Is it appropriate to test an ESD garment on an operator? This question was asked of Larry Levit during an "Ask the Experts" panel discussion at ESTECH, the annual technical meeting of the IEST (Institute of Environmental Science and Technology). Larry teaches a tutorial "ESD the Unseen Contaminant in the Cleanroom" for the IEST. Larry answered "Yes, but the results will be different when the ESD garment is worn versus when tested at the laundry for surface resistivity and triboelectric charging. Different environments will give different results." q(velence)CV where "q" is the charge on the garment; "V" is the voltage that is read on the static meter, and "C" is the capacitance of panel to ground. If the panel is near a person or grounded metal piece then "C" is a large numeric value. If there is nothing near the panel such as the garment hanging on a plastic hanger with three-foot spacing around the garment, then "C" is a very small number indeed. Therefore, "V" is a large or small numeric value depending upon the environment and the ESD garment will respond differently when worn on a person rather than on a hanger. The person is the capacitance. For that reason, it is not reasonable to set the same voltage limits on a garment on the rack because it does not represent the conditions in which the garment is used. Garments on the rack OVERESTI-MATE the amount of charging that a garment will exhibit when worn, sometimes even by a factor of 10.
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