Under pressure, tire pressure that is…

By Steve Schaeber,  MACS Technical Training Specialist

I’ve had this one tire pressure gauge in my tool box for several years now, and while it’s not the most accurate gauge I have, it’s small and handy, and I don’t mind lending it out to someone (especially if I don’t know that I’ll get it back).


It was one of those ‘freebees’ that I got at a motorcycle show in New York City.  The other day, I went to get a wrench out of my box, and this gauge’s  key chain got caught on it.  Across the shop it flew, landed on the floor, and busted apart.


Man, that stinks.  I picked up all the pieces, went over to the trash can, and was just about to throw it away, when I thought, this would make a great blog article.  Sure, it’s nothing too impressive, or too important, and there’s not much to write about a tire pressure gauge.  We all have them, and don’t really care much about them (that is, until we need them and can’t find them, and they become that most important tool again).

So, I decided to snap a few pictures of this tool, all busted apart, and reassembled to its full glory.  Why?  Because even though it’s one of those cheap, stupid tools, it is still, just like all the rest, one of the most important tools we have in our toolbox.  Also, because even after all these years that I’ve spent in shops and working with tools, I’ve never seen one of these things fall apart before.  Again, even though It’s a cheap tool, it’s still important, and I would never take one apart just to see how it worked, or just to see what’s inside of it.  Since this one came apart by accident, I thought I would take advantage of the opportunity, and share this with all of you.

Tire pressure measurements are made relative to atmospheric pressure at the location of testing.  In other words, gauge pressure is zero-referenced against ambient air pressure.  When you connect the gauge to the valve stem, the air pressure from within the tire is allowed to act against a rubber diaphragm within the tool.  This diaphragm then acts against a spring, which is calibrated to display the correct measurement on the tool in Pounds per Square Inch, commonly referred to as (PSI).

However, while the air pressure from within the tire is acting on one side of the diaphragm, the air pressure from the area around the tool is acting on the other side of the diaphragm.  It is this difference in pressure that is being measured by the gauge.  A search of Google patents shows me that the earliest mention of a pencil-type tire pressure gauge first shows up around 1920.  But the oldest pressure measuring device, called a manometer, was actually a column filled vertically with liquid mercury and was invented by Evangelista Torricelli in 1643.

What surprises me the most is just how much pressure 32 PSI really is.  We never think about it, because it’s such a commonly referenced specification, and when compared to some higher pressure tires (such as trucks which sometimes require 80 PSI or more), 32 PSI doesn’t seem to be very much.  But when I ‘simulated’ the pressure by compressing the spring/gauge assembly by hand, it was actually difficult to hold the spring in that position during the time it took to take a few pictures.

Anyway, I’m going to clean up this tool, reassemble it, and then go find the rest of my air pressure gauges, and test a few tires to compare my readings.  I’m sure this is not the most accurate gauge I own, but it’s still too important to just throw away.

The Mobile Air Conditioning Society’s blog has been honored as the best business to business blog in the Automotive Aftermarket by the Automotive Communications Awards and the Car Care Council Women’s Board!

When having your mobile A/C system professionally serviced, insist on proper repair procedures and quality replacement parts. Insist on recovery and recycling so that refrigerant can be reused and not released into the atmosphere.

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The 34th annual Mobile Air Conditioning Society (MACS) Worldwide Training Conference and Trade Show, Power Up will take place January 16-18 2014 at the Sheraton New Orleans.


About macsworldwide

Mobile Air Conditioning Society (MACS) Worldwide Founded in 1981, MACS is the leading non-profit trade association for the mobile air conditioning, heating and engine cooling system segment of the automotive aftermarket. Since 1991, MACS has assisted more than 600,000 technicians to comply with the 1990 U.S. EPA Clean Air Act requirements for certification in refrigerant recovery and recycling to protect the environment. The Mobile Air Conditioning Society (MACS) Worldwide’s mission is clear and focused--as the recognized global authority on mobile air conditioning and heat transfer industry issues. www.macsw.org
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