The part salesman gives a micron rating for his fuel filter. Your high school physics teacher told you that the sun’s gravity at the distance of Pluto is a few microns per second squared. The tool salesman tries to sell you a micron gauge for your HVAC repairs. What is going on here? How are filters, vacuum pumps, and (ex)planets related?

The micron is actually a lot less complicated than you might think. The complication is in the application. A micron is nothing more than a millionth of a meter. Yes, once you start using millions, not to mention billions and trillions, your head may start to swim. Humans often have trouble grasping large and small numbers accurately. So, let’s convert this into something more common. In class, I often use a piece of paper as an illustration of measurement. A standard sheet of copy paper is 3-4 thousandths of an inch (0.003-0.004”). This is also 0.10-0.12 mm. That same sheet of paper is 100 microns thick. So, for those of us who grew up with thousandths, relating it to an item we see every day makes sense.

Notice first that a micron is a lot smaller than 0.001” (a thousandth of an inch). 100 of them = 0.0039”, around 4 thousandths. This alone may make it harder for us to grasp. But if we learn to use microns on a regular basis, the discomfort will go away.

When seen this way, it is not difficult to see why the filter salesman speaks about microns. If a particle is 4 microns in diameter and the filter stops 98% of them, you have a 4-micron rated filter. The sun’s gravity at the distance of Pluto would take up too much space in this article.

How does a micron relate to evacuating an HVAC system? Buckle up, this will take a little time.

It is considered an industry “Best Practice” to pull a vacuum of 500 microns. For that, we need to talk about atmospheric pressure. Air pressure in our industry can be a counter-intuitive pain. And, this is our fault. Our traditional jargon has persisted despite 120 years of technological improvement.  

In the automotive world, we measure pressure in:

Inches of mercury (in Hg) This is our traditional way of measuring intake manifold vacuum.

• Millimeters of mercury (mmHg)
• Pounds per square inch (PSI) Tires.
• Inches of water column (in WC) Fuel tank/Evap pressure.
• Kilopascals (kPa) Intake manifold pressure, metric
• Megapascals (mPa) Diesel fuel injection pressure.
• Hectopascals (hPa) European tires and turbo boost.

Plus, we have two separate scales of measurement. Since we live at the bottom of the ocean (of atmosphere) we set some gauges to zero when they are at the bottom of that ocean (sea level). That was our choice. I’m not suggesting this is a bad idea. You just have to get your head wrapped around it properly, or you will make mistakes.

At the bottom of this atmospheric ocean, we an find atmospheric pressure of 14.7 pounds per square inch (psi). Your tire that shows 30 psi on your tire gauge using Gauge Pressure actually has 44.7 psi Absolute Pressure. Your customer is content to blithely believe that 30 psi is a real number and it is… kind of. But today, “kind of” can lead you astray.

Gauge pressure messes with your mind. Once you set your zero point above zero you add complications. Primarily, you find that going below zero gives you negative numbers. But we often fail to speak in negative numbers. A good running Chevrolet 350 from the 1990s idles at about 20 inHg. Did you catch that mistake? It’s not 20 inHg. It’s -20 (minus 20) inHg. “Vacuum” is how far we are below Gauge Pressure of zero. It is this complication that causes technicians in class to make the following mistake. I ask, “If a small block Chevrolet is idling normally, and the Tech snaps the throttle, does the pressure in the manifold go up or down?” I have been asking this question for many years, and about 90 percent of the students say, “Down.” The reason for this is that if you have ever used a vacuum gauge under these circumstances, you see the needle go from 20 to zero. It takes conscious effort to remember that it is going up from -20 to zero.

To continue, we need to clear up what pressure actually is. Pressure is a force. A coffee cup sitting on a table exerts a force equal to its weight, spread over the area of the table covered by the bottom of the cup. This is the force of gravity. The cup is a solid. If you pick up a solid, you will experience the weight only in the down direction.

The atmosphere, also held down by gravity, is not a solid, but a fluid, and as such exerts pressure differently. We experience (not the same thing as paying attention) air pressure every moment. When we expand our chest, air is forced into our lungs by the weight of the atmosphere. When we open the throttle of an idling engine, we allow air to rush into the manifold, raising the pressure. Because the atmosphere is literally all around us, we get used to it and lose our awareness of it and tend to ignore it. The force on our left is balanced by the force on our right. Also, up and down, front and back. We are being squeezed from every direction by the pressure generated from the weight of the air. We only sense the air if the wind is blowing or if we are moving rapidly.

One more question: pressure is a force that acts over a measured area, 14.7 psi for example. Area is a two-dimensional measurement. How can we measure pressure with a one-dimensional number like inches of mercury (in hg)?

Oops, I’m out of space. We’ll have to cover that in the next article.