Manometers, atmospheric pressure measurement method

There are a number of different methods used to express a pressure measurement. Some of them are natural units, i.e., based on a force per unit area, as with pound (force) per square inch (abbreviated Ibf/in or psi) or dyne per square centimeter (dyn/cm ). Others are based on a fluid height, such as inches of water (in H2O) or millimeters of mercury (mm Hg) units such as these are convenient when the pressure is indicated by a difference between two levels of a liquid as in a manometer or barometer. This measurement is based on the pressure at the base of a column (height) of fluid. Barometric pressure and atmospheric pressure are synonymous and measure the ambient air pressure. Standard barometric pressure is the average atmospheric pressure at sea level, 45° north latitude at 32°F. It is used to define another unit of pressure called the atmosphere (atm). Standard barometric pressure is 1 atm and is equivalent to 14.696 psi and 29.921 in Hg. As one might expect, barometric pressure varies with weather and altitude. 

The diameter of the capillary tube must be known with accuracy and the cross-section must be truly circular. It is not as a rule easy to obtain tubing of uniform circular bore, but in default of time for the tedious process of calibrating tubing the difficulty may be overcome by the following method due to Ferguson Proc. Phys. Soc. XXXVI. 37, 1923) Lengths of capillary tube are examined under a micrometer until one is found whose end cross-section is circular. This tube is then used so that the measured end dips downwards into the liquid the upper end, instead of being open to the atmosphere is connected to a source of pressure and a manometer, and the meniscus is forced down until its lowest point is level with the end of the tube, so as to be observed at the only point where its curvature is accurately known. If then p be the pressure in dynes/cm. recorded by the manometer we have... 

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