Tham khảo tài liệu 'volume 07 - powder metal technologies and applications part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Outer tube case of thief sampling device Fig. 1 Sampling spears Inner tube split into separate containers The spear is thrust into the powder with the inner chamber closed off and when in position the outer tube is rotated to allow powder to fall into the inner chamber. When the chamber is full the inner tube is turned to the closed position and the spear is withdrawn. Possible segregation throughout the bed may be investigated with type 3 an average value for the length of the spear with type 2 and a spot sample with type 3. Frequently the spears are vibrated to facilitate filling and this can lead to an unrepresentative quantity of fines entering the sample volume. The sampling chambers also have a tendency to jam if coarse particles are present because they can get lodged between the inner and outer chambers. Coning and Quartering. In industry it is common practice to sample small heaps by coning and quartering. The powder is formed into a heap which is first flattened at the top and then separated into four equal segments with a sharp-edged board or shovel. The segments are drawn apart and frequently two opposite quadrants are recombined and the operation is repeated until a small enough sample has been generated. This practice is based on the assumption that the heap is symmetrical and since this is rarely so the withdrawn sample is usually nonrepresentative. Sampling from Trucks or Wagons. Consignment sampling is carried out on a single consignment . a truck or wagon load . In sampling from a truck or a wagon it is recommended that eight samples be extracted Ref 1 . No increment should be taken at less than 12 in. below the surface this avoids the surface layer in which segregation can have occurred due to vibration Fig. 2 . Care needs to be taken to prevent powder sliding down the slope created due to removal of surface material. Fig. 2 Sampling points for a wagon or a truck Sampling Stored Free-Flowing Material. It is practically impossible to .
