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Chapter 8 -- Measurement of Small Signals--Hints for System and Cell Design
Shunt resistance in the cell arises because of imperfect insulators. No material is a perfect insulator (one
with infinite resistance). Even Teflon, which is one of the best insulators known, has a bulk resistivity of
12
about 10
ohms
m. Worse yet, surface contamination often lowers the effective resistivity of good
•
insulators. Water films can be a real problem, especially on glass.
Shunt capacitance and resistance also occur in the potentiostat itself. The Interface 1000 Potentiostat
Mode specifications in Appendix A contain equivalent values for the potentiostat's R
values can be measured by an impedance measurement with no cell.
In most cases, the cell's shunt resistance and capacitance errors are larger than those from the potentiostat.
Hints for System and Cell Design
The following hints may prove helpful.
Faraday Shield
A Faraday shield surrounding your cell is mandatory for very low-level measurements. It reduces both
current noise picked up directly on the working electrode and voltage noise picked up by the reference
electrode.
A Faraday shield is a conductive enclosure that surrounds the cell. The shield can be constructed from
sheet metal, fine mesh wire screen, or even conductive paint on plastic. It must be continuous and
completely surround the cell. Don't forget the areas above and below the cell. All parts of the shield must
be electrically connected. You will need an opening in the shield large enough to allow a cell cable to
enter the shield.
Gamry offers a Faraday shield called the Vista Shield that features a transparent window so you can see
your electrochemical test running. It can be used with any potentiostat.
The Faraday shield must be electrically connected to the Interface 1000's floating ground terminal.
An additional connection of both the shield and the Interface 1000 floating ground to an earth ground may
also prove helpful.
Only connect the Interface 1000 ground to earth ground if all conductive cell
components are well isolated from earth ground. A glass cell is usually well isolated.
An autoclave is generally not well isolated.
Avoid External Noise Sources
Try to keep your system away from electrical noise sources. Some of the worst are:
Fluorescent lights
•
Motors
•
Radio transmitters
•
Computers and computer monitors
•
Try to avoid AC powered or computerized apparatus within your Faraday shield.
Cell Cable Length and Construction
The Interface 1000 is shipped with a 60 cm shielded cell cable. We also offer extended length cables and
a special cable for low impedance cells as extra cost options.
Cell cables longer than 1 meter will result in degraded instrument performance. Increased noise and
decreased stability both can occur. However, with most cells, the instrument will work acceptably with an
extended cell cable, so our advice is go ahead and try it. As a rule, you should not attempt to use current
interrupt IR compensation with cell cables longer than 5 meters.
NOTE
8 - 5
and C
. These
shunt
shunt
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