How many measurements are necessary when traversing a 10 inch round duct?

To determine how many measurements are necessary while traversing a 10-inch round duct, one must consider the principles of airflow measurement and duct performance assessment. In HVAC practices, a common guideline is to take measurements at multiple points across the duct to accurately capture the variations in velocity and ensure a representative sampling.

For a round duct, the standard approach often recommends taking measurements at specific intervals around the circumference and along the length. Specifically, the rule of thumb for circular ducts suggests taking at least 12 measurement points, which typically includes measuring at a minimum of 4 points across the diameter of the duct (top, bottom, left, right) and at least 3 points along the length of the duct for a more comprehensive assessment.

Taking 12 measurements allows for effective assessment of the airflow conditions within the duct, accounting for potential variations and the overall performance. This makes option C the correct choice, as it aligns with established practices for comprehensive airflow measurement in round ducts.