In a catch-can test, distribution uniformity is determined by comparing which portion of readings to the zone average?

• A. The highest quarter to the zone average.
• B. The lowest quarter of readings to the zone average.
• C. All readings to the median.
• D. The middle half to the average.

Answer: (B)

Distribution Uniformity in a catch-can test is determined by comparing the lowest quarter of readings to the zone average. This focuses on the driest spots in the area, since how evenly water is distributed is revealed by how close those low readings are to the overall average. If the bottom 25% averages near the zone mean, uniformity is high; if they're much lower, uniformity is poor. The other options don’t reflect how DU is defined: using the highest quarter would highlight over-watering areas, not uniformity; using all readings with the median or the middle half with the average doesn’t capture the critical low-end spread that DU measures.