What type of device is used to measure pressure differences in a fluid?

A manometer is specifically designed to measure pressure differences in fluids by comparing the pressure in a system to a reference pressure, typically atmospheric pressure. It does this using a column of liquid within a U-shaped tube; the height difference of the liquid in the two sides of the tube reflects the pressure differential. This instrument is commonly used in various applications, including HVAC systems, to ensure safe and efficient operation.

While other devices mentioned in the options serve important roles in engineering, they are not intended for measuring pressure differences. A thermometer is used for measuring temperature, a controller is used for regulating process variables (like temperature or pressure) but does not measure directly, and a transducer converts one form of energy into another and is often used in the context of measurements but does not directly measure pressure differences in the way a manometer does. Thus, the manometer stands out as the most appropriate choice for this specific application.