We fluid power industry professionals probably take pressure for granted. If I asked any of you to define pressure, you would say force over a predetermined area. We all typically use similar units, such as pounds per square inch (psi) or Newtons per square metre (Pa), but pressure isn’t confined for use in the fluid power industry alone (no pun intended). Pressure exists everywhere at all times, and even the most remote location in outer space will still have a few atoms of matter loitering around to ruin a perfect vacuum.
I’m going recap week one of hydraulics night class, so forgive me if this is redundant. The gases held close to the Earth’s crust by gravity have enough mass to create pressure around us at all time. This atmospheric mass is enough to exert 14.7 lb on every single square-inch plane equally in every direction. Think about that for a moment. If you held your hand out flat with your palm up and we could magically create a vacuum under the other side of your hand, you would have in the realm of 400 lb pushing down on your hand.
Go ahead, hold out your hand. You, in fact, do have 400 lb pushing down on your hand (give or take, depending if you have hands like a toddler or pro-wrestler). You do not feel 400 lb pushing down on your hand because there is also 400 lb pushing up on your hand, and that is the nature of fluid pressure as define by Blaise Pascal: “when there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.”
Around you right now—if you live at or near sea level—are 14.7 lb per every single square inch of your body, in every direction inward to your body and every direction outward from inside your body. Force from an infinite number of directions pushes equally and opposite to an infinite number of places on your body due to the isotropic nature of a liquid or gas. But why? Why does the mere existence of air molecules around you cause them to push on each other and your body?
There is some epic science going on at the atomic level to explain the nature of pressure. You can’t see air molecules up close because they are smaller than the wavelengths of visible light passing around them, although with enough air molecules—we’re talking an atmosphere worth—you can see the compound result of countless diffraction events creating our beautiful blue sky. However, just because we can’t see air particles, it doesn’t mean they’re not busier than a border collie with ADHD and a caffeine addiction. I’m going to refer to air atoms and molecules as particles from here on out.
The motion of atoms in a fluid is dynamic and incessant. Their motion is complex, but air particles essentially travel in a straight path until they smack into a solid object or hit other air particles. After hitting something, the particles deflect, reflect and generally bounce around like undiminishing-energy squash balls trapped in a box, as seen here. Their energy to create pressure, for the most part, remains constant. Energy is not wasted as heat in the “Brownian” chaos of particle movement; any kinetic motive being converted to heat as a result of these smashing particles results in an equivalent excitement of motion, conserving energy entirely. Particle movement creates heat, and heat creates particle movement, both of which maintain pressure.
The pressure created from a mass of fluid is different from the pressure created from a solid. A solid object will apply pressure only in the direction of its force vector, such as an anvil being pulled to the centre of the Earth, usually landing on Wile E. Coyote’s noggin. However, if you put your finger to the side of the anvil, the only pressure is a result of the force applied by your finger, and only in the direction of the push.
Atmospheric fluid pressure (remember, air is a fluid, as it can take the shape of its container) is a result of the mass of the air column extending up to outer space. If you placed a weightless, one-inch square tube extending to the top of the troposphere, used the giant vacuum from Spaceballs to suck away the rest of our atmosphere, and then weighed your tube, you would find it weighed 14.7 lb. I find that fact amazing, especially considering the air around me provides no perceivable effect of mass on me whatsoever.
Pressure in a fluid power system isn’t much different from atmospheric pressure, only that there is more of it. By packing more matter into a smaller area, we are able to create more pressure. In a pneumatic system, the increase in pressure is directly proportional to the number of particles crammed into the same sized space. Twice the particles moving with the same energy means twice the impacts on the walls of the container, exerting twice the pressure.
A hydraulic system doesn’t need nearly the same increase in particles to create exponentially more pressure. In a sealed pressure vessel measuring zero gauge pressure, we need just one percent more molecules to be packed into the same space to increase gauge pressure to 2,000 psi. Doubling the number of molecules of hydraulic oil in this pressure vessel would result in an increase in pressure to well over 200,000 psi on our pressure gauge. Fun fact: if we sank this pressure vessel to the bottom of an 86,580-ft sea on a remote planet in a galaxy far, far away, gauge pressure would drop to zero as it equalized with the 200,000-psi of surrounding water pressure.
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