A U-tube manometer is a type of fluid pressure measuring device that consists of a U-shaped tube filled with a liquid. The two arms of the U-tube are open to the atmosphere and connected to the points of measurement. The pressure at a point in a fluid is proportional to the height of the fluid above that point, and in a U-tube manometer, the fluid level in one arm of the U-tube is higher or lower than the fluid level in the other arm depending on the pressure difference between the two points. The difference in fluid level is proportional to the pressure difference, and can be measured using a scale or a differential pressure transducer.

U-tube manometers are commonly used to measure the pressure of gases, liquids, and vapors in a variety of applications, including HVAC systems, fluid power systems, and chemical processing plants. They are also used to measure pressure drops across filters, valves, and other flow-restricting components. U-tube manometers are relatively simple and inexpensive devices and can be used in a variety of pressure ranges, from low to high.