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Chapter 9 -- Measurement of Small Current Signals--Floating Operation
Cell Construction
If you need to measure small currents or high impedances, make sure that your cell construction does not limit
your response.
A cell where the resistance between the electrodes is only 10
impedances. In general, glass and Teflon are the preferred cell construction materials. Even glass may be a
problem when it is wet.
You also must worry about C
placing electrodes close together or parallel with each other if you are measuring high impedances.
Reference Electrode
Keep your reference electrode impedance as low as possible. High impedance reference electrodes can cause
potentiostat instability and excessive voltage noise pickup.
Try to avoid:
Narrow bore or Vycor tipped Luggin capillaries.
•
Poorly conductive solutions - especially in Luggin capillaries.
•
Asbestos thread and double junction reference electrodes.
•
Reference electrodes often develop high impedances as they see use. Anything that can clog the isolation frit
can raise the electrode impedance. Avoid using saturated KCl based references in perchlorate ion solutions
Instrument Settings
There are several things to remember in setting up a very sensitive experiment.
In EIS, use the largest practical excitation. Don't use a 10 mV excitation on a coated specimen that
•
can handle 100 mV without damage.
Avoid potentials where large DC currents flow. You cannot measure 1pA of AC current on top of
•
1 mA of DC current.
EIS Speed
In EIS, do not expect the Reference 3000 to measure 10
listed above limit the performance.
As a rule of thumb, the product of Impedance, Z, times frequency, f, should be less than 10
measurements with a Reference 3000.
Z · f < 10
Ancillary Apparatus
Do not use the Reference 3000 with ancillary apparatus connected directly to any of the cell leads. Ammeters
and voltmeters, regardless of their specifications, almost always create problems when connected to the
Reference 3000 cell leads.
Floating Operation
The Reference 3000 is capable of operation with cells where one of the electrodes or a cell surface is at earth
ground. Examples of earth grounded cells include: autoclaves, stress apparatus, pipelines, storage tanks and
battleships. The Reference 3000's internal ground is allowed to float with respect to earth ground when it works
with these cells, hence the name floating operation.
. Make the "inactive" portion of your electrodes as small as possible. Avoid
shunt
9
ΩHz
10
ohms cannot be used to measure 10
10
ohm impedances at 1 kHz. Many of the factors
9 - 7
13
ohm
9
ΩHz for good EIS
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