Baron P A, Deye G J
Division of Physical Sciences and Engineering, National Institute for Occupational Safety and Health, Cincinnati, OH 45226.
Am Ind Hyg Assoc J. 1990 Feb;51(2):51-62. doi: 10.1080/15298669091369330.
Electrostatic charge can cause errors during sampling of airborne asbestos fibers and other particles. The change in particle trajectories caused by charge effects during sampling can result in nonuniform deposits on the collecting filter surface and net loss of sample. The degree of these electrostatic effects depends on particle charge, sampler charge, sampler conductivity, and sampling flow rate and direction. The purpose of this research was to evaluate the dependence of sampling efficiency and sample uniformity on these variables. Humidity has been postulated as a primary determinant of particle charge during aerosol generation. Measurements of particle charge and concentration were made as a function of relative humidity with chrysotile fibers generated from a fluidized bed. A strong increase in charge and a decrease in concentration of fibers was noted as the relative humidity was decreased below 15%. The effects of conductive versus nonconductive samplers and sampling flow rate were measured as a function of particle and sampler charge levels. Nonconductive samplers can carry a large and variable charge distribution on their surfaces. This can result in a biased and highly variable particle deposit on the filter when sampling charged particles. Conductive cowls spread any acquired charge over the entire surface and produce a more symmetrical and less biased charged particle deposit. Increasing the sampling flow rate will improve sampling efficiency and decrease deposit variability because the charged particle has less time to interact with the field produced by the sampler. These results suggest that sampling problems caused by electrostatic charge interactions are most likely to occur under low humidity conditions of dust generation, that sampling should be done at as high a flow rate as possible to reduce these effects, and that analysts should select fields toward the center of the filter to minimize bias and variability.
静电电荷会在空气中石棉纤维及其他颗粒采样过程中导致误差。采样期间电荷效应引起的颗粒轨迹变化会导致收集过滤器表面沉积不均匀以及样品净损失。这些静电效应的程度取决于颗粒电荷、采样器电荷、采样器电导率、采样流速及方向。本研究的目的是评估采样效率和样品均匀性对这些变量的依赖性。湿度被假定为气溶胶生成过程中颗粒电荷的主要决定因素。利用流化床生成的温石棉纤维，测量了颗粒电荷和浓度随相对湿度的变化。当相对湿度降至15%以下时，观察到纤维的电荷大幅增加而浓度降低。测量了导电与非导电采样器以及采样流速对颗粒和采样器电荷水平的影响。非导电采样器表面会带有大量且分布可变的电荷。在对带电颗粒进行采样时，这会导致过滤器上的颗粒沉积出现偏差且高度可变。导电罩会将获取的电荷分散到整个表面，从而产生更对称且偏差更小的带电颗粒沉积。提高采样流速会提高采样效率并降低沉积变异性，因为带电颗粒与采样器产生的场相互作用的时间更短。这些结果表明，由静电荷相互作用引起的采样问题最有可能在低湿度的粉尘生成条件下出现，采样应尽可能在高流速下进行以减少这些影响，并且分析人员应选择过滤器中心区域进行采样，以尽量减少偏差和变异性。
