hi all ,

am working with annular capacitors ,
made a simulated annular plates capacitor in Comsol

but my experimental results are not matching with the experimental results
experimental capacitance's are far smaller than the simulated ones

ex: actual capacitance for 3 mm distance is 1.4 pf
by simulation we am getting around 14pf

in Comsol i used : electrostatics module in AC/DC -3D module

i made 2 annular plates and 2 guard rings around them , and one more big box surrounding all of them
(guard rings are just 2 grounded rings surrounding both the capacitor plates , one with +ve potential other with -ve potential to reduce the effect of external disturbances on capacitance )

in sub domain settings i have deactivated all except the big box in which i have put field and other stuff active , have put epsilon(relative) as 1 .

now for boundary settings i gave zero charge or symmetry for the bigger box , ground for both guard rings , -1V for one plate , and set port as input for other plate ,

when i simulate this i am getting results which are not matching with experimental values

so somebody plzz help me out