Chapter 2: Prerequisites
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 → |
| 105 Ω per square | Skin, Metals | |
Static dissipative | 105 | → | 109 Ω | per square | Glass |
Antistatic | 109 | → | 1012 Ω | per square | Polyethylene bag |
Insulative | 1012 Ω per square→ | Acrylic, Packing foam, Styrofoam | |||
Location and Air Current
Requirements
The XPert 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 quality management, 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.
Exhaust and Blower
Requirements
XPert
System
The XPert Filtered Balance System uses an integral motorized impeller to draw room air past the operator and through the enclosure. This contaminated air is then pushed through the HEPA filter. The
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