33 Pressure
Pressure Measurements
Pressure: The force exerted by an object on another object, or the force exerted by an object per unit area. That area could be any area you could come up with, but for the most part we use a square inch or a square metre. When dealing with pressure our starting point is going to be pounds per square inch (psi). Pounds per square inch is the imperial version of pressure, and the one most of us are most likely familiar with. We use this measurement when dealing with things like pumping up car or bike tires, or when dealing with water pressure.
The most common metric version of pressure is known as the Pascal (Pa), and quite often we put Pascals into groups of 1000, so it is then called a kilopascal (kPa). Now, as pressure is defined as force per unit area, we should define what a Pascal actually is. One Pascal is equal to a pressure of one Newton per metre square (n/m2) where a Newton is a measure of force.
Newton: The force needed to move (or accelerate) an object weighing one kilogram at one meter per second squared.
[latex]1\text{ psi}=6.895\text{ kilopascals}[/latex]
[latex]\dfrac{1\text{ psi}}{10\text{ psi}}=\dfrac{6.895\text{ kPa}}{(X)\text{ kPa}}[/latex]
[latex]1\times(X)=10\times6.895[/latex]
[latex](X)=68.95[/latex]
Answer = [latex]68.95\text{ kPa}[/latex]
One pound per square inch equals: 6.895 kilopascals (kPa)
Air pressure at different Altitudes:
- Sea level – 14.7 psi
- 10,000 feet – 10.2 psi
- 20,000 feet – 6.4 psi
- 30,000 feet – 4.3 psi
- 40,000 feet – 2.7 psi
- 50,000 feet – 1.6 psi
To check the reading on the tire pressure gauge, simply read the slide ruler for manual pen gauges, the dial for dial pressure gauges, or the digital screen for digital pressure gauges. The ruler, dial, or screen should display the current tire pressure in PSI.
Understanding Pressure
Let’s say you take a 1-inch by 1-inch piece of wood that’s 3 feet long, and let’s say this piece of wood weighs 1 pound. If you were to stand that piece of wood on-end on your foot, it would place 1 pound of force on your toe.
Since its cross-section is 1 square inch, it exerts 1 pound per square inch of force (1 psi) on your toe.
If you were to take a 30-foot-long piece of the same wood and balance it on your foot, it would apply 10 psi of pressure.
If it were 300 feet long, it would apply 100 psi, and so on.
Water that is 1 foot deep exerts 0.43 psi, so if you are a mile underwater there’s about 2,270 psi being exerted. That is, a 1-inch-square column of water a mile high weighs 2,270 pounds.
Air works the same way. The atmosphere is about 50 miles “deep,” and at sea level it exerts 14.7 psi. That is, a 1-inch-square column of air 50 miles high weighs 14.7 pounds. Our bodies think 14.7 psi of air pressure is completely normal.
Exerting Pressure
For every 10° fluctuation in air temperature, vehicle tire pressure will vary by about 1 psi. The way a gas exerts pressure inside a container like a tire or a balloon is through the action of the