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4.5 Studies on influence of humidity Measurements have been made of the variation of capacitance loading and decay time with humidity for paper, a cotton handkerchief and cleanroom garment fabrics with 2.5 and 5mm grid spacing, without and with antistat treatment. The paper studied showed a strong increase in capacitance loading and little significant change of decay time with increasing humidity. In contrast, a cotton handkerchief showed a small increase in capacitance loading and a sharp fall in decay time. Cleanroom fabrics including grids of core conductive threads showed higher capacitance loading with an antistat treatment and with closer spaced grids. Decay times tended to be longer with an antistat surface finish and tended to increase with humidity. This is because of the higher capacitance loading with an antistat finish.
4.6 Tribo and corona charging comparison The results of studies carried out under low humidity conditions on a variety of fabrics (listed in 3.6 above) with 'scuff' tribocharging and with corona charging are summarised in Table 1. The following points are noted: - the ranking orders of decay time and capacitance loading performance are very similar for the two methods of test. - the values of decay times and the values for capacitance loading are generally comparable (although higher values for capacitance loading are often observed with corona charging) - the lowest initial peak surface voltages are associated with the highest values of capacitance loading - as would be expected Table 1: Comparison of tribocharging and corona charging studies at BTTG (Jan 1999, 23C 25%RH)
Studies recently reported [8] compared three different electrostatic test methods using the same set of materials as in Table 1. The first method was that developed at NASA by Dr Gompf [9] involving mechanical rubbing with a Teflon pad of a tensioned disc of material with an earthed outer boundary. The second method was the modified Shirley Method 18 [10] involving rubbing an isolated tensioned disc of material. The third method was the above 'scuff charging' method – referred to as the ‘JCI ad hoc Method’ (see 3.2). Despite the differences in the methods, all three gave similar ranking orders in terms of the peak surface voltage generated. Some differences between the NASA method and 'scuff charging' may well have arisen from differences in the time to measure the initial peak voltage when the decay time is short. So far very comparable results have been observed on the modest size samples used for tribo and corona charging studies. The next stage of work is to see if similar behaviour is observed with large area samples and with full and inhabited cleanroom garments. The studies will include garments unearthed as well as earthed at likely connection positions, so the effect of garment seams can be directly assessed. These studies will show whether there is opportunity for quick, easy and fair assessment of material applications from small scale corona charge decay and sample charge measuring instrumentation [6]. It needs to be noted that the new approaches described do not answer all aspects of electrostatic risks from materials. For example, they do not cover shielding against electric field transients or the opportunity to draw sparks from charged materials isolated from earth. These aspects require additional tests by other appropriate methods. |
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