Labconco XPert Series, Руководство пользователя

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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
Ω
per square
Skin, Metals
Static dissipative
10
5
→
10
9
Ω
per square
Glass
Antistatic
10
9
→
10
12
Ω
per square
Polyethylene bag
Insulative
10
12
Ω 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.