WHAT CAUSES THE FAKE KURTOSIS AND HOW TO AVOID IT IN GRAIN-SIZE MEASUREMENT
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摘要: 粒度分析时同样的前处理方法,黄土、黏土质粉砂、粉砂、极细砂样上机时易出现大于100 μm由气泡形成的假峰。取样量过大导致遮光度高于13%,液体黏滞系数大,气泡不易排出是气泡假峰出现的原因之一;上机时水量少于800 mL一方面使液体的黏滞系数大,另一方面激起的气泡多也是气泡假峰出现的原因;转速不合理、上机时超声波振荡时间少于2.5 min就开始测量也会导致假峰出现。根据不同类型样品合理取样,黏土质粉砂约0.1 g,黄土、中粉砂约0.2 g,粗粉砂取样约0.3 g,极粗粉砂约0.4 g,极细砂约0.6 g,细砂约1 g;使遮光度在8%~12%之间,上机时水量达到1 000 mL,上机时延长超声波振荡时间到2.5 min以上再开始测量,一般可避免假峰出现。较细样品(中值粒径小于10 μm)即使水量1 000 mL也可能出现假峰,可通过改变泵的转速来判断和消除假峰,不能稳定出现的大于100 μm的峰值为假峰,遮光度低的样品降低转速到2 000 r/min假峰一般会消失;遮光度高的样品升高转速到3 200 r/min假峰一般会消失。Abstract: A false mode>100 μm often occurs in laser grain-size measurement. The causes of the false mode may be that the water amount <800 mL leads to a high liquid viscosity and in turn stirs more bubbles. Another cause may be that larger quantity of a sample may induce a larger shading level of light (>13%) and a higher liquid viscosity in turn may release bubbles with difficulty. An inappropriate pump speed or an ultrasonic dispersion time shorter than 2.5 min also may result in false modes appearing. Several principles to avoid false modes are concluded. The shading level of light (obstruct light) should be 8%~13%. The amount of samples is determined according to their types. The appropriate quantity is~0.1 g for clay samples, 0.2 g for loess and fine silty sand samples, 0.4 g for coarse silty sand samples, 0.6 g for fine sand samples and 1.0 g for sand samples in order to keep the light shading level of 8%~12%. The water amount of~1 000 mL and the ultrasonic dispersion time of>2.5 min before measuring are necessary to eliminate false modes of grain size distribution. For clay or fine silty sand (median size<10 μm), false modes may occur even though water amount is more than 1 000 mL. In the case, false modes can be identified and eliminated by changing stir speed. The mode whose median size is>100 μm and does not occur in all conditions may be a false mode. The false mode may disappear when the stir speed is reduced to 2 000 for a sample with lower light shading level. The false mode may disappear when the stir speed is increased to 3 200 for a sample with higher light shading level.
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Keywords:
- grain-size distribution /
- false mode /
- laser grain-size measurement
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