Monroe Electronics Charge Plate - Analyzer - model 268A - Manual (Page 11)

APPENDIX II
Application Note APNE-0017
Plate Self Decay of Charged-Plate Monitors
Introduction
Charged-Plate Monitors (CPM’s) evaluate the ability of air ionizers to neutralize
static charge. A CPM charges an isolated, conductive plate that is charged to a
known level. Then the air ions produced by the ionizer neutralize the charge on
the isolated plate, and the time required to discharge the isolated plate is meas-
ured.
In an ideal, traditional CPM, the plate is perfectly isolated from ground and
represents a perfect capacitor. Once the plate is charged to a known level, it
would hold that charge indefinitely. The only cause for the isolated plate to dis-
charge would be by impingement of air ions.
In practice, other factors may exist that cause the isolated plate to discharge. In
the absence of air ions, any discharge of the isolated plate is known as plate self
decay or plate isolation.
The Problem
As the isolated plate in a traditional CPM is supported from ground by one or
more insulating supports, a common cause of self decay is leakage through
these insulative supports.
The two major contributors to leakage across these supports are:
• High humidity, and,
• Lack of cleanliness.
Any substance (contamination) that is deposited on the surface of the insulative
supports, such as salts and oils from fingers, and dust, may increase the
conductivity of the support and increase the leakage from the isolated plate to
ground. Moisture in the air can be absorbed into the surface layers of the support
and further increase the surface conductivity of the material. Moisture may also
activate ionic impurities within the insulating supports.