Hi, I am using a rotational rheometer and I am investigating the smallest optimal gap size using a parallel plate on silicone oil (0.990Pa.s +-1%). I am using a flow ramp test where my inputs are temperature of 25 degrees Celsius, shear rate of 10-100 1/s, duration of 120s and gap size starting from 400um going down in increments of 50um.

For some unknown reason, I keep getting a viscosity value of 0.930Pa.s at 400um,350um and 300um but at 250um, 200um and 150um a viscosity of 0.925-0.910Pa.s.

I don't understand why i am getting such a low value. I know my filling is fine considering i have asked several of my colleagues to check it.

Do you guys have any advice? Am I doing something wrong or am I missing something?