problem does not came form the noise (so no osc mapping is neccesary). each geometry has his own beraing friction correction. the bearing friction correction must be donde with the geometry installed.
please, send me a original file .rsl of whatever experiment and I will check if the bearing friction correction is activated and also I will explain you how to correct the other problems that you explained
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