Gauge vs Absolute Confusion [E203]

Thanks to our Partner, NAPA Autotech Training and Pico TechnologyWatch Full Video EpisodeIn this episode, Matt Fanslow takes on a common source of confusion in automotive diagnostics: pressure, vacuum, and the difference between gauge and absolute readings.What starts as a discussion on PSI and inches of mercury quickly expands into how technicians interpret scan tool PIDs, why definitions matter, and where misunderstandings creep in—especially in the U.S., where we often mix measurement scales. Along the way, Matt also detours (in true Matt fashion) into physics, quantum mechanics, and the origins of universes from near-perfect vacuums.Yes, really. And yes, it still ties back to cars.Key Topics Covered:The two main pressure scales used in the U.S.:PSI (pounds per square inch) above atmosphericInches of mercury below atmospheric (vacuum)Where confusion starts:Vacuum gauges vs scan tool dataPSI vs inches of mercury and how both can technically read positive or negativeGauge vs Absolute pressure:Gauge pressure treats atmospheric as zeroAbsolute pressure references a sealed, near-vacuum chamberWhy MAP (Manifold Absolute Pressure) sensors often confuse techsRules of thumb:2 inHg ≈ 1 PSIAtmospheric pressure at sea level ≈ 14.7 PSI (≈ 30 inHg, ≈ 100 kPa)Engine vacuum at idle ≈ 18–20 inHg (≈ mid-30 kPa absolute)How to tell what your scan tool is showing you:Quick test: Key On, Engine Off → atmospheric pressure value reveals if it’s gauge or absoluteMetric perspective:Why kilopascals (kPa) often simplify thingsThinking of vacuum as “low pressure,” not “negative pressure”Physics detour (because Matt can’t help himself):Schrödinger’s Cat and quantum absurditiesHeisenberg’s uncertainty principleAbsolute zero pressure, absolute zero temperature, and why universes may form from vacuum energyWhy It MattersA clear understanding of how pressure is defined, displayed, and measured allows technicians to:Interpret scan tool PIDs correctlyAvoid misdiagnosis caused by unit confusionCommunicate more precisely with peers and customersGain confidence when moving between PSI, inHg, and kPaNext time someone’s arguing inches of mercury vs PSI on a forum, remember: it’s all about knowing whether you’re looking at gauge or absolute. And if the conversation stalls, just casually mention that universes may have formed from absolute zero pressure. It probably won’t help you win the argument—but hey, it’s a good story.Thanks to our Partner, NAPA Autotech TrainingNAPA Autotech’s team of ASE Master Certified Instructors are conducting over 1,200 classes covering 28 automotive topics. To see a selection, go to napaautotech.com for more details.Thanks to our Partner, Pico TechnologyAre you chasing elusive automotive problems? Pico Technology empowers you to see what's really happening. Their PicoScope oscilloscopes transform your diagnostic capabilities. Pinpoint faults in sensors, wiring, and components with unmatched accuracy. Visit PicoAuto.com and revolutionize your diagnostics today! Contact InformationEmail Matt: [email protected] the Aftermarket A - Z YouTube Channel Subscribe & Review: Loved this episode? Leave a 5-star review on Apple Podcasts and SpotifyThe Aftermarket Radio Network: https://aftermarketradionetwork.com/Remarkable Results Radio Podcast with Carm Capriotto: Advancing the Aftermarket by Facilitating Wisdom Through Story Telling and Open Discussion. https://remarkableresults.biz/Diagnosing the Aftermarket A to Z with Matt Fanslow: From Diagnostics to Metallica and Mental Health, Matt Fanslow is Lifting the Hood on Life. https://mattfanslow.captivate.fm/Business by the Numbers with Hunt Demarest: Understand the Numbers of Your Business with CPA Hunt Demarest. https://huntdemarest.captivate.fm/The Auto Repair Marketing Podcast with Kim and Brian Walker: Marketing Experts Brian & Kim Walker Work with Shop Owners to Take it to the Next Level. https://autorepairmarketing.captivate.fm/The Weekly Blitz with Chris Cotton: Weekly Inspiration with Business Coach Chris Cotton from AutoFix - Auto Shop Coaching. https://chriscotton.captivate.fm/Speak Up! Effective Communication with Craig O'Neill: Develop Interpersonal and Professional Communication Skills when Speaking to Audiences of Any Size. https://craigoneill.captivate.fm/