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TPMS reading @ sea level vs. higher altitudes

Dolemite

is my name
Sep 19, 2019
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Seattle, WA
Let's assume TPMS is calibrated @ sea level (which I'm pretty sure it is), and you have a super-accurate handheld digital gauge. Target pressure is 42 PSI all-around.

If you fill your tires to 42 at sea level, everything is in-sync: TPMS reads 42, and so does your handheld gauge. FYI - handheld gauges measure pressure relative to ambient pressure, TPMS measures in absolute since it's in a closed system (the tire). Which is why you shouldn't trust TPMS for absolute measurements.

Anyway, let's then assume you drive up 5,000 feet in altitude, where the atmospheric pressure drops. You look at your TPMS screen, and it reads 44 all-around, because of the lower ambient pressure.

If you now take a reading with your gauge at this new +5,000 ft altitude, I believe it would also match the TPMS reading @ 44???

However, if you adjust your tire pressure at this new +5000 ft altitude back down to 42 using your handheld gauge, your TPMS would read lower - say ~40 PSI? If my understanding here is correct, this is where I'm getting confused... why is it a 1:1 on your way up, but not 1:1 once you adjust your pressures to the correct PSI at your new +5,000 ft home? Unless the gauge would initially read even higher than the TPMS reading prior to adjustment, say 46... in which case this would all make sense.
 
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So I understand that the TPMS would continue reading the same PSI set at sea level, regardless of altitude (all else being equal). Is it then "correct" to adjust your tire pressure to this new altitude, as the Tire Rack article suggests? If you take this to the extreme and you're driving around at 40,000 feet, where the PSI is 2.7, would it still hold? I'd imagine so...?

There are two pressure measurement systems to be cognizant of:

PSIA - pounds per square inch absolute ... the pressure of the gas in PSI referenced to a pure vacuum. 0 PSIA = pure vacuum.

PSIG - pounds per square inch gauge ... the pressure of the gas in PSI referenced to current atmospheric pressure. 0 PSIG = same pressure as the atmosphere.

TPMS...

Dolemite

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Sep 19, 2019
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medphys3

Member
Feb 10, 2020
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Northwest Ohio
Hypothetical. Forgot to say - this is everything else being held constant.


wat?


“Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure”

That article is talking about measuring the tire pressure with an external pressure gauge. As far as I know, the TPS measurement is absolute so the measured value won’t change with altitude so your statement that the TPS reading changing from 42 to 44 at altitude is incorrect.

The TPS would still read 42, but an external measurement would be somewhat higher.
 
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Dolemite

is my name
Sep 19, 2019
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Seattle, WA
“Most pressure gauges (including all tire pressure gauges) are designed to measure the amount of pressure above the ambient atmospheric pressure”

That article is talking about measuring the tire pressure with an external pressure gauge. As far as I know, the TPS measurement is absolute so the measured value won’t change with altitude so your statement that the TPS reading changing from 42 to 44 at altitude is incorrect.

The TPS would still read 42, but an external measurement would be somewhat higher.
I think that makes sense… so what you’d ultimately want is for the gauge to read 42 - consequently making the TPMS read a little bit lower? Since that’s what the tire pressure would be at that altitude “fo real.”
 
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techlogik

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Mar 22, 2021
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Although the partial pressure of air at altitude, meaning the air molecules are less compressed at altitude than sea level, this doesn't have any effect of tire pressure.

The tire is a sealed (mostly) system. Compare a tire to an airplane fuselage at altitude, they normally maintain "altitude" of around 10,000ft msl with pressurization (bleed air off the engines to pressurize the cabin) while flying at 30+k msl.

Same goes for a tire, since it is a sealed system and pressurized, the gauge will read exactly the same at sea level depending on the pressure in the tire.
 
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techlogik

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Mar 22, 2021
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The tire pressure will not change with respect to the external air pressure. It will change with respect to temperature & volume.

PV=nRT
Yes, but that isn't the question, as a tire "heats" up for example, the air will expand inside the tire increasing the pressure. But, the tire pressure gauge will directly read that pressure as what it actually is despite of the altitude.
 
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Piffle

Member
Jul 14, 2021
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Correct - their are different types of sensors, some are vented to give an accurate reading for atmospheric differences.
I am sure the TPMS sensors are sealed, and work VIA an algorithm that was setup a sea-level.

These aren't lab quality sensors, and are more to let you know if your tire is about to go flat. I wouldn't be surprised if the have a +or- of 5psi accuracy.
 
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Dolemite

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Sep 19, 2019
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Seattle, WA
Thanks for the input - I think I grossly overcomplicated this as usual.

So I understand that the TPMS would continue reading the same PSI set at sea level, regardless of altitude (all else being equal). Is it then "correct" to adjust your tire pressure to this new altitude, as the Tire Rack article suggests? If you take this to the extreme and you're driving around at 40,000 feet, where the PSI is 2.7, would it still hold? I'd imagine so...? Don't those high-altitude research balloons start off really shriveled-up on the ground but then expand to actually look like a balloon as they reach their target altitude? The same principle stands with a tire? I'm just not sure whether there are any additional variables, considering the tires are supporting a 2-ton car.

