What cannot be used to determine the relative air quantity delivered to the furnace during normal boiler operation?

The Venturi flow meter serves a specific purpose by measuring the flow rate of liquids and gases based on pressure differentials. While it provides accurate data for volumetric flow, its primary function is not to directly measure the air quantity delivered to the furnace in a boiler system. Instead, it calculates flow based on the principles of fluid dynamics, requiring careful alignment with other systems and conditions to confidently assess air quantity in a combustion scenario.

In contrast, a pressure gauge measures the pressure within the system, which can be related to airflow under certain conditions, while a thermocouple can help in gauging temperature changes indicative of combustion parameters. A rotameter, on the other hand, is specifically designed to measure flow rates and, therefore, can be more directly associated with the volume of air supplied to the furnace during operation.

Understanding the functions of each device helps clarify why the Venturi flow meter does not serve as a direct indicator of air quantity in this specific context.