When the free area cannot be determined, the percentage of free area is approximately what range of the total area?

When the free area cannot be determined, it is generally accepted that the percentage of free area falls within the range of 70 to 85% of the total area. This understanding comes from empirical data and experience in system performance measurements that relate to how air flow or fluid dynamics function in various applications.

In many systems, especially when dealing with ducts or fluid channels, the available free area directly influences the efficiency and effectiveness of airflow or fluid movement. While the exact figure may vary based on specific circumstances, a range of 70 to 85% is often utilized by professionals in the field for practical estimations.

This range allows engineers and technicians to make informed decisions when designing or evaluating systems where the actual dimensions of components have not been accurately measured or when there is uncertainty regarding obstructions within the system that could affect free flow. Hence, the correct answer provides a realistic approximation that maintains operational integrity while remaining flexible to variations in specific designs or circumstances.