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Old 05-13-2020, 01:41 PM   #1
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Default Tire pressure changes with TPMS

A recent thread on aftermarket TPMS systems reminded me to do something I have kept forgetting about.


Executive summary first, explanation (long) after;


Always adjust your tire pressures for temp if more than you like your pressures to be off of baseline.


Never adjust your tire pressures due to elevation change.



We all have been around the perpetual discussions about tire pressure changes due to temperature changes, and they are quite accurate and predictable changes. On the other hand, pressure changes due to elevation change are often treated the same way, but probably should not be. Put on top of that the fact that TPMS systems don't all measure and/or display the tire pressures in the same units of pressure, and it gets even weirder.


The two different units used are the "gauge" pressure (psig) and the "absolute" pressure (psia). The difference is the starting point of the scale, with psia starting zero at a complete vacuum and psig starting zero at whatever atmospheric pressure is at the time and place measured. At sea level psig is zero and psig is 14.7psi.


If you already know about psig vs psia, jump to the end to see about the TPMS stuff.



The way psig is measured is how your hand tire gauges work. They just measure how much pressure you have above ambient atmosphere. To get psia you need to measure against a standard reference that never changes, which is much less practical in the real world and quite expensive to do.


How this all relates to tire pressures and elevation is not particularly instinctive, I think, and pretty misunderstood and often argued. Without going into the details of gas laws it comes down to that if no air is put in our let of fixed volume container, like a tire, the psia does not change with elevation. Psia will be the same at sea level as at 10,000 feet. Psig using exactly the same container and no air in or out will be about 4.6psi higher at 10,000 feet. The only difference is the gauge reference has changed for psig from 14.7 psi at sea level to about 10.1 psi at 10,000 feet. Basically psia minus atmospheric pressure equals psig.


For tires, all the air pressure does is support the weight on the tire by pushing back on the road though the contact area. The contact area part of the tire doesn't see atmospheric pressure so no allowance for any change needed. The psia in the tire hasn't changed at high elevation, so the contact area hasn't changed at all. This would all indicate that you should not reduce your tire pressure measured with a hand gauge when you go to high elevation, because the pressure that counts and is needed to support the vehicle hasn't changed for the elevation change. Mention this to many people and they will not believe it, as it would mean that the recommended tire pressure read on a gauge should be adjusted up for elevation increases.


Now put into the mix that most internally mounted TPMS sensors measure in psia as they have no external atmospheric reference unless it is done in the receiver, and it is better anyway as it is psia that counts. It also makes it so you don't get an overpressure alarm if you go up in elevation. It also makes it so if the temperature also changes a bunch, you will get an accurate underpressure alarm if it gets really cold at the elevation. Very confusing is that psig from going up in elevation can be just about equal to the psi (both psig and psia) lost because of the temp drop. You put your hand gauge on the tires that just gave a low pressure alarm and it looks just like it always has and where it was set at low elevation and high temp. You say it is a false alarm and take off on underinflated tires.


Now you take the case of the external sensors that may read in psig, like our TST, and it all changes as the TPMS will read just like your hand gauge. The TPMS will read higher pressure at high elevations, and you may get a high pressure alarm, that really is a error alarm as you should not change the tire pressures for elevation. Or, if the temp has fallen, you will not get any alarm so go merrily on your way when you should increase the pressure for the temp change, even if it sets off the high pressure alarm.


The ultimate confusion comes in something most of us won't see with class b's but is interesting, and I have actually been asked about this as it can get noticed. You have a tow vehicle with a factory tpms on it, and are pulling a trailer with a TST external sensors unit on it. You go up to 12,000 feet and the TST says the pressure is too high. You diligently let out air back until you hand held gauge reads what you thing is right (but isn't) and check the truck tires which also show high and you also air down then. Then the truck TPMS, which is looking at psia, trips a low pressure alarm which is actually correct as the tires are now actually too low. Almost nobody in the real world is going to know which is right, but nearly everyone in the real world will go with the two out three logic and have the hand gauge and TST overrule the factory TPMS. You go on your way with an alarm on the truck and underinflated tires on both the truck and trailer, but at least the TST quit beeping


There are a few disclaimers on this, though. If you can spare the capacity loss, if your tires are at max like most B van single rears are, it is questionable if you should reduce the pressure or not, as it is the casing you worry about. I posed this question to a couple of manufacturers and they said the amount they go above from elevation is small enough to not be and issue and overloading due to low psia support is very bad.


And, yes, the casing does grow a very small amount with the reduced atmospheric pressure on the outside, but it is a very small amount and the change in psia inside the tire does not change enough to put into the calculations, per the the manufacturers.



For me, if we are going to go through a high pass I will look at the temp difference we will see, and if it is more than about 30*F I will adjust pressure by about half of what the temp change would cause in pressure. That will cover the whole day without being more than a little over at the bottom and a little under at the top, no matter what the hand gauge and TST say.


