Barometer vs. Manometer

Difference Between Barometer And Manometer

Barometer

Barometer, an instrument for measuring the pressure of the atmosphere. Since atmospheric pressure decreases with altitude above sea level, a barometer can also be used to measure altitude. Barometers are of two general types: mercury and aneroid. In the mercury barometer, pressure is measured by the height of a column of mercury supported by the atmosphere, whereas in the aneroid the pressure is measured by a metallic diaphragm.

Mercury Barometer

There are two types of mercury barometers: fixed cistern and adjustable cistern. In the former the container, or cistern, is fixed in relation to the glass tube. The mercury level in the cistern rises and falls slightly with changing atmospheric pressure, so that the height of the column in the tube is measured from a changing reference point in the cistern. In the adjustable cistern type, the mercury level in the cistern is adjusted by varying the volume of the cistern. As a result, measurement of the height of the column in the tube always starts from a constant reference point built into the top of the cistern.

The mercury column is about 30 inches (76 cm) high at sea level, making for a usable-sized instrument. A water barometer would be about 34 feet (102 meters) high. The low vapor pressure of mercury is also important, since any residual vapor in the vacuum above the liquid in the tube detracts from the accuracy of measurement. Since the density of mercury varies with temperature, and the materials used in the barometer’s structure expand and contract with temperature, corrections must be made for variations from the temperature at which the barometer has been standardized. A mercury barometer has a thermometer mounted on it for this purpose. The barometer is also equipped with a vernier scale to facilitate accurate reading. It is usually housed in a protective case, and must hang vertically. Further refinements for precise work involve correcting the readings for the local value of gravity, which varies slightly with both latitude and altitude, and correcting for the convex shape of the meniscus on the top of the mercury column.

Manometer

Manometer, an instrument for measuring the pressure of a gas. Manometers are typically more accurate for recording extremely low pressures than the more common Bourdon gauges. The simplest form is a U-tube containing mercury or other liquid that stands at the same height in the two legs of the U when the pressure of the gas in each column is equal. One end of the tube is connected with the source of the gas, and the other usually is open to the atmosphere. The difference between the levels gives the pressure of the gas being measured, with the atmospheric pressure taken as 1. The reading is made on a scale between the two legs.

Another simple, but highly accurate, form of manometer is the inclined-tube type. In this type one leg, to which the scale is attached, is inclined at a large angle from the vertical. A small change in pressure produces a small change in the vertical level of the manometer fluid but a large and easily measured change in the length of fluid in the inclined leg.

 

 

 

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