Hello all,
I'm new here
I'm Instrumentation Engineer working in oil & gas field
in our site we have a lot of mass flow meters and calibration is done periodically
from 2 weeks ago I calibrated a rosemount mass flow meter, the initial error was 0.1% after adjusting the mass calibration factor and took 3 consecutive runs the error reduced to 0.02% and this is acceptable
then from 4 days ago I wanted to make a verification on the same mass flow meter and I found the error increased again to 0.1 % !!
could anyone make it clear to me why this happens !?

Note: the flow rate is constant in all runs (20 kg/min) and also the fluid density....

Waiting a helpful feedback.

Thanks,
boxyman