Inches of mercury (inHg) to bar (bar) conversion

What is Inches of mercury?

The "inches of mercury" (inHg) is a unit of pressure commonly used in the United States. It's based on the height of a column of mercury that the given pressure will support. This unit is frequently used in aviation, meteorology, and vacuum applications.

Definition and Formation

Inches of mercury is a manometric unit of pressure. It represents the pressure exerted by a one-inch column of mercury at a standard temperature (usually 0°C or 32°F) under standard gravity.

The basic principle is that atmospheric pressure can support a certain height of a mercury column in a barometer. Higher atmospheric pressure corresponds to a higher mercury column, and vice versa. Therefore, the height of this column, measured in inches, serves as a direct indication of the pressure.

Formula and Conversion

Here's how inches of mercury relates to other pressure units:

• 1 inHg = 3386.39 Pascals (Pa)
• 1 inHg = 33.8639 millibars (mbar)
• 1 inHg = 25.4 millimeters of mercury (mmHg)
• 1 inHg ≈ 0.0334211 atmosphere (atm)
• 1 inHg ≈ 0.491154 pounds per square inch (psi)

Historical Context: Evangelista Torricelli

The concept of measuring pressure using a column of liquid is closely linked to Evangelista Torricelli, an Italian physicist and mathematician. In 1643, Torricelli invented the mercury barometer, demonstrating that atmospheric pressure could support a column of mercury. His experiments led to the understanding of vacuum and the quantification of atmospheric pressure. Britannica - Evangelista Torricelli has a good intro about him.

Real-World Applications and Examples

• Aviation: Aircraft altimeters use inches of mercury to indicate altitude. Pilots set their altimeters to a local pressure reading (inHg) to ensure accurate altitude readings. Standard sea level pressure is 29.92 inHg.

• Meteorology: Weather reports often include atmospheric pressure readings in inches of mercury. These readings are used to track weather patterns and predict changes in weather conditions. For example, a rising barometer (increasing inHg) often indicates improving weather, while a falling barometer suggests worsening weather.

• Vacuum Systems: In various industrial and scientific applications, inches of mercury is used to measure vacuum levels. For example, vacuum pumps might be rated by the amount of vacuum they can create, expressed in inches of mercury. Higher vacuum levels (i.e., more negative readings) are crucial in processes like freeze-drying and semiconductor manufacturing. For example, common home vacuum cleaners operate in a range of 50 to 80 inHg.

• Medical Equipment: Some medical devices, such as sphygmomanometers (blood pressure monitors), historically used mmHg (millimeters of mercury), a related unit. While digital devices are common now, the underlying principle remains tied to pressure measurement.

Interesting Facts

• Standard Atmospheric Pressure: Standard atmospheric pressure at sea level is approximately 29.92 inches of mercury (inHg). This value is often used as a reference point for various measurements and calculations.

• Altitude Dependence: Atmospheric pressure decreases with altitude. As you ascend, the weight of the air above you decreases, resulting in lower pressure readings in inches of mercury.

• Temperature Effects: While "inches of mercury" typically refers to a standardized temperature, variations in temperature can slightly affect the density of mercury and, consequently, the pressure reading.

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 ($10^5 Pa$). The Pascal (Pa) is the SI unit of pressure, defined as one Newton per square meter ($N/m^2$). Therefore:

$1 \, bar = 100,000 \, Pa = 10^5 \, N/m^2$

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).