Bar (unit) The kilobar, equivalent to 100 MPa, is commonly used in geological systems, particularly in experimental petrology. "Bar(a)" and "bara" are sometimes used to indicate absolute pressures and "bar(g)" and "barg" for gauge pressures. This usage is deprecated and fuller descriptions such as "gauge pressure of 2 bar" or "2-bar gauge" are recommended. Bar (unit) The bar is a metric unit of pressure, but is not approved as part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa, which is slightly less than the current average atmospheric pressure on Earth at sea level. The
Standard state Standard state In chemistry, the standard state of a material (pure substance, mixture or solution) is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry (IUPAC) recommends a conventional set of standard states for general use. IUPAC recommends using a standard pressure "p" = 10 Pa. Strictly speaking, temperature is not part of the definition of a standard state. For example, as discussed below, the standard state of a gas is conventionally chosen to be unit pressure (usually in bar)