I am relatively certain there will be lots of folks that don't agree with any of this, but that is fine. It would be interesting to hear any other information.
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Old 05-13-2020, 04:14 PM   #2
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Thanks for the info.

That recent TPMS topic had me checking out options on Amazon and AliExpress. My interest is internal sensors not external and I see several internal sensor options now other than TST's internal rim band.

The timing of your info is great for me.
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Old 05-13-2020, 04:35 PM   #3
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I used to think I wanted internal sensors--until I got some (on a long-gone Volvo). They are a nightmare. Between having tire jockeys bust them to having to move them when replacing tires to having to open the tire to replace a battery to having to reprogram the system if tire position changes... never again.
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Old 05-13-2020, 05:09 PM   #4
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Those are downsides for sure (and some bad luck).

Replaced the internal sensors on my Ford SUV at 9.5 years, 3rd set of tires, not a problem.

I just really dislike not using a tire valve as designed and dislike even more the idea of depending on a valve cap for system safety. I'm not trying to convince anyone of anything though. Just personal preference.
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Old 05-13-2020, 05:27 PM   #5
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I didnít have problems with pressure sensors on my van yet. We will need new tires next year so this would be good time to replace batteries, I hope this will be painless.

On my Honda tires pressure difference between wheels is measured indirectly by wheel diameters derived from revolutions per distance or time. Anytime I pump tires or rotated I need to calibrated the tire pressure monitoring system. It takes a few seconds to get through the menu to get the car into the self-calibrating mode for up to 30 min. of driving. It is rather painless, and no batteries!

I donít like gadgets requiring difficult battery replacement, I think I am not alone as automatic mechanical watches are coming back. I had a few quartz watches loosing waterproof quality after the first battery change.
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Old 05-13-2020, 06:21 PM   #6
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I've never understood why somebody doesn't make an internal TPMS that harvests parasitic power from the tire rotation. I would think that a piezo device and a little pendulum charging an ultracapacitor would work fine. It sure seems like there is plenty of energy available.
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Old 05-13-2020, 07:04 PM   #7
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Quote:
Originally Posted by avanti View Post
I've never understood why somebody doesn't make an internal TPMS that harvests parasitic power from the tire rotation. I would think that a piezo device and a little pendulum charging an ultracapacitor would work fine. It sure seems like there is plenty of energy available.

That certainly makes sense to me. I think I even saw a patent application a long time ago about that, but I don't recall any details.
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Old 05-13-2020, 07:16 PM   #8
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That certainly makes sense to me. I think I even saw a patent application a long time ago about that, but I don't recall any details.
It is another point of failure. Does it make sense? Maybe some wireless source of power! Obviously.
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Old 05-13-2020, 08:15 PM   #9
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Quote:
Originally Posted by avanti View Post
I've never understood why somebody doesn't make an internal TPMS that harvests parasitic power from the tire rotation. I would think that a piezo device and a little pendulum charging an ultracapacitor would work fine. It sure seems like there is plenty of energy available.
Mechanical clocks which donít need winding are commercially built since late 1920 (Atmos by Jaeger LeCoultre Ė 400 days mechanism) with amount of energy available in a rotating wheel and a tire a power generator design shouldnít be difficult. https://en.wikipedia.org/wiki/Atmos_clock

A small diameter magnetic rod loosely moving in reciprocating motion in a coiled cylinder could be very simple to build.

But I still like simplicity of an indirect measurement of a wheel diameter like my Hondaís system.
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Old 05-13-2020, 08:50 PM   #10
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Quote:
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A small diameter magnetic rod loosely moving in reciprocating motion in a coiled cylinder could be very simple to build.
I can't quite see how to make that design work. There is no available energy inside a uniformly spinning wheel--it is just like being in a gravity field. I don't see how to make your piston move much. The energy would have to come in CHANGES of speed and (more plausibly) bumps and shakes. That's why I though a piezo generator and a spring-loaded pendulum would be better.

Interesting design challenge.
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Old 05-13-2020, 08:57 PM   #11
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Quote:
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I can't quite see how to make that design work. There is no available energy inside a uniformly spinning wheel--it is just like being in a gravity field. I don't see how to make your piston move much. The energy would have to come in CHANGES of speed and (more plausibly) bumps and shakes. That's why I though a piezo generator and a spring-loaded pendulum would be better.

Interesting design challenge.

You could probably just use a a magnet filled spinner inside the tire on the wheel. Just like the old spinner hubcaps, it spins at a different speed than the wheel during and speed change, and could induce charge to a capacitor setup.
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Old 05-13-2020, 09:37 PM   #12
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Quote:
Originally Posted by avanti View Post
I can't quite see how to make that design work. There is no available energy inside a uniformly spinning wheel--it is just like being in a gravity field. I don't see how to make your piston move much. The energy would have to come in CHANGES of speed and (more plausibly) bumps and shakes. That's why I though a piezo generator and a spring-loaded pendulum would be better.