Correct - their are different types of sensors, some are vented to give an accurate reading for atmospheric differences.
I am sure the TPMS sensors are sealed, and work VIA an algorithm that was setup a sea-level.

These aren't lab quality sensors, and are more to let you know if your tire is about to go flat. I wouldn't be surprised if the have a +or- of 5psi accuracy.
You'd be surprised - they're pretty spot-on. I have an accurate pressure gauge and when I set them all exactly to the same thing, they all read the same number except for one, which reads a little high. So in my experience they're good to about ~1 PSI. Still, a good handheld gauge should be the standard of measure.
 
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SomeJoe7777

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Mar 28, 2015
2,200
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Houston, TX
So I understand that the TPMS would continue reading the same PSI set at sea level, regardless of altitude (all else being equal). Is it then "correct" to adjust your tire pressure to this new altitude, as the Tire Rack article suggests? If you take this to the extreme and you're driving around at 40,000 feet, where the PSI is 2.7, would it still hold? I'd imagine so...?

There are two pressure measurement systems to be cognizant of:

PSIA - pounds per square inch absolute ... the pressure of the gas in PSI referenced to a pure vacuum. 0 PSIA = pure vacuum.

PSIG - pounds per square inch gauge ... the pressure of the gas in PSI referenced to current atmospheric pressure. 0 PSIG = same pressure as the atmosphere.

TPMS measures PSIA of the interior of the tire, then calculates the PSIG by subtracting atmospheric sea level pressure. Example: Interior tire pressure is 56.7 PSIA, TPMS subtracts 14.7 (sea level atmospheric pressure in PSIA) and reports 42 PSIG to the driver.

Tire manufacturers and auto manufacturers reference all of their recommended pressures and maximum pressures in PSIG, because that pressure directly corresponds to the actual forces on the tire. E.g. if the maximum rated pressure for the tire is 50 PSIG and that's what you've inflated it to, that means that there is 50 PSI more inside the tire than outside the tire, and thus the forces on the rubber are at the maximum recommendation at that point. If the outside pressure goes down (like when you drive up in altitude), then you need to bleed pressure from inside the tire to maintain 50 PSIG and bring the forces on the rubber back down.

So what happens if you have tires at 42 PSIG at sea level and then drive to 10,000 feet? Atmospheric pressure drops roughly 1 PSIA for every 2000 feet of altitude. So you can expect atmospheric pressure at 10,000 feet to be approximately 9.7 PSIA.

Compute actual tire pressure at 10,000 feet (also what a standard tire pressure gauge will read):
1. Convert tire pressure to absolute: 42 PSIG + 14.7 PSIA at sea level = 56.7 PSIA inside the tire.
2. Compute tire pressure at altitude: 56.7 PSIA - 9.7 PSIA at 10,000 feet = 47 PSIG.

TPMS will not reflect the altitude change, and will still report 42 PSIG to the driver.

What do you do to the tires? For the same ride, we need to reduce the tire back to 42 PSIG. Set this with a standard tire pressure gauge, you will need to bleed 5 PSI out of the tire. Once that is done:

1. Compute internal tire pressure in PSIA: 42 PSIG + 9.7 PSIA at 10,000 feet = 51.7 PSIA.
2. Compute what TPMS will report to the driver: 51.7 PSIA - 14.7 PSIA at sea level = 37 PSIG.

If you don't bleed air, the ride will be harsh, equivalent to the ride you would get at sea level with 47 PSIG in the tires.

When you bleed air, TPMS may report that tires are low. Compensate by recalibrating TPMS in the Service menu.

Remember that all of this assumes constant temperature. In reality, tire pressures vary 1 PSI for every 10 degree Fahrenheit change in temperature (temperature goes down 10F, pressures go down 1 PSI and vice versa). Sometimes, when driving to altitude, the altitude change and the temperature change will compensate for each other. E.g. driving from sea level to 6,000 feet where the temperature has dropped from 80F to 50F will cause the tires to have constant pressure:

Initial Condition: 80F, sea level, 42 PSIG, inside tire 56.7 PSIA, TPMS reads 42 PSIG.
Change due to +6000 feet altitude (but still at 80F): inside tire still 56.7 PSIA, TPMS reads 42 PSIG. External pressure gauge would read 45 PSIG.
Change due to -30F temperature (but still at sea level): inside tire down to 53.7 PSIA, TPMS reads 39 PSIG. External pressure gauge would read 39 PSIG.
Change with +6000 feet and -30F: inside tire down to 53.7 PSIA, TPMS reads 39 PSIG. External pressure gauge would read 42 PSIG.

Summary / TL;DR: External pressure gauge is the gold standard, and should be what you use to maintain tire pressures. Tires may need air adjustments at altitude and in seasonal temperature changes, use the external pressure gauge to maintain proper pressure. TPMS is useful to warn of improper inflation, but do not use it to set actual tire pressures -- use the external gauge.
 
