What are devices that use water columns for measuring small pressure changes called?

Devices that utilize water columns for measuring small pressure changes are known as manometers. Manometers are specifically designed to measure pressure differences by balancing the pressure being measured against a column of liquid, typically water or mercury. The height of the liquid column corresponds to the pressure difference, allowing for precise measurements of small variations in pressure.

Manometers are particularly useful in HVAC applications where measuring the slight pressure changes within a system can help ensure optimal operation and safety. The sensitivity of manometers makes them ideal for such tasks, as even minor fluctuations can significantly impact system performance.

In contrast, other devices listed, like barometers, measure atmospheric pressure using different principles; hygrometers are used for measuring humidity levels; and altimeters primarily measure altitude based on changes in atmospheric pressure, without the specific application of monitoring small pressure changes within a controlled environment.