Manometer
A manometer is a device that is able to measure the pressure of a medium (a liquid, or a gas), it is also used to check the pressure of cylinder but it can only check the inner pressure not outer. In most cases, the relative pressure will be measured. A barometer can measure the absolute pressure, compared to a vacuum.A common simple manometer consists of a U-shaped glass tube filled with a liquid. It is a simple pressure gauge and it's measure difference in pressure exerted at the two ends of the glass tube. The name ("manometer") comes from Ancient Greek, μανός manós  -thin and μέτρον métron measurement, or ruler. Many animals have a tympanic membrane (or eardrum), which only reacts to changes in pressure similar to a manometer.