The extensive early use of mercury in manometers led to the widespread adoption of mmHg as a convenient unit of pressure. He found that atmospheric pressure could sustain a column of mercury of about 760mm. What is a Torr?Ītmospheric pressure was first measured by the Italian scientist, Evangelista Torricelli, using a mercury-filled glass tube. In vacuum measurement, pressures are typically given in millibar (mbar) although Torr or millimetre of mercury (mmHg) are also used (see below). The bar is commonly used in weather forecasting and engineering. This is slightly lower than standard atmospheric pressure (101325 Pa). What is a Bar?Ī bar is defined as 100,000 Pa (100 kPa). This suction limit can only be overcome by pressurising the supply vessel or by using multiple pumps and intermediate reservoirs. In reality, the limit is only about 7-8m because of pump inefficiencies, frictional losses, elevation and temperature differences. The pressure exerted by a 10m column of fresh water is roughly equal to atmospheric pressure and this is the maximum height to which water can be raised by a pump using suction. For example, a water pressure of 3 ata consists of 1 atm of air pressure and 2 atm of water pressure. The abbreviation ata denotes an absolute measurement of the total pressure of the system, including atmospheric pressure. For example, the atmospheric pressure in Denver, Colorado, is only approximately 12.1 psi. In reality, atmospheric pressure varies quite widely with elevation, weather, temperature and humidity. 1 atm is equal to 101.325 kPa or 14.7 psi, which corresponds to atmospheric pressure at mean sea level. The standard ‘atmosphere’ (abbreviated to atm) is a convenient unit for measuring pressures. From the original definition, other units can be substituted (g for kg cm or mm for m) to produce a whole range of combinations such as gf/m², kgf/cm², and gf/mm². For specifying industrial pressures, the kilopascal is used when SI units are preferred (1000 kPa = 145 psi). Similar suffixes and notes are sometimes applied to SI units, for example 101 kPa (abs).Īs the pascal is a very small unit, it is commonly quoted in vacuum applications. This distinguishes it from an absolute pressure measurement (lba/in 2, psia), which is relative to vacuum. For example, in the USCS system, lbf/in 2 (the ‘f’ stands for force) or psig (the ‘g’ stands for gauge) shows that the value is relative to ambient atmospheric pressure. Sometimes, pressure units are appended with letters to show how the value has been measured.
14.7 PSI TO KPA PLUS
Most pressure measurements (gauge pressures) are made relative to ambient air pressure – the gauge shows a zero reading when exposed to atmospheric pressure.Īn absolute pressure is referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure (Torr is an absolute unit).Ī differential pressure is the difference in pressure between two places in a system (Head values are differential pressures). Pressure values can be stated in three ways: Pressure units of the SI and USCS systems Measurement System The standard unit of pressure in this system is pound per square inch (PSI): the pressure resulting from a force of one pound applied to an area of one square inch. This is based on Imperial units such as the pound (lb) and inch (in) or foot (ft).
In North America, however, the US Customary System is preferred. The basic unit of pressure is the pascal, defined as the pressure exerted by a force of one newton perpendicularly upon an area of one square metre. It is widely accepted and used across the world. The SI system of units is the International System of Units (Système International) derived from the metric system and is based on the kilogram and the metre. This wide variation is partly down to historical or cultural differences, or a particular method of defining and measuring pressure is more convenient, intuitive and useful in some applications but not in others.
Many terms, abbreviations and acronyms are used to describe pressure and values can be quoted in a host of different units.
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