An instrument called a manometer measures the pressure of a gas or vapor; some consist of a U-shaped tube with a moving column of liquid, others have an electronic design. Manometers see use in industrial, medical and scientific equipment, allowing an operator to monitor gas pressure by reading marks on the device. They are essential to maintain the efficiency of gas-related processes and avoid excessive pressures that might cause an explosion.

A U-tube manometer is comprised of a U-shaped hollow glass column containing a small amount of colored water, mercury or other liquid. A pressure difference between the open ends of the “U” pushes the liquid towards the side with lower pressure. The column has marks to indicate the amount of pressure difference. Variations on the U-tube manometer design include the McLeod and well gauge designs.

A capacitance manometer is a chamber divided in two sections by a flexible membrane. One side contains a sealed reference vacuum, the other opens to a tube connected to the system under measurement. Pressure changes in the tube cause the membrane to flex, changing its electrical capacitance. An electronic circuit measures the capacitance and translates it into a pressure reading on a digital or analog display. It can also be part of an automatic system that may open valves or otherwise respond to pressure changes without operator intervention.