Interesting design challenge.
At slow speed gravity would force the magnet rod movement up and down every 180 degree of wheel rotation. Above a certain speed centrifugal force would keep the rod stationary due to increased friction. There is plenty enough energy at low wheel speed rotation to charge a battery.

Interesting project, not from the perspective if it can be done, but how to do it at low cost and with high reliability.
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Old 05-13-2020, 09:55 PM   #13
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Try this one for a good read:

https://www.intechopen.com/books/sma...rotating-wheel
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Old 05-13-2020, 10:23 PM   #14
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Comment on external TPMS. I had them. I also had both of the rear valve stems develop leaks when running in the desert heat with three year old valve stems. These were high pressure valve stems, BTW.

Apparently, the combination of heat and extra weight that the stems are not designed for did them in.

BTW, I was able to get four new stems installed at Walmart for a total of $12 + tax. They didn't have a separate computer code for installation labor. They are just priced as an add on to a regular tire install. So for less than $13, they removed the tires, installed fresh high pressure valve stems, and re installed the tires with two different employees verifying lug nut torque with a torque wrench. I was very pleasantly surprised.
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Old 05-13-2020, 10:52 PM   #15
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Apparently, the combination of heat and extra weight that the stems are not designed for did them in.
Yep, that is pretty much inevitable with any rubber stems, no matter how "heavy duty". After I lost an MB OEM stem, I replaced all 6 with metal stems and grommets to keep them rigid. There are kits for this. Should be considered mandatory with external TPMS setups.
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Old 05-14-2020, 01:40 AM   #16
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Seems as we touched a tip of the iceberg. As Booster wrote, there is a patent somewhere.
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Old 05-14-2020, 12:27 PM   #17
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Yep, that is pretty much inevitable with any rubber stems, no matter how "heavy duty". After I lost an MB OEM stem, I replaced all 6 with metal stems and grommets to keep them rigid. There are kits for this. Should be considered mandatory with external TPMS setups.
It was the short metal stems that failed, apparently the rubber seals that are squeezed against the rim degraded. If I put a little sideways pressure against the stem it leaked more. Just sitting still in the lot I could not hear a leak. I diagnosed the stem failure as I was hooking up the air hose to the low tire in the parking lot at a Walmart and heard the hissing as I was attaching the hose fitting.
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Old 05-14-2020, 12:36 PM   #18
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It was the short metal stems that failed, apparently the rubber seals that are squeezed against the rim degraded. If I put a little sideways pressure against the stem it leaked more. Just sitting still in the lot I could not hear a leak. I diagnosed the stem failure as I was hooking up the air hose to the low tire in the parking lot at a Walmart and heard the hissing as I was attaching the hose fitting.

Were they the bolt in style? Short versions of them have been very reliable from all I have ever seen or heard.
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Old 05-14-2020, 02:27 PM   #19
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Somehow this topic got off Booster's initial posting so here is my attempt to bring it back. Booster probably has made this table, or has it figured out in his head. As booster said, as you go up in altitude 2 things change, atmospheric pressure and temperature. The table on the left is how those typically change. Of course nature isn't that predictable and the temperatures can change more or less, and the pressures can change some too from these "Standard" values. But assuming these standard conditions, and not accounting for heating of the tires due to driving, going from sea level to 10,000-ft, the absolute pressure (psia) in the tire drops by 6.6 psi due to the large temperature drop (see Temperature in left table). However the pressure measured by the external TPMS or a handheld gage only drops 2.0 psi when you go to 10,000 ft (see PSIG in right table); this is due to the atmospheirc pressure drop. So the first thought is: add air so my gage reads 80 psig. But that only bumps the absolute pressure up a little bit, to 90.1 psia. You need to bump the pressure up more (to 84.6 psig on your gage) to maintain the absolute pressure at 94.7 psi, which is the absolute pressure you are trying to maintain. As booster said, taking air out is the opposite of what should be done.

This is probably not worth worrying about unless you are in the ragged edge of load capacity in your tires. I have a Roadtrek 210 that is at max gross weight and the rear tires are near their max rating, so I try to pay attention to my pressures. Thanks to booster for pointing out the issue. If I am ever in an extreme altitude situation I will pay closer attention to the pressures. Now I know what to be aware of and how to adjust.
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Old 05-15-2020, 04:07 PM   #20
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Quote:
Originally Posted by avanti View Post
I've never understood why somebody doesn't make an internal TPMS that harvests parasitic power from the tire rotation. I would think that a piezo device and a little pendulum charging an ultracapacitor would work fine. It sure seems like there is plenty of energy available.
They exist. A few years ago I read about sensor on the inside of tire treath build in, that gives nit only pressure and temperature, but also sends trough what the weight is on the wheel.
I think that weight is determined on the deflection, so sensor measures distance to rimm when on tge ground and half a cycle further no deflection.
Also the energy was generated vy the bending of tire, so mayby with piŽzo technic like you suggest.
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