What unit of measurement are manometers calibrated in?

Manometers are specifically designed to measure pressure in fluids. The most common unit of measurement for manometers is inches of water, which provides a very effective way to express small pressure differences in a fluid. This unit is particularly useful because it reflects how high a column of water can be lifted in the manometer due to a pressure differential.

Using inches of water allows for a direct understanding of the pressure exerted by the fluid in terms of a physical height, which can be more intuitive for certain applications compared to other units. While pounds per square inch is also a recognized unit for measuring pressure, it is not typically used in manometers, which focus on fluid columns instead. The other options, degrees Celsius and gallons, pertain to temperature and volume measurement respectively and do not relate to pressure measurement at all. Therefore, inches of water is the correct and most applicable unit for calibrating manometers.