Chapter 5 -- Measurement of Small Signals -- Hints for System and Cell Design
5-5
are common. This shunt capacitance cannot be distinguished from "real" capacitance in the cell. If you are
measuring a paint film with a 100 pF capacitance, 5 pF of shunt capacitance is a very significant error.
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 about 10
14
ohms. 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 PCI4 Potentiostat Mode
specifications in Appendix A contain equivalent values for the potentiostat's R
shunt
and C
shunt
. These 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 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 the PCI4 cell cable to enter the shield.
The shield must be electrically connected to the PCI4's floating ground terminal.
An additional connection of both the shield and the PCI4 floating ground to an earth ground may also prove
helpful.
NOTE: Only connect the PCI4 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 PCI4 is shipped with a 1.5 meter cell cable. We also offer extended length cables as extra cost options.
