Experiment Artifacts while measuring
impedances
Experiment:
Mutual Induction (High current applications)
The main source of
errors measuring impedances on high current applications is the so-called mutual
induction: due to the (high) current through the cables and the alternating
electrical field, an alternating magnetically field is induced. This alternating
magnetic field reveals itself in an impedance spectrum as a parasitic, inductive
contribution in the high frequency region.
Background:
The mutual induction
effect may be reduced significantly but is not eliminated completely. Therefore,
the current- as well as the potential conducting lines have to be drilled. As a
result, the magnetic fields eliminate themselves best.
Objective of the
experiment:
The contribution of
the mutual induction effect is determined, measuring a 10 mΩ resistor using
different arrangement of the measuring cables. In addition, a so-called coaxial
shunt is measured. Due to the geometrical arrangement of the resistive layer of
the latter one, almost all of the mutual induction effect is eliminated.
Note:
Provided you are not
aware of the mutual induction effect and its origin, you will get impedance
spectra which are hardly reproducible in the high frequency area because a
change in the arrangement of the cables may result in a remarkable change in the
magnetic effect.
Experiment: Coupling Effect (High- and Low
current application)
Concerning a
‘conventional’ electrochemical setup with a three electrode arrangement, the low
conductivity of the solution as well as the high input impedance of the
reference electrode may be the source of undesired parasitic effects during an
impedance measurement. Both contributions lead to a ‘pseudo-inductive’ behavior
in the high frequency part of an impedance spectrum (coupling effect).
Considering
‘classical’ electrochemical methods like cyclic voltammetry (CV), this effect is
not recognizable due to the different time scale (CV is much ‘slower’).
Goal of the
experiment:
The order of magnitude
of the coupling effect as well as the frequency dependence has to be determined,
using two different reference electrodes exhibiting different input resistances.
Note :
Similar to the mutual
induction, the coupling effect can not be eliminated completely. But, taking
appropriate steps, this effect can be shifted towards higher frequencies.
Experiment:
Shielding (Low current application)
Measuring high Ohmic
objects, for instance insulators, the main source of error is the so called
electromagnetic pollution. This parasitic effect reveals itself in the low
frequency region of an impedance spectrum. One can overcome this problem using
an appropriate shielding. As a rule, one can profit by applying shielding
techniques provided the current is in the order of magnitude of 1 µA or below.
Goal of the
experiment:
Different stages of
shielding are applied on measuring an equivalent circuit and the corresponding
influence of electromagnetic pollution is checked on the impedance spectra.
Note :
One of the coworkers
of Zahner-elektrik is involved in an international working group which deals
with the standardization of impedance measurements on high Ohmic systems. During
this standardization, a set of different equivalent circuits (dummy cells) is
measured worldwide in a Round Robin test.