Are the units displayed on the dash PSIG or PSIA?
Thanks.
J.
Here’s what I found:
PSIG (Gauge Pressure):
Measures the pressure difference between the inside of the tire and the surrounding outside air. A tire that is completely flat (equal to the outside air) reads 0 PSIG.
Your car's door placard and owner's manual list the recommended pressure in PSIG
Based on that, I would venture that the display is registering PSIG.
Thanks for that. I asked because I live at 5600ft. If I set the pressure at 30PSIG with my gauge (very accurate), the TPM display registers 28PSI, which happens to be about two to three PSI lower because of the difference in atmospheric pressure at this altitude. In Los Angeles they both agree perfectly. I was wondering if
Porsche
intended it that way, that is displaying the pressure in PSIA.
I sometimes overthink **** like this🤪
I don't agree with
@WTFChuck
's assumption (that PSIG is displayed). I can't see how the TPMS sensor could measure the ambient (outside) air pressure while mounted within the tire. Only the stem and nut reside outside of the pressurized tire. I guess the delta could be computed externally by the firmware using a separate pressure sensor. But I doubt it.
I know the TPMS sensor's raw RF signal can be read directly using special tools. I wonder if it agrees with the pressure displayed on the dash. I suspect it does.
Granted, this is a confusing concept. Below is the full explanation of the difference between the two. It’s as clear as mud to us regular folk. Although if I understand this sufficiently, gauge pressure is relative to the number of air molecules in the tire, which I suspect the
TPMS
is only capable of reading. When we put the gauge on our stem, that is the pressure we are reading, Gauge pressure. But since our tire pressure changes during the day due to temperature swings, or changes from a cold tire to one that’s been driven and heated up, there will never be an absolute pressure reading from one day to the
next
, or one hour to the
next
. Someone with an engineering background, which I do not possess, could probably better explain it. But
tires
are fairly forgiving, and a few pounds either way most likely isn’t a huge deal, except for on the track or at high or low altitudes. The pressure is changing as we drive anyway. This from a Google search:
PSIA
(Pounds per Square Inch Absolute) measures pressure relative to a perfect vacuum, including the weight of Earth's atmosphere.
PSIG
(Pounds per Square Inch Gauge) measures pressure relative to the surrounding ambient air, ignoring atmospheric pressure. In short, PSIG shows what a standard pressure gauge reads, while PSIA is total pressure. [
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The Core Difference
Zero Reference:
PSIA starts at a complete vacuum (0 PSIA). PSIG starts at ambient atmospheric pressure, which means an open pressure gauge reads 0 PSIG.
At Sea Level:
Atmospheric pressure is approximately 14.7 psi. Therefore, 0 PSIG is exactly equal to 14.7 PSIA. [
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How to Convert Between Them
Because PSIA accounts for the atmosphere, its value is always roughly 14.7 higher than PSIG. [
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PSIG to PSIA:
Add atmospheric pressure to the gauge pressure.
\(P_{psia} = P_{psig} + 14.7\) [
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PSIA to PSIG:
Subtract atmospheric pressure from the absolute pressure.
\(P_{psig} = P_{psia} - 14.7\) [
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Common Use Cases
When to use PSIG:
Used for everyday applications like inflating your car tires, using an air compressor, or reading liquid pressure lines. These tools measure the
difference
between the inside and the outside air.
When to use PSIA:
Used in scientific calculations, thermodynamic data, vacuum systems, and gas/steam volume flow formulas. [
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Wow. This is activating some long dormant brain cells (and it hurts)! Long ago I was an Air Force weatherman. In reality, I never used most of the stuff learned in school. We used barometric pressure (mb or hPa), I can't recall ever converting millibars to PSI. But this brings a further complication into the picture. Meteorological conditions. Atmospheric pressure at sea level (or anywhere on Earth) isn't a constant. I'm wondering where atmospheric conditions come into this PSI/PSIA stuff. Probably just ignored due to the complication. Especially for PSIA.
Mr Google helped tells me that "for Flat Rock, NC, your current barometric reading is approximately
30.19 inches of mercury (inHg)
."
Here is how that measurement converts into different common units of pressure:
Millibars / Hectopascals:
~ 1019.7 mb or hPa
Pounds per Square Inch:
~
14.83 psi
Oddly, that is a higher PSI than the 14.7 pounds at sea level mentioned above, but I am at around 2500'. Anyway this is just a diversion from the original topic. I believe, as Chuck stated, "tires are fairly forgiving, and a few pounds either way most likely isn’t a huge deal". I bet the sill sticker and any specs from tire manufactured is just boiler-plate CYA to avoid getting sued when someone gets a blow-out. Humm, I wonder if they let the air out of the tires of that Tesla that Elon launched into space?
BTW, until a few years ago I lived on a barrier island in the Gulf of (insert your favorite name here) off Florida. Now I'm in the Blue Ridge Mountains. I haven't noticed any significant difference between my fairly accurate hand tire gauge and what the Porsche states (at same ambient temp). PSIA or just plain PSI, I don't know.
Funny thing is, I’ve checked pressures with pencil gauges, good quality dial gauges and good digital gauges. And I have two portable compressors with built in digital gauges and there is always a differential of .5 to 1.5 psi. Question is which one is the right reading? Short of an expensive scientific gauge all these readings are close enough for government work. And being P-car owners we tend to check our tire pressures far more than the average Joe on the road. I would venture to guess that most cars on the road are several pounds off at any given time.
