A manometer is read in:

A manometer is a device used to measure the pressure of gases or liquids, and it can be calibrated in different units, depending on its application. The most common units in which manometers are read include inches of water, inches of mercury, and sometimes Pascals. When it comes to standard manometer readings, especially in low-pressure applications typically found in HVAC systems, inches of water is often used because it is convenient for measuring small pressure differences.

In contrast, while inches of mercury is also a legitimate unit for measuring pressure, it is typically reserved for higher pressure measurements, such as barometric pressure monitoring. Pascals are a SI unit of pressure and may not be as practical in all settings involving manometers, particularly in situations with low pressure where a water column provides a more intuitive reading. Barometric pressure is a specific form of pressure measurement and does not pertain directly to the function of a manometer but rather focuses on atmospheric measurements.

By reading in inches of water, the manometer provides a direct and understandable representation of pressure that is suitable for many practical engineering applications, especially in systems affected by minor fluctuations in pressure.