Sample thief

Schematic diagram of a sample thief. Rotating the inner cylinder opens and closes the openings along the outer cylinder s shaft.

Regardless of the method of evaluating the solids mixture, the sampling procedure is vital. Often a sampling thief, or other special device, is used to remove samples from the mixture without excessive disturbance of the batch. If an easier method of sampling is obvious and will bring less contamination to the batch, it should be used. 

Manual sampling should be carried out by grounded personnel, preferably using grounded metal equipment such as a metal sample thief bonded to the tank via metal cord. A satisfactory alternative is to place one or more glass sample bottles in a grounded metal cage (5-S.4.2). 

Fig. 18 Sampling errors introduced by an end-sampling thief differ from those of a side-sampling thief, but persist nonetheless. In this type of thief, a window is opened at the bottom of the sampling tube and grains are forced into a cavity by further insertion of the thief This eliminates the bias toward grains that passively fill a cavity more easily than others, but on the other hand, (A) these thieves entrain more grains during insertion and (B) their performance again suffers from substantial systematic error.

Disturbing of powder minimized over sample thief Porous plate prevents too many fines because of strong air current Simple... 

An alternative technique, which reduces segregation, is a unit-dose compacting sample thief (Fig. 2). Bias is still likely with this method because of the invasive nature of any sample thief... 

---