Inches of mercury (inHg) to pounds per square inch (psi) 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 pounds per square inch?

Pounds per square inch (psi) is a unit of pressure that's commonly used, especially in the United States. Understanding what it represents and how it's derived helps to grasp its significance in various applications.

Definition of Pounds per Square Inch (psi)

Pounds per square inch (psi) is a unit of pressure defined as the amount of force in pounds (lbs) exerted on an area of one square inch ($in^2$).

$$Pressure (psi) = \frac{Force (lbs)}{Area (in^2)}$$

How psi is Formed

Psi is derived by dividing the force applied, measured in pounds, by the area over which that force is distributed, measured in square inches. It's a direct measure of force intensity. For example, 10 psi means that a force of 10 pounds is acting on every square inch of the surface.

Applications and Examples of psi

• Tire Pressure: Car tires are typically inflated to 30-35 psi. This ensures optimal contact with the road, fuel efficiency, and tire wear.

• Compressed Air Systems: Air compressors used in workshops and industries often operate at pressures of 90-120 psi to power tools and equipment.

• Hydraulic Systems: Hydraulic systems in heavy machinery (like excavators and cranes) can operate at thousands of psi to generate the immense force needed for lifting and moving heavy loads. Pressures can range from 3,000 to 5,000 psi or even higher.

• Water Pressure: Standard household water pressure is usually around 40-60 psi.

• Scuba Diving Tanks: Scuba tanks are filled with compressed air to pressures of around 3,000 psi to allow divers to breathe underwater for extended periods.

Pascal's Law and Pressure Distribution

Pascal's Law is relevant to understanding pressure in fluids (liquids and gases). Blaise Pascal was a French mathematician, physicist, and philosopher. Pascal's Law states that pressure applied to a confined fluid is transmitted equally in all directions throughout the fluid. This principle is fundamental to hydraulics and pneumatic systems where pressure is used to transmit force. Pascal's Law can be summarized as:

A change in pressure at any point in a confined fluid is transmitted undiminished to all points in the fluid.

More formally:

$$\Delta P = \rho g \Delta h$$

Where:

• $\Delta P$ is the hydrostatic pressure difference (in Pascals or psi)
• $\rho$ is the fluid density (in $kg/m^3$ or $lbs/in^3$)
• $g$ is the acceleration due to gravity (approximately $9.81 m/s^2$ or $32.2 ft/s^2$)
• $\Delta h$ is the height difference (in meters or inches)

For more information, you can refer to this excellent explanation of Pascal's Law at NASA