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EIS - impedance measurements with very low conductivity

Posted 27 janv. 2012, 06:34 UTC−5 Low-Frequency Electromagnetics, Results & Visualization, Studies & Solvers Version 4.0a, Version 4.2a 5 Replies

Ebia
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I've bee working for a while on a EIS model, so models of electrical impedance spectroscopy on various shape of electrodes (2D model properly setting the d [m] )

(Terminal 1 and 2 in voltage configuration, frequency analysis)

It happens that at a very low frequency the real part of the impedance change into negative....

I can't find a physical reason to obtain this so I suppose it is something related to the precision of the calculation

I did a lot of tests and I found out that randomly, a very low conductivity setting to an interface or to a materials results in a negative real term of impedance (1/ec.Y11).

...? a bug? a not proper numerical precision?

help please
best

5 Replies Last Post 22 juin 2012, 09:53 UTC−4
Edgar Kaiser
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Posted: 1 decade ago 27 janv. 2012, 06:55 UTC−5
How small are the numbers? Is it approaching machine precision?
How small are the numbers? Is it approaching machine precision?
Ebia
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Posted: 1 decade ago 27 janv. 2012, 08:06 UTC−5

We expect to obtain Real Z value under 10e+8 Ohm
we deal with conductivity around 10e-10 S/m
( this values are very common in the EIS field )

we check the machine precision on Matlab and we found a eps = 2.2 e-16


but actually, depending on the complexity of the model, we obtained very consistent values also as 1e+30 ( real Z).

Is it possible that we go under the machine precision during the calculation from conductivity -> admittance?

is there any section in the documentation , helping us to understand the mathematical model used by Comsol to reach the result?

thanks
We expect to obtain Real Z value under 10e+8 Ohm we deal with conductivity around 10e-10 S/m ( this values are very common in the EIS field ) we check the machine precision on Matlab and we found a eps = 2.2 e-16 but actually, depending on the complexity of the model, we obtained very consistent values also as 1e+30 ( real Z). Is it possible that we go under the machine precision during the calculation from conductivity -> admittance? is there any section in the documentation , helping us to understand the mathematical model used by Comsol to reach the result? thanks
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 27 janv. 2012, 08:57 UTC−5
Hi

If you type in Parameter epss = "eps" (without the "", you get the COMSOl precision, for me 2.220E-16. but you should not forget that most physics imply second order formulas and differences and sqrt(eps) = 1.49E-8

And eps is valid for the difference from "1" if you are scaled far from "1" the smallest difference is much much greater

Check any book on binary representation and numerical precisions

--
Good luck
Ivar
Hi If you type in Parameter epss = "eps" (without the "", you get the COMSOl precision, for me 2.220E-16. but you should not forget that most physics imply second order formulas and differences and sqrt(eps) = 1.49E-8 And eps is valid for the difference from "1" if you are scaled far from "1" the smallest difference is much much greater Check any book on binary representation and numerical precisions -- Good luck Ivar
Parmender Singh
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Posted: 1 decade ago 28 mai 2012, 04:31 UTC−4
Hello

Can you send me your model . i am also working on Battery analysis though magnetic filed. so i have no idea how to couple AC/Dc module with battery model. Please help me
Hello Can you send me your model . i am also working on Battery analysis though magnetic filed. so i have no idea how to couple AC/Dc module with battery model. Please help me
Richie Guo
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Posted: 1 decade ago 22 juin 2012, 09:53 UTC−4
Did you recieve the Model？
Did you recieve the Model？

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