Chapter 2: Prerequisites
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5
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. Impurities within the air impinging upon surfaces dictate
the polarity and magnitude of the charge. The process, triboelectrification, occurs
when the dust particles contact the surface, creating friction 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 by balance manufacturers 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 line of Filtered Enclosures have been designed to contain hazards by
negating typical cross drafts and turbulence within the opening. Air movement
does not affect most modern balances with draft shields. However, as a
precautionary safety measure and a higher level of control, it is recommended that
the enclosure be placed in 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.
Do not position the unit so that it is difficult to operate the main
disconnect device.
Ne placez pas l'appareil de sorte qu'il est difficile de faire
fonctionner le dispositif principal de déconnexion.
!
