Chapter 2: Prerequisites
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Background on Electrostatics or Static Electricity
Electrostatic charges on a surface such as the wall of a balance enclosure are not
created by moving air. Gases do not cause the charge. Impurities within the air
impinging upon surfaces dictate the polarity and magnitude of the charge. The
process, triboelectrification, occurs when the friction of the dust particles contact
the surface and electrons move across the interface.
The ability of a material to become polarized is a property known as permittivity.
On highly insulative materials like acrylic, ions or charged molecules are strongly
bound to the surface by polarization forces. The higher the force, the higher is the
permittivity value of the material. It is suggested that the use of high permittivity
materials, such as plastic be avoided.
Since static electricity is a surface phenomenon, materials can also be classified by
their surface resistivity measured in ohms per square. The table below lists the
surface resistivity of various classes of material.
Surface Resistivity Table
Material
Surface Resistivity
Example
Conductive
0
10
5
per square
Skin, Metals
Static dissipative
10
5
10
9
per square
Glass
Antistatic
10
9
10
12
per square
Polyethylene bag
Insulative
10
12
per square
Acrylic, Packing foam, Styrofoam
Location and Air Current Requirements
The XPert Balance Enclosure and XVS Ventilation Station both have been
designed to contain hazards by negating typical cross drafts and movements within
the opening. Air movement does not affect most modern balances with draft
shields. However, as a precautionary measure of safety and a higher level of
quality management, it is recommended that the enclosure be placed in such an
area to avoid:
High traffic areas where walking might cause an air disturbance or be a
nuisance to balance readings.
Overhead or wall HVAC diffusers, fans, radiators or other lab equipment
producing air currents.
Next to doorways or windows that may be opened.
