Which instrument is not used to measure the quantity of combustion air?

The implementation of instruments in measuring combustion air is an essential aspect of boiler operation and efficiency. A rotometer, while often used to measure fluid flow rates in various applications, is not typically categorized as a primary instrument for measuring combustion air specifically. Combustion air measurement usually requires devices that can provide accurate readings of air flow and pressure.

The other instruments mentioned serve more direct purposes in this context. A manometer measures pressure differences, which can help in understanding air flow dynamics. A flow meter quantifies the flow of gases, including combustion air, allowing operators to ensure the right amount of air is supplied for efficient combustion. A barometer, while it measures atmospheric pressure, isn't used to assess combustion air quantity specifically but may provide contextual atmospheric pressure readings that can influence combustion efficiency.

Thus, the rotometer stands out as the instrument not typically employed for measuring combustion air quantity, as its application is broader and does not focus specifically on the needs of combustion processes in boiler operations.