Stupid Question - Air Pressure Readings?? (1 Viewer)
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An increase in altitude should cause an increase in gauge pressure because the outside pressure goes down. Ever travel with flimsy bottled water and notice that the bottle gets firmer going up in elevation and collapses if you go lower … same idea
I thought that the air pressure could only go up if the amount of air in the container (tire) was increased or the temperature went up causing the air in the tire to expand. I thought that your bottle expanded because there was less pressure out side but the pressure inside doesn’t go up. Nothing changes inside the tire, does it?
TheGrrrrr
GOLD Star
Alright, so regardless of all the environmental variables, my external gauge should be accurate? Like, my indeflate gauge or whatever would be roughly accurate for the moment in time and place. If it says 37 and isn’t miscalibrated or broken, it’s probably damn close to 37?
RET2
SILVER Star
I think you may have exposed a gorilla in the room, temperature.
bloc
SILVER Star
The gauge being surrounded by less air pressure is the issue.
The senders within the tire are “absolute” pressure. Meaning they are sealed and don’t compare the pressure within the tire to the atmosphere surrounding it. At sea level you’d subtract 14.7psi from an absolute pressure reading to get a usable reading.
Virtually all tire pressure gauges are “gauge” pressure meaning they compare the pressure from the valve stem to the atmosphere surrounding the tire gauge. A given pressure in a tire will read higher on a gauge if you take it to altitude, because the pressure surrounding the gauge drops. For our TPMS systems this will look like cluster readings that may closely match a tire gauge near sea level, but as you go higher and higher up the difference between the cluster reading and the gauge will increase.
If tire pressures are actually lowering at altitude as a result of less atmospheric pressure surrounding them, it will be reflected on the cluster readout.
Thing is, what determines how a tire acts physically (and therefore whether it will overheat) is the difference in pressure from the inside to the outside. So a tire gauge is actually a more accurate way to dial in tire behavior. This means someone going to altitude should be safe, as their relative pressure will increase. However someone that sets pressure at altitude could have underinflated tires if they travel to sea level.
For the record your in-tire TPMS transmitters also broadcast their temperature and (I think) battery percentage to the vehicle. This data is readable with Techstream.
The senders within the tire are “absolute” pressure. Meaning they are sealed and don’t compare the pressure within the tire to the atmosphere surrounding it. At sea level you’d subtract 14.7psi from an absolute pressure reading to get a usable reading.
Virtually all tire pressure gauges are “gauge” pressure meaning they compare the pressure from the valve stem to the atmosphere surrounding the tire gauge. A given pressure in a tire will read higher on a gauge if you take it to altitude, because the pressure surrounding the gauge drops. For our TPMS systems this will look like cluster readings that may closely match a tire gauge near sea level, but as you go higher and higher up the difference between the cluster reading and the gauge will increase.
If tire pressures are actually lowering at altitude as a result of less atmospheric pressure surrounding them, it will be reflected on the cluster readout.
Thing is, what determines how a tire acts physically (and therefore whether it will overheat) is the difference in pressure from the inside to the outside. So a tire gauge is actually a more accurate way to dial in tire behavior. This means someone going to altitude should be safe, as their relative pressure will increase. However someone that sets pressure at altitude could have underinflated tires if they travel to sea level.
For the record your in-tire TPMS transmitters also broadcast their temperature and (I think) battery percentage to the vehicle. This data is readable with Techstream.
kcjaz
SILVER Star
I think there is something funky in your math. I'm not sure what you mean by "actual" pressure. There is only absolute pressure (psia) and gauge pressure (psig). (Pabs = Pgauge + Patm).
What you want in your tires is the RCTIP as measured by a hand held gauge on the valve stem no matter your elevation, weather/temperature affect on local barometric pressure. Its the gauge pressure inside the tire that correlates to the tire tread on the road. Your TPMS will always be off by the difference between the local barometric pressure and standard sea level elevation pressure (14.7 psia). As others mentioned, local barometric pressure is a function of not only elevation but also local weather (temperature and other atmospheric conditions) and of course the air temp in the tire is a massive factor.
