Final answer:
The batch must be sampled at four or more regularly spaced intervals during the discharge of the middle portion to ensure accurate representation and consistency of the mixed materials.
Step-by-step explanation:
When sampling from a stationary mixer, the batch must be sampled at four or more regularly spaced intervals during the discharge of the middle portion of the batch. This ensures that the samples accurately represent the entire batch without the variability that may occur at the beginning or end of the discharge. Regularly spaced sampling is critical in quality control processes to determine the consistency and uniformity of the mixed materials.