Bar (unit)

The bar (symbol bar) is widely used in many countries as a unit of pressure. It is not an SI unit, nor is it a cgs unit, but it is accepted for use with SI units by NIST.  Except for the power of ten, the definition of bar fits in the sequence of SI units (Pa, kPa, MPa), namely, 1 bar ≡ 100,000 Pa = 100 kPa = 0.1 MPa. This is in contrast to the well-known unit of pressure, atmosphere, which now is defined to be 1.01325 bar exactly. As a rule of thumb, a bar is (almost) equal to an atmosphere.

The bar and the millibar were introduced by the British meteorologist William Napier Shaw in 1909. William Napier Shaw was the director of the Meteorological Office in London from 1907 to 1920.

Definition

The bar, decibar (symbol dbar), centibar (symbol cbar) and millibar (symbol mbar or mb) are defined as:

• 1 bar = 10⁵ Pa (Pascals) = 10⁶ dyn/cm² (dynes per square centimetre) = 0.987 atm (atmospheres)
• 1 dbar = 0.1 bar = 10⁴ Pa = 10 kPa
• 1 cbar = 0.01 bar = 10 ³ Pa = 1 kPa
• 1 mbar = 0.001 bar = 10² Pa = 1 hPa (hectopascal)

Usage

Although millibars are not an SI unit, meteorologists and weather reporters worldwide have long measured air pressure in millibars. After the advent of SI units, some meteorologists began using hectopascals (symbol hPa) which are numerically equivalent to millibars. For example, the weather office of Environment Canada uses hectopascals on their weather maps.

Atmospheric air pressure is often expressed in millibars and sea level atmospheric air pressure is defined as 1013.25 mbar which is equivalent to 1 atm.

In water, there is an approximate numerical equivalence between the change in pressure in decibars and the change in depth from the sea surface in metres. Specifically, an increase of 1 decibar occurs for every 1.019716 metre increase in depth close to the surface. As a result, decibars are commonly used in oceanography.

Many engineers worldwide use the bar as a unit of pressure because, in much of their work, using pascals would involve using very large numbers.

Absolute pressure and gauge pressure

Bourdon tube pressure gauges, vehicle tire gauges and many other types of pressure gauges are zero referenced to atmospheric pressure, which means that they measure the pressure above atmospheric pressure. However, absolute pressures are zero referenced to a complete vacuum. Thus, the absolute pressure of any system is the gauge pressure of the system plus atmospheric pressure.

Absolute pressures expressed in bar are often referred to as bara, whereas gauge pressures expressed in bar are often referred to as barg.

In the United States, where pressures are still often expressed in pounds per square inch (symbol psi), gauge pressures are referred to as psig and absolute pressures are referred to as psia.

Sometimes, the context in which the word pressure is used helps to identify it as meaning either the absolute or gauge pressure. However, in truth, whenever a pressure is expressed in any units (bar, Pa, psi, atm, etc.), it should be denoted in some manner as being either absolute or gauge pressure to avoid any possible misunderstanding. One recommended way of doing so is to spell out what is meant, for example as bar gauge or kPa absolute.