What units are used to measure pressure?

When we look at the unit of measure called the Pascal (denoted as Pa), we're talking about a very small amount of pressure. It's officially defined as one newton of force acting on one square meter. To visualize it better, imagine the weight of a small apple pressing down on a surface as large as a door; that's roughly equivalent to one Pascal.

Understanding that this is the SI (International System of Units) standard for pressure, Pascals can be found in scientific and engineering equations to calculate forces in fluids or gases. Since they measure such a small amount of pressure, they're often used in multiples like kilopascals (kPa) or megapascals (MPa) for practicality in real-world applications, such as tire pressure or atmospheric conditions.

An atmosphere (atm) is another common unit of pressure, which originates from measuring the weight of the air at sea level on Earth. One atmosphere is equivalent to the pressure exerted by a 1-meter column of mercury at sea-level conditions, which is approximately 101,325 Pascals.

It is a very intuitive unit because it's based on the everyday experience of pressure we're all subject to. When divers go underwater, they often refer to atmospheres to describe the pressure they feel because it increases reliably with depth. Likewise, when we talk about outer space being a vacuum, it's said to have a pressure of virtually zero atmospheres.

Moving on to the bar, which is similar but not exactly equivalent to an atmosphere. One bar is exactly 100,000 Pascals, making it slightly less than the pressure of an average atmosphere at sea level. It was once aimed to be closely aligned with atmospheric pressure to provide an easy reference point for meteorologists and engineers, often showing up in weather forecasts and reports.

Despite its convenience, it is not the official SI unit, yet still prevalent due to its historical and practical usage. For example, you might see bar used as a unit of measure for air pressure in car tires in Europe.

Torr is an intriguing unit of pressure because its name pays homage to the inventor of the barometer, Evangelista Torricelli. One torr is defined as 1/760 of an atmosphere, which directly relates back to the way early barometers used a column of mercury (Hg) to measure pressure.

An interesting aspect of the torr is how it bridges the gap between two worlds: the practical, hands-on method of measuring pressure with a mercury barometer, and the standardized units like atmospheres that we use in calculations. In medicine, for example, blood pressure is sometimes measured in millimeters of mercury, which is conceptually tied to the torr.

Commonly known as psi, pounds per square inch is a unit of pressure that is particularly prominent in the United States, primarily in industrial, automotive, and aviation areas. One psi represents the amount of force exerted by one pound-force applied to an area of one square inch.

This unit resonates with everyday experiences, especially for activities such as inflating tires or measuring the compression inside engines. It may seem like a straightforward concept but is crucial for ensuring the safe and efficient operation of a myriad of systems. Since the United States hasn't fully adopted the SI system, psi remains a significant unit of pressure in various applications within the country.