Here's a table showing what you want to see on your TPMS to get 37 psig TCTIP:
| elevation | Patm (psia) | TPMS | Hand held tire gauge PSIG | "Actual" Absolute Pressure PSIA | TPMS error (PSI) | |
| Sea Level | - | 14.7 | 37.0 | 37.0 | 51.7 | 0.0 |
| Grrrr's home | 1,234 | 14.1 | 36.4 | 37.0 | 51.1 | 0.6 |
| Moab | 4,026 | 12.7 | 35.0 | 37.0 | 49.7 | 2.0 |
| Ouray | 7,792 | 11.0 | 33.3 | 37.0 | 48.0 | 3.7 |
Here's what happens if you inflate to 37 as displayed by TPMS:
| elevation | Patm (psia) | TPMS | Hand held tire gauge PSIG | "Actual" Absolute Pressure PSIA | TPMS error (PSI) | |
| Sea Level | - | 14.7 | 37.0 | 37.0 | 51.7 | 0.0 |
| Grrrr's home | 1,234 | 14.1 | 37.0 | 37.6 | 51.7 | 0.6 |
| Moab | 4,026 | 12.7 | 37.0 | 39.0 | 51.7 | 2.0 |
| Ouray | 7,792 | 11.0 | 37.0 | 40.7 | 51.7 | 3.7 |
and yes, the tables assume that its 59F ambient as well as tire temp and no cold front has just blown through.
So, if you inflate to 37 psi as shown by TPMS at sea level and then drive to Ouray, your TPMS will not notice anything but your tire will be slightly over inflated. In theory, you should let 3.7 psi out when you get there. But, when you get back home, you will be under inflated. Another way of explaining this change is that your tire was 37 psi above the ambient atmospheric pressure at sea level. But the same tire is 40.7 psi above the ambient atmospheric air pressure in in Ouray. The same number of air molecules is in the tire the whole time.
kcjaz
SILVER Star
^^^ this!
TheGrrrrr
GOLD Star
Ok, I think I'm starting to understand. I was basing my calculations off of a calculator I found online and just reversing the logic:
Im so sorry, but im still confused though 
.
If sealed system internal in the tire is reading xPSI against absolute tpms gauge, shouldn’t that still be a “true” measure? Against sea level or whatever temp it was recommended to be set to?
Sure it will drop or increase given ambient, but that dropped reading isnt “accurate” anymore?
Wouldnt all the above mean that people in denver would have totally different door placards?
.If sealed system internal in the tire is reading xPSI against absolute tpms gauge, shouldn’t that still be a “true” measure? Against sea level or whatever temp it was recommended to be set to?
Sure it will drop or increase given ambient, but that dropped reading isnt “accurate” anymore?
Wouldnt all the above mean that people in denver would have totally different door placards?
Yeah those calculations won't work for air pressure inside a "sealed" container like a tire. I'm not sure where you got them but they appear to be describing what happens to atmospheric air pressure as altitude increases which causes the inverse ... air pressure decreases.
As discussed as the atmospheric air pressure decreases with altitude (or temp etc) the same amount of air in the tire will show an INCREASE in tire pressure (less atmospheric pressure pushing on the outside of tire) at the gauge (psig). The table @kcjaz made up looks good but as he said it only really works under "standard" conditions which, as a pilot, I can assure you are unusual.
bloc
SILVER Star
See the part kcjaz quoted above. The important number is tire pressure in relation to the atmosphere surrounding it, because that determines the physical behavior as you go down the road. An external tire gauge does a better job of reporting this.Im so sorry, but im still confused though
If sealed system internal in the tire is reading xPSI against absolute tpms gauge, shouldn’t that still be a “true” measure? Against sea level or whatever temp it was recommended to be set to?
Sure it will drop or increase given ambient, but that dropped reading isnt “accurate” anymore?
Wouldnt all the above mean that people in denver would have totally different door placards?
There’s no reason you couldn’t make adjustments once you get to altitude then note what pressures that results in on the display, and even do your TPMS reset/learn based on those numbers, so it would alert you if a tire gets out of the new range.
But our senders being absolute pressure is simply the easiest way to do it. If they had a reference to atmosphere to be more accurate, you’d have to find a reliable and cost effective way to provide that seal/path for measurement. I totally see why they don’t do that.
