SQOD Squad - Stupid Question Of the Day

[Sajan]

At altitude you can trust an old school dial gauge. The reason for that is it would be comparing the pressure within the tire to the pressure outside of it.. which is what will determine how the tire flexes and performs. Digital gauges often are “absolute” gauges.. as is your TPMS.


If you’d like a better explanation than I can quickly write up look into the difference between absolute and relative pressure gauges.

This is what I used to deflate it to 20 psi.

https://over-sand.com/products/tire-deflators

This is my digital gauge. I didn't have it with me in Silverton so I am not sure what it would have shown.

 

[GrouchyTech]

The reason for that is it would be comparing the pressure within the tire to the pressure outside of it.. which is what will determine how the tire flexes and performs. Digital gauges often are “absolute” gauges.. as is your TPMS.

I have a few minutes to explain while this terrible movie wraps up. “Jurassic World: Rebirth.”
“Afterbirth” would be more appropriate.
(I do apologize, that was pretty gross...)

The sensor reading absolute pressure is the reason shoving the entire sensor inside a pressure vessel (like a PVC pipe) and pressurizing it works. It doesn’t need to detect the pressure inside AND outside the tire (though that would be most accurate).

The tire pressure sensor detects the “absolute pressure” inside the tire by measuring the movement of a tiny piezo diaphragm, subtracts the standard atmospheric pressure of around 14.7 psi (at MSL) and what you’re left with is “gauge pressure”—which is what we’re all used to seeing.

I can’t believe I just watched Scarlett Johansson get chased by dinosaurs for 2 hours and got far more excited discussing TPMS sensors.

 

[gasman4u]

If you want more clarity, look up the differences between psia and psig. That may help you figure out what your guage is reflecting.

 

[2HundyJTex]

Same as temps, FWIW I used to overthink this. Follow your tire gauge.

One reason we Texans go to the Rockies in the summer is to cool off - which is also why we don’t automatically bleed off 3-4 psi upon arriving at 9,000 ft from near sea level to compensate for the ~4 psi lower atm pressure. The 20-30 degrees temp difference tends to offset what would otherwise show up as a psi differential across the sidewall. If you watch TPMS only, you’re blissfully unaware. It starts showing up when we air-down or change pressures, then air up, then head back down the mountain… To keep myself sane, I leave the spare pressure alone on mountain road trips to serve as a reference and carry a good tire gauge.

 

[Sajan]

Got it. Good info guys. Thanks.
I knew about temperature and PSI but didn’t know about absolute vs relative pressures.

@bloc you sure the digital gauge is absolute.
Reading otherwise.