millimeters of mercury (mmHg) to technical atmospheres (at) conversion

What is millimeters of mercury?

Millimeters of mercury (mmHg) is a unit of pressure, often used in medicine (especially blood pressure) and meteorology. It represents the pressure exerted by a column of mercury one millimeter high at a standard temperature. Let's delve into its definition, history, and applications.

Definition and Formation

Millimeters of mercury (mmHg) is a manometric unit of pressure. Specifically, it's the pressure exerted at the base of a column of mercury exactly 1 millimeter high when the density of mercury is 13,595.1 kg/m³ and the local acceleration of gravity is exactly 9.80665 m/s². It's not an SI unit, but it is accepted for use with the SI.

While not an official SI unit (Pascal is the SI unit for pressure), mmHg remains widely used due to its historical significance and practical applications, especially in fields like medicine.

History and Torricelli's Experiment

The unit originates from Evangelista Torricelli's experiments in the 17th century. Torricelli, an Italian physicist and mathematician, invented the mercury barometer in 1643. He filled a glass tube with mercury and inverted it into a dish of mercury. The mercury column would fall, leaving a vacuum at the top, and the height of the column was proportional to the atmospheric pressure. This led to the standardized measurement of pressure using the height of a mercury column. Read more about it in Britannica.

Relation to Other Units

• Pascal (Pa): The SI unit of pressure. 1 mmHg is approximately equal to 133.322 Pascals.

  $$1 \, mmHg \approx 133.322 \, Pa$$

• Atmosphere (atm): A standard unit of pressure. 1 atm is equal to 760 mmHg.

  $$1 \, atm = 760 \, mmHg$$

• Torr: Named after Torricelli, 1 Torr is very close to 1 mmHg. For most practical purposes, they are considered equivalent.

  $$1 \, Torr \approx 1 \, mmHg$$

Real-World Examples and Applications

• Blood Pressure: In medicine, blood pressure is commonly measured in mmHg. For example, a blood pressure reading of 120/80 mmHg indicates a systolic pressure of 120 mmHg and a diastolic pressure of 80 mmHg. The first number represents the pressure in the arteries when the heart beats (systolic pressure) and the second number represents the pressure in the arteries between beats (diastolic pressure).

• Atmospheric Pressure: Meteorologists often use mmHg to report atmospheric pressure. Standard atmospheric pressure at sea level is 760 mmHg. Changes in atmospheric pressure are often precursors to changes in weather.

• Vacuum Gauges: Many vacuum gauges, particularly older or specialized instruments, display pressure in mmHg. Low pressures in vacuum systems, such as those used in scientific experiments or manufacturing processes, are often expressed in mmHg or fractions thereof (e.g., milliTorr, which is approximately 1/1000 of a mmHg).

• Aircraft Altimeters: Aircraft altimeters use atmospheric pressure to determine altitude. While the actual scale on the altimeter might be in feet or meters, the underlying pressure measurement is often related to mmHg.

Important Considerations

While mmHg is widely used, it's essential to be aware of its limitations:

• Temperature Dependence: The density of mercury varies with temperature, so precise measurements require temperature corrections.
• Local Gravity: Although standardized, the local acceleration due to gravity can vary slightly depending on location, potentially affecting accuracy.

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%.