Bar (unit)

A bar is a metric unit used to measure pressure. Pressure is the force applied to a surface over a specific area. While the bar is not part of the International System of Units (SI), it remains a vital tool for many scientists. It is used to understand everything from deep ocean depths to the air around us. The name itself comes from an ancient Greek word meaning "weight."

To understand the scale of a bar, we can look at our own atmosphere. One bar is slightly less than the average atmospheric pressure at sea level on Earth. That average pressure is approximately 1.013 bar. Using the barometric formula, we can find another way to visualize this value. One bar is roughly the atmospheric pressure found at an altitude of 111 metres when the temperature is 15 °C. This makes the bar a very useful reference point for studying the Earth's environment.

Scientists define the bar using the SI derived unit known as the pascal. One bar is exactly equal to 100 kilopascals (kPa) or 1,000 hectopascals (hPa). This unit can also be expressed in older terms like the barye, which is a unit from the CGS system. Because the pascal often involves very large numbers, engineers frequently prefer using the bar. This preference helps them work with more manageable figures in their daily calculations.

There are many different versions of the bar used for different scales of measurement. These are created by adding metric prefixes to the base unit. For example, a decibar (dbar) is one-tenth of a bar, and a centibar (cbar) is one-hundredth. A millibar (mbar) is one-thousandth of a bar. On the much larger side, a kilobar (kbar) represents 1,000 bars, and a megabar (Mbar) represents 1,000,000 bars. In geology, scientists use the kilobar to study high-pressure systems in experimental petrology.

The history of the bar is tied to the birth of modern weather science. The unit was introduced by Vilhelm Bjerknes, a Norwegian meteorologist. Bjerknes was a founder of modern weather forecasting practices. He originally defined the bar as one megadyne per square centimetre. Although the SI brochure now omits the bar, it has been legally recognized in the European Union since 2004. In the United States, the National Institute of Standards and Technology (NIST) deprecates its use, except for limited use in meteorology.

Different fields use these units in very specific ways. In oceanography, the decibar is a common tool. In fresh water, an increase of one decibar is roughly equal to a one-metre increase in depth. For sea water, scientists use a more complex empirical formula to convert pressure into depth. Scuba divers also rely on the bar to measure pressure. A full standard scuba tank often holds about 200 bar of air.

Meteorologists also use these units to track the weather. Many weather reporters worldwide use the millibar because the values are convenient. In the United States, Americans often see millibars used in reports about hurricanes and cyclonic storms. In aviation, most countries use the hectopascal (hPa) as the standard unit for barometric pressure. These units are numerically equivalent to the millibar, which makes the transition easier for many professionals.

You can find the bar in many mechanical and industrial settings. Outside the United States, people often describe turbocharger boost in bar. Drivers also use it to check tire pressure.

In heavy industry, hydraulic machinery components are often rated by their maximum oil pressure. This pressure is typically measured in hundreds of bar, with 300 bar being common for fixed machinery. Even in the oil industry, versions like "BarG" for gauge pressure and "BarA" for absolute pressure are still widely used.