Main Catalogue - VUOTOTECNICA - #9

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8 VACUUM DEFINITION, MEASUREMENT UNIT AND CLASSIFICATION IN THE CONTAMPORARY AGE The term “vacuum” refers to the physical situation that occurs in an environment in which the gas pressure is lower than atmospheric pressure. Since the pressure represents a force per unit area, its unit is the Pascal (symbol Pa), which represents the pressure there is when the force of a newton (symbol N) is exerted on the unit area, measured in square metres (symbol m2). Therefore: pascal = newton/m2 ==> Pa = N/m2 The Pa is adopted dal International System of Units (abbreviated SI). However, alternative measurement units are also used, such as the millibar (mbar) equal to 100 Pa and the torr or mmHg, equal to 133.322 Pa. The latter is a non-SI unit, but it is tolerated for measuring blood pressure. VACUUM LEVELS Depending on whether the pressure is slightly or much lower than atmospheric pressure (i.e., 101325 Pa), different phenomena can take place, and different means can be used to obtain and measure that pressure. There are different vacuum levels, each with a specific designation according to the intervals of sub-atmospheric pressure, as indicated in the table below: - Rough vacuum (RV): from 105 to 102 Pa - Medium vacuum (MV): from 102 to 10-1 Pa - High vacuum (HV): from 10-1 to 10-5 Pa - Ultra-high vacuum (UHV): from 10-5 to 10-9 Pa - Extremely-high vacuum (EHV): < 10-9 Pa In the industrial field, vacuum is divided into three application fields, depending on the required vacuum level. - Low vacuum: Generally used in applications requiring a high air intake flow. This application field usually makes use of impeller type electro-mechanical pumps, such as centrifugal fans, side channel blowers, flow generators etc. Fabric silk-screen printing, for instance, falls in this category of applications requiring a low vacuum level. - Industrial vacuum: The term “industrial vacuum” refers to a vacuum level between -20 and -99 KPa. This range includes most applications. Industrial vacuum is mainly produced by rotary, liquid ring, piston, and lobe vacuum pumps, all activated by electric, internal combustion or hydraulic engines and compressed air-powered vacuum generators based on Venturi’s principle. Industrial vacuum applications include handling by vacuum cups, thermoforming, vacuum clamping, vacuum packaging, etc. - Process vacuum: Refers to a vacuum level over -99 KPa. The main generators of this vacuum level are the two-stage rotary pumps, eccentric rotor pumps, Roots pumping units, turbomolecular pumps, diffusion pumps, cryogenic pumps, etc. The most common applications include freeze-drying, metalisation and vacuum thermal treatments. As for the scientific applications, on the other hand, these include space simulations. The highest vacuum level on Earth is far from the value of absolute vacuum, which remains a purely theoretical value. In fact, even in space, and therefore in absence of atmosphere, there is a minimum presence of atoms.