Which instrument is used to measure air pressure in a duct?

The correct choice is the manometer, which is specifically designed to measure air pressure in a duct. Manometers utilize a column of liquid that moves in response to the pressure exerted by the air within a duct. By comparing the height of the liquid in the manometer against a scale, operators can determine the air pressure accurately. This instrument is commonly used in HVAC applications to ensure proper airflow and system performance.

An altimeter, on the other hand, measures altitude by determining the changes in atmospheric pressure as one ascends or descends. Although it also relates to pressure, its application is not suitable for measuring air pressure within ducts.

A barometer is similar in that it measures atmospheric pressure, but it is primarily used for weather forecasting and does not provide the specific localized pressure measurements required in duct systems.

A thermometer measures temperature and does not provide any information about air pressure. This is distinctly different from what is needed in the context of duct measurements. Thus, for measuring air pressure within a duct system, the manometer is the most appropriate and specialized instrument.