What device is used to measure draft in a boiler?

The correct answer is the manometer, as it is specifically designed to measure pressure differences, including draft in a boiler. Draft refers to the movement of air or gases in the combustion process, which is crucial for efficient boiler operation. A manometer can detect the pressure variations between the inside of the boiler and the outside atmosphere, providing important data for optimizing combustion efficiency and ensuring safety.

Other devices listed serve different purposes; for example, pressure gauges measure static pressure but are not as sensitive to the small pressure changes associated with draft. Flow meters are designed to measure the flow rate of liquids, making them unsuitable for measuring gas draft in a boiler. Thermocouples are temperature sensors and do not provide information about pressure or draft at all. Thus, the manometer is tailored for this specific application, making it the appropriate choice for measuring draft in a boiler.