Under Pressure: Why So Many Units?

Unless you’re talking about the David Bowie song, understanding that something is under pressure requires some precise measurement.  However, in measuring pressure, there are many units that must be taken into consideration.  But why?  Why do there need to be so many units in order to measure one thing?  To understand that, it’s important to understand what pressure is.

What is Pressure?

As an equation, pressure is equal to the force divided by the area.  In other words, it’s the force applied when considering the area over which that force is applied.

What can be Measured?

Once you realize that the measurement of pressure involves area, you can begin to see where the confusion starts.  Think about all of the different measurements that humans have come up with for measuring a given space.  This doesn’t just refer to the difference between metric and imperial units.  Measuring an area of land is often different from measuring a volume of water or a cloud of a gas.

What are the Common Measurements?

• Pascal (Pa) – This is the SI unit for pressure. It can be adapted into other forms such as the kilopascal (kPa) or hectopascal (hPa).  Prior to being adopted in 1971, the standard unit was Newtons per Square Meter.  Pascals are not always convenient for a given volume, so other kinds of measurement have been retained.
• Bars and Millibars (bar)(mbar) – One pascal is equal to 0.01 miilbars, and one bar is equal to 100,000 Pascals. These terms are often used for discussing atmospheric air pressure.
• Standard Atmosphere (atm) – One atmosphere is a constant equal to the air pressure at earth mean sea level.
• Pounds per Square Inch (psi) – This is the pressure that results from one pound of force applied to an area of one square inch. This is commonly used in tire pressure.