The tire is "sealed" and at a giving atmospheric pressure the TMPS will record an accurate drop due to a temp change but not a change in outside pressure.Im so sorry, but im still confused though
If sealed system internal in the tire is reading xPSI against absolute tpms gauge, shouldn’t that still be a “true” measure? Against sea level or whatever temp it was recommended to be set to?
Sure it will drop or increase given ambient, but that dropped reading isnt “accurate” anymore?
Wouldnt all the above mean that people in denver would have totally different door placards?
No the same recommended tire pressure is used regardless of altitude.
I was born and raised in CO and just moved back after living in GA for 10 years. I have always used a high quality dial tire gauge and in GA both my wife's and my Lexus TPMS matched my tire gauge but coming back to CO they are off (I bought the LC here so no comparison for alt but doesn't match the gauge). Interestingly my Porsche TMPS matches my gauge here (I don't really have an explanation as I haven't done the research to see it that system compensates).
I always use a tire gauge to set my pressure and the TPMS is just a reference for variations from tire to tire which I double check with a gauge. Living at sea level (ish) you likely see no difference between the two
TheGrrrrr
GOLD Star
The net here seems to be that TPMS should not be used to determine actual tire pressure and if I want to actually run at 37psi at any given altitude, I should just use an external gauge to check and apparently just need to get over the fact that every gauge I have shows a different pressure. Is there a good reliable gauge out there I should rely on? Do I need to spend $400 on a precision gauge to be sure?
Porsche owners are even more anal than LC owners and a lot of us use a longacre dial gauge
Amazon product ASIN B00DL6T2YK
kcjaz
SILVER Star
No not exactly.
TPMS is an absolute pressure transducer. You can not actually measure pressure directly. You can only measure pressure difference between two things by the deflection of a diaphragm. In an absolute pressure transducer one side of the diaphragm is exposed to the pressure you are trying to measure and the other side connected to a sealed chamber that has been evacuated to zero pressure.
What your TPMS is really measuring then is the pressure in your tire plus the atmospheric pressure on the outside. I agree that the TPMS is in a "sealed" tire but that tire is still being squeezed by the atmosphere. So when your tire is filled to 37 psig (hand held gauge) and the barometric pressure is 14.7, your TPMS would measure 51.7 psia. No matter where you take your tire to, Ouray or outer space, the TPMS would measure 51.7 psia (as long no air leaks and the temperature in the tire doesn't change).
The ECU simply subtracts a constant 14.7 from the measured TPMS value and it will display 37 psi the whole time (as long no air leaks and the temperature in the tire doesn't change). Its the constant 14.7 that causes the error here.
You should inflate your tire to RCTIP at whatever elevation you are at. The reality though is that 3 or 4 psi isn't that big of a deal in most cases. When you are down to 15 psi accuracy starts to matter.
If I'm still not making any sense, just google, lots of info out there probably explaining better what I'm trying to say here.
How Do You Adjust Your Tire Pressure for High Altitude?
There's an easy way to set your vehicle's tire psi while road tripping across elevations.
www.motorbiscuit.com
Temperature is by far the bigger deal. RCTIPs are "cold" pressures. After I wheel all day and go to air up my tires are not cold. My RCTIP is 42 psi. I've just learned to go to ~45 when I air up and then in the morning, I'm pretty close to 42. Conversely, if you air down, with your tires hot, and don't air up at the end of the day, you will be lower than you think in the morning.
Even more rabbit hole:
I believe it is possible to program a PID in OBDFusion or possible other OBDII apps, to take the TPMS readings, add back the constant 14.7 and then subtract the OBD barometric pressure. This could potentially give you accurate TPMS data.
scottm
SILVER Star
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kcjaz
SILVER Star
A Wiseman once told me that a man with one watch knows what time it is but a man with two watches is confused.
I see no need for $400 tire pressure gauges but $3 ones are crap. I went through a bunch of them as I also noticed that all my cheapos were all different from each other. I now have a decent $30 gauge and a digital ARB gauge that I got suckered into buying with my air compressor (Slee said, "want fries with that" and I said "sure!"). They match other and I call it good. It is possible to calibrate gauges or at least test them. The calibration equipment isn't cheap but you can also send gauges places to be tested.
