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"Composite Primary Unit" (CPU) is a term commonly used in sampling. A CPU is a composite unit consisting of multiple sub-samples that are combined to form a representative sample.

In the sampling of bulk materials, this could mean that several sub-samples are taken from different areas of the bulk material and then merged to create a single, representative sample. For example, a CPU from a bulk material sample could be formed by taking samples from various depths and locations within the material. By combining these sub-samples, a representative overall sample is obtained, which serves as the basis for further analyses.

This process helps capture the variation within the entire bulk material and ensures that the resulting sample is representative of the whole material. It is particularly important for bulk materials where the properties of the material (such as moisture, particle size, chemical composition, etc.) can vary from one area to another.