Calibration of the - 9 _ D. M. Poole and 



Encry Settling Tube W. S. Butcher 



for Sand Analysis 



Pour splits of the same sand were run, each having a different weight, 

 2, 3. ^, and 5 grams. The total probable error of the median of these runs 

 vpis only 0.6-^. Since the tube error is 0.87a, it can be stated that there is 

 no appreciable effect caused by differences in weight of the sample used. 

 Figure 7 shovis the results of these runs. 



Influence of Particles Coarser Than 1 ram. 

 on the Median Diameter 



To determine the effect of particles coarser than 1 mm. on the median 

 diameter of the sample, two splits of the s?.me beach sand were prepared, one 

 having the particles greater than 1 mm. removed. Figure 8 shows a plot on 

 logarithmic probability paper of the two runs and also a sieve analysis of 

 the same sample for comparison. To make the ciirves strictly similar, the 

 v/eight percentage greater than 1 mm. has been added (at the 1 mm. grade) to 

 the sample having the coarser material removed. 



It is to be noted that the sample containing the sand particles coarser 

 than 1 ram. shows, vrith few exceptions, larger diameters for the sa.me percen- 

 tage of the total. This difference can be interpreted as a result of the 

 carrying dovm of the finer particles with the coarser. The difference in 

 the median diameter of the sample having the fraction coarser than 1 mm._ 

 removed R,nd the sieve analysis can be attributed to the entraining of_ the 

 finer particles by the coarser in the tube analysis. In addition, the 

 differences arc due to the difficulty of a.ccurately reading the scale divi- 

 sions on the settling tube when the suspension-sand interface changes rapidly, 



