Sampling Techniques

The most important step in any particle size analysis, but especially by microscopic techniques, is sampling from the bulk. Because an extremely small quantity of material is used to determine the particulate size, an accurate analysis cannot be obtained if the bulk material is not properly sampled. Particles tend to segregate according to size. If handling has caused vibration of the sample, coarse material tends to collect near the surface. When free-flowing material is poured into a pile, the coarse portion collects near the outside of the pile, and the fines concentrate in the center. The following sampling techniques are recommended for obtaining a valid representation of particle sizes in a laboratory sample.

Chute rifflers (Fig. 3) consist of V-shaped hoppers that feed a series of chutes, which alternately feed into trays on opposite sides of the hopper. A sample that is properly poured into the hopper is halved, and the process is repeated until a suitable sample size is obtained. Sampling efficiency depends on an even feed of the sample into the hopper and on the width of the chutes. Numerous narrow chutes provide more accurate sample statistics than a few wide chutes, provided the powder can flow easily through them.

Fig. 3 Chute riffler

Spinning rifflers (Fig. 4) divide a sample by feeding it into a number of containers that rotate on a table under the feed. This method is efficient, provided the table rotates at least 100 times during the flow of the sample.

Fig. 4 Spinning riffler

Oscillating Sampler. The feed hopper oscillates rapidly between two sampling cups, thereby halving the sample. Feed times must be long compared to oscillation times for optimum efficiency.

Cone and Cup Sampler. In this device, the sample is fed onto a cone that disperses the powder in all directions along the cone surface. A sample cup is rotated around the base of the cone, gathering portions of the sample at every position. The width of the cup and length of the cone periphery determine the sample reduction.

Sampling tables (Fig. 5) are used to reduce larger quantities of the sample. Sampling tables consist of an inclined plane with a series of holes that halve the sample in each of the four stages, leaving lti of the original volume as a final sample.

Fig. 5 Sampling table diagram

Cone and quartering involves pouring the sample into a cone-shaped pile on a glass plate, flattening the top, and dividing it into quarters with a thin metal blade. One of the quarters can then be mixed, repoured, and requartered until a suitably small sample is obtained. This method requires some technique to ensure symmetry of particle sizes in the pile and accurate quartering.

Scoop sampling is the least precise, but perhaps the most frequently used, sampling technique. The sample container is shaken, and a small portion is extracted with a scoop or spatula. However, segregation can be imparted to the sample by the shaking method. The scoop sampling technique that provides the least variance is shown in Fig. 6 and consists of shaking the container back and forth while rotating. Because particle sampling is the initial and most important step in particle size analysis, miniature sampling devices should be used whenever possible.

Fig. 6 Scoop sampling apparatus showing direction of shaking
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