Question

What are the common units of pressure? List them in order of smallest to largest unit.

Short answer

The common units of pressure from smallest to largest are: Pascal (Pa), Torr, millimeters of mercury (mmHg), pounds per square inch (psi), bar, and atmosphere (atm).

Step-by-step solution

Understanding Pressure

Pressure is defined as the force applied perpendicular to the surface of an object per unit area over which that force is distributed. It is commonly measured in several different units.

List Common Units of Pressure

Common units of pressure include Pascals (Pa), bars, atmospheres (atm), Torr, millimeters of mercury (mmHg), and pounds per square inch (psi).

Arrange Units by Size

Order the units from smallest to largest: Pascal (Pa), Torr (Torr), millimeters of mercury (mmHg), pounds per square inch (psi), bar (bar), and atmosphere (atm). To compare these units, you need their equivalent values. One atmosphere is standard atmospheric pressure at sea level and is equal to 101,325 Pascals, 760 millimeters of mercury, 760 Torr, 14.696 pounds per square inch, and 1.01325 bar.

Final Ordering

Based on the equivalent values, the order of units from smallest to largest is: Pascal (Pa), Torr, millimeters of mercury (mmHg), pounds per square inch (psi), bar, and atmosphere (atm).

Key concepts

Pascals

Pascals (Pa), named after the French mathematician Blaise Pascal, are the SI (International System of Units) unit of pressure. A Pascal is defined as one newton of force applied over an area of one square meter.

In simpler terms, if you were to evenly spread a force of one newton over a square that is one meter on each side, the pressure exerted would be one Pascal. It's worth noting that since Pascal is a relatively small unit, it's common to see pressures expressed in kilopascals (kPa), which are 1,000 Pascals.

Atmospheres

The atmosphere (atm) measures pressure based on the average atmospheric pressure at sea level on Earth. One atmosphere is equivalent to the pressure exerted by a 760 mm column of mercury or about 14.696 psi.

This unit is handy for expressing pressures relative to the Earth's atmosphere, which is a familiar reference point. For example, divers use atmospheres to gauge water pressure levels when they're below the surface because this measure directly correlates with the weight of the overlying water.

Torr

Named in honor of the Italian physicist Evangelista Torricelli, Torr is a unit of pressure where one Torr is approximately equal to the pressure exerted by a millimeter of mercury (mmHg).

Differentiating Between Torr and mmHg

While both Torr and millimeters of mercury are often used interchangeably because of their close values, there is a slight difference: one atmosphere equals 760 mmHg but slightly more, 760.0013 Torr. This minor difference is usually negligible for everyday applications but could matter in scientific research where precision is paramount.

Millimeters of Mercury

Millimeters of mercury (mmHg) is a manometric unit of pressure, commonly used in medicine to measure blood pressure. It indicates the height of a column of mercury that exerts equivalent pressure.

Under standard gravity, one mmHg is nearly equal to the pressure exerted by a 1 mm column of mercury. In most practical situations, 760 mmHg equals one atmosphere. This unit relates pressure readings to mercury's weight, a method that dates back to the first barometers.

Pounds per Square Inch

Pounds per square inch (psi) is a widely used unit of pressure in the British and US customary systems of measurement. It describes the pressure resulting from one pound-force applied to an area of one square inch.

A common example where psi is used is in tire pressure readings. Cars, bicycles, and other inflatable objects often specify required pressures in psi, allowing for consistent performance and safety standards. In terms of relative size, 14.696 psi is equal to one atmosphere.

Pressure Measurement

Pressure measurement is fundamental in a wide array of applications, from weather forecasting to medical diagnostics. That's why it's essential to comprehend the different common units of pressure and their equivalents to communicate measurements accurately.

Importance of Accurate Pressure Measurement

Accurate pressure measurement enables scientists and engineers to create safer and more efficient systems, such as high-performance car engines where proper tire inflation or the accurate calibration of medical devices for blood pressure monitoring can be life-saving.