Manometer, << muh NOM uh tuhr, >> is an instrument used to measure the pressure of a gas or vapor. There are several types of manometers. The simplest kind consists of a U-shaped tube with both ends open. The tube contains a liquid, often mercury, which fills the bottom of the U and rises in each of the arms. The person using this type connects one of the arms to the gas whose pressure is to be measured. The other arm remains open to the atmosphere. In this way, the liquid is exposed to the pressure of the gas in one arm and atmospheric pressure in the other.
If the pressure of the gas is greater than that of the atmosphere, the liquid rises in the arm of the tube exposed to the air. The user measures the difference between the heights of the liquid in the two arms to determine the pressure of the liquid. The liquid pressure is equal to the product of the measured height difference and the specific weight (weight per unit volume) of the liquid. The sum of the liquid pressure and the atmospheric pressure is the gas pressure.
In some manometers, the air is removed from one arm of the tube and that end is sealed. This eliminates the need for corrections due to changes in atmospheric pressure. The difference between the levels of the liquid in the arms shows the pressure of the gas. Gas pressure is often measured in units of the height of the liquid. For example, manometers called barometers commonly measure atmospheric pressure in centimeters of mercury (see Barometer ). Some manometers work on the principle of a spring attached to an indicator. The indicator moves in front of a graduated scale that gives direct pressure readings. Physicians use a type of manometer called a sphygmomanometer to measure blood pressure.