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Solution

Dolemite

is my name
Sep 19, 2019
1,372
1,733
Seattle, WA
There are two pressure measurement systems to be cognizant of:

PSIA - pounds per square inch absolute ... the pressure of the gas in PSI referenced to a pure vacuum. 0 PSIA = pure vacuum.

PSIG - pounds per square inch gauge ... the pressure of the gas in PSI referenced to current atmospheric pressure. 0 PSIG = same pressure as the atmosphere.

TPMS measures PSIA of the interior of the tire, then calculates the PSIG by subtracting atmospheric sea level pressure. Example: Interior tire pressure is 56.7 PSIA, TPMS subtracts 14.7 (sea level atmospheric pressure in PSIA) and reports 42 PSIG to the driver.

Tire manufacturers and auto manufacturers reference all of their recommended pressures and maximum pressures in PSIG, because that pressure directly corresponds to the actual forces on the tire. E.g. if the maximum rated pressure for the tire is 50 PSIG and that's what you've inflated it to, that means that there is 50 PSI more inside the tire than outside the tire, and thus the forces on the rubber are at the maximum recommendation at that point. If the outside pressure goes down (like when you drive up in altitude), then you need to bleed pressure from inside the tire to maintain 50 PSIG and bring the forces on the rubber back down.

So what happens if you have tires at 42 PSIG at sea level and then drive to 10,000 feet? Atmospheric pressure drops roughly 1 PSIA for every 2000 feet of altitude. So you can expect atmospheric pressure at 10,000 feet to be approximately 9.7 PSIA.

Compute actual tire pressure at 10,000 feet (also what a standard tire pressure gauge will read):
1. Convert tire pressure to absolute: 42 PSIG + 14.7 PSIA at sea level = 56.7 PSIA inside the tire.
2. Compute tire pressure at altitude: 56.7 PSIA - 9.7 PSIA at 10,000 feet = 47 PSIG.

TPMS will not reflect the altitude change, and will still report 42 PSIG to the driver.

What do you do to the tires? For the same ride, we need to reduce the tire back to 42 PSIG. Set this with a standard tire pressure gauge, you will need to bleed 5 PSI out of the tire. Once that is done:

1. Compute internal tire pressure in PSIA: 42 PSIG + 9.7 PSIA at 10,000 feet = 51.7 PSIA.
2. Compute what TPMS will report to the driver: 51.7 PSIA - 14.7 PSIA at sea level = 37 PSIG.

If you don't bleed air, the ride will be harsh, equivalent to the ride you would get at sea level with 47 PSIG in the tires.

When you bleed air, TPMS may report that tires are low. Compensate by recalibrating TPMS in the Service menu.

Remember that all of this assumes constant temperature. In reality, tire pressures vary 1 PSI for every 10 degree Fahrenheit change in temperature (temperature goes down 10F, pressures go down 1 PSI and vice versa). Sometimes, when driving to altitude, the altitude change and the temperature change will compensate for each other. E.g. driving from sea level to 6,000 feet where the temperature has dropped from 80F to 50F will cause the tires to have constant pressure:

Initial Condition: 80F, sea level, 42 PSIG, inside tire 56.7 PSIA, TPMS reads 42 PSIG.
Change due to +6000 feet altitude (but still at 80F): inside tire still 56.7 PSIA, TPMS reads 42 PSIG. External pressure gauge would read 45 PSIG.
Change due to -30F temperature (but still at sea level): inside tire down to 53.7 PSIA, TPMS reads 39 PSIG. External pressure gauge would read 39 PSIG.
Change with +6000 feet and -30F: inside tire down to 53.7 PSIA, TPMS reads 39 PSIG. External pressure gauge would read 42 PSIG.

Summary / TL;DR: External pressure gauge is the gold standard, and should be what you use to maintain tire pressures. Tires may need air adjustments at altitude and in seasonal temperature changes, use the external pressure gauge to maintain proper pressure. TPMS is useful to warn of improper inflation, but do not use it to set actual tire pressures -- use the external gauge.
hell ya. Thanks
 
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David.85D

Active Member
Oct 29, 2016
1,513
1,280
USA
FWIW, I’ve compared my VDO (Siemens) TPMS with a 3-point (very well) calibrated analog gauge and they agree within 1 psi. They are surprisingly good.

for the rest of it, not sure it really matters. I see 5 psi swings due to outside temperature changes (including 2 psi differences for tires in direct sunlight compared to shade). Atmospheric pressure changes roughly 1 psi for every 2000 feet of elevation change.

for the question raised about the flimsy weather ballons - the material is extremely thin and light and essentially unable to support stress/pressure. They are under inflated so they never get to the fully inflated stage as the gas inside expands (equilibrium says the gas inside will expand to match the outside pressure in the absence of a container squeezing it in). Tires can resist the pressure (steel belts, etc) and don’t change size very much.
 
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