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 air atoms colliding with the sides of their container. Imagine that you have a single atom of nitrogen in a sealed container. That atom is in constant motion ricocheting off the sides of the container. The speed of the atom’s motion is controlled by the temperature. At 0 degrees Kelvin (absolute zero) the atom has no motion, and at higher temperatures the speed increases.

By its collisions with the sides of the container, the atom exerts an outward pressure. So there are two ways to increase the pressure inside the container:

• Raise the temperature of the atoms inside the container – The hotter the atoms, the faster they move.
• Put more atoms in the container – The more gas atoms you put in the container, the more collisions you get and the greater the pressure they exert on the sides of the container.

When you blow up a tire on a car or a bike, use a pump to increase the pressure of the air inside the tire by increasing the number of atoms inside the tire. A car tire typically runs at 30 psi, and a bike tire might run at 60 to 100 psi. There is no magic here — the pump simply stuffs more air into a constant volume, so the pressure rises.

Absolute Pressure

Absolute pressure is the sum of gauge pressure and atmospheric pressure.

Gauge Pressure + Atmospheric Pressure = Absolute Pressure

Atmospheric Pressure is a unit of pressure defined as equivalent to 14.696 PSI (Pounds per square inch).

If your tire gauge reads 34 psi (pounds per square inch), then the absolute pressure is 34 psi plus 14.7 psi (atm in psi), or 48.7 psi (equivalent to 336 kPa).