technical atmospheres (at) to bar (bar) conversion

What is the technical atmosphere?

The technical atmosphere (at) is a non-SI unit of pressure equal to one kilogram-force per square centimeter. It was widely used in engineering, especially in Europe, before the pascal became standard.

Definition

The technical atmosphere is defined as the pressure of one kilogram-force acting on one square centimeter:

$$1\ \text{at} = 98066.5\ \text{Pa}$$

This equals exactly 1 kgf/cm² (98,066.5 Pa), because standard gravity is 9.80665 m/s². It is close to, but distinct from, the standard atmosphere (atm = 101,325 Pa) and the bar (100,000 Pa).

Origin and History

The technical atmosphere arose from the gravitational metric system of units, which used the kilogram-force rather than the newton. Expressing pressure as kilogram-force per square centimeter was intuitive for engineers weighing loads, and the unit became common in mechanical and hydraulic engineering, boilers, and pneumatics through much of the 20th century.

Law and Notable Facts

The technical atmosphere is not part of the SI and its use is discouraged in favor of the pascal or bar. It is easily confused with the standard atmosphere; the two differ by about 3.3%. Gauge and absolute variants were often written "atü" (gauge) and "ata" (absolute) in German-language engineering.

Real-World Examples and Conversions

• 1 at = 1 kgf/cm² = 98.0665 kPa ≈ 0.9678 atm.
• 1 at ≈ 14.223 psi, close to but slightly below the 14.696 psi of one standard atmosphere.
• A pressure of 10 at (about 981 kPa) is a common rating benchmark for industrial hydraulic components.
• 1 at ≈ 0.980665 bar, so the bar and technical atmosphere differ by under 2%.

What is bar?

The bar is a metric unit of pressure, widely used in science, engineering, and industry. It's a convenient unit because it is close to standard atmospheric pressure on Earth. Below is detailed information about bar, it's origin, and some real-world examples.

Definition of Bar

The bar is defined as exactly $100,000$ Pascals ($10⁵ Pa$). The Pascal (Pa) is the SI unit of pressure, defined as one Newton per square meter ($N/m^2$). Therefore:

$1 \, bar = 100,000 \, Pa = 10⁵ \, N/m^2$

Origin and History

The bar was introduced by British physicist Sir Napier Shaw in 1909. The goal was to have a unit of pressure that was close to atmospheric pressure but based on the metric system. The term "bar" comes from the Greek word "βάρος" (baros) meaning "weight."

Relation to Atmospheric Pressure

Standard atmospheric pressure at sea level is approximately $1.01325$ bar. Because of this proximity, the bar and millibar (1 mbar = 0.001 bar) are frequently used in meteorology to measure atmospheric pressure. Historically, meteorologists used millibars, but now the SI unit, the hectopascal (hPa), is also widely used (1 hPa = 1 mbar).

Real-World Examples and Applications

• Tire Pressure: Car and bicycle tire pressures are often measured in bar or PSI (pounds per square inch). For example, a car tire might be inflated to 2.5 bar.
• Weather Reports: Atmospheric pressure in weather reports can be given in millibars or hectopascals, where 1013.25 mbar is standard atmospheric pressure.
• Scuba Diving: Divers often use bar to measure the pressure of compressed air in their tanks. A typical scuba tank might be filled to 200 bar.
• Industrial Processes: Many industrial processes, such as hydraulic systems and pressure testing, use bar as a convenient unit of measurement.
• Geology: Pressures deep within the Earth are often measured in kilobars (kbar), where 1 kbar = 1000 bar.
• Vacuum: While bar is not commonly used for measuring high vacuum, it's relevant when discussing rough or backing vacuum levels. For high vacuum, units like Torr or Pascal are more typical.

Interesting Facts

• The bar is a metric unit but not an SI unit. The SI unit for pressure is the Pascal (Pa).
• The millibar (mbar) is commonly used in meteorology.
• 1 bar is approximately equal to 0.987 atmospheres (atm).