Assessments - 3, GPA: 3.7 ( )

Fabricante : Gentek Arquivo Tamanho: 1.29 mb Arquivo Nome : Língua de Ensino:

The advantages to the glass and epoxy-coated metal construction are twofold: 1. The enclosure does not contribute high electrostatic forces affecting the precision of the balance. 2. The attraction and ultimate accumulation of powders, (hazardous or nuisance), are minimized on the inside of the enclosure. To correct or ensure against electrostatic issues, the following additional measures may be prescribed to improve weighing operations. • Maintain a humidity level between 45 and 60%. The ability to sustain this humidity range can be challenging depending upon the regional climate and HVAC system. • Ionizers in various forms (guns, bars and blowers) are effective ways to flood an area with ions and essentially “neutralize” electrostatic electricity. 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 dust particles contact the surface, creating friction and electrons move across the interface. 7 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 . per square Skin, Metals Static dissipative 105 > 109 . per square Glass Antistatic 109 > 1012 . per square Polyethylene bag Insulative 1012 . 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 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 t...

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