My car has a dashboard indicator that tells me when the tires are under-inflated. I was discussing this with some friends and one said that she hates those indicators because her husband puts nitrogen in her tires and the indicator is on all of the time because of that.
That can’t possibly be right can it? Pressure is pressure and I’m highly skeptical that the chemical composition of the gases affects the sensor somehow.
It isn’t right. Either her tires are low or there is a fault in the system.
Not necessarily. Not all tire-pressure sensors sense pressure directly; some infer it from other data. Some of the methods actually in use could be thrown off by the difference in average molecular weight.
What are the other methods of tire pressure measurement that could tell the difference between normal 80% Nitrogen air and 100% Nitrogen?
The indirect sensors that I’m aware of look at tire revolutions, a low-pressure tire will be slightly smaller so will rotate slower. Sometimes they use a vibration sensor too.
I’m not aware of, nor can even think of a way to measure tire pressure in a way that can detect a few grams of difference between air and Nitrogen. Possibly if the tire was off the ground, a sensor could detect how much force it takes to spin the tire and maybe it could sense a difference between air and nitrogen, but I’m not sure that would work in the real world. A little mud on the tire could more than make up for the difference in mass.
For that matter normal variations in the weight required to balance the wheel and tire assembly would far outstrip the weight difference between air and N2
If her tire pressure monitoring system is inference based (tire rotation or suspension loading) there should be a reset function. Independently confirming correct tire pressure and resetting the computer may correct her problem.
If the TPMS is direct pressure measuring (internal or external valve stem sensor/transmitters) she likely has a bad sensor since nitrogen inflation (with much lower water vapor and oxygen content) usually reduces flaky sensor readings, especially cold weather false alerts.
Thanks for reminding me of another N2 advantage to add to my list.
It’s interesting that you mention “cold weather false alerts”. This is the first car I’ve had that had a pressure monitoring system. One thing I’ve noticed is that the alert comes on when we start getting winter temperatures.
But, in my case at least, the alerts aren’t false. When I check the pressure, all four are below where they should be. Bringing them back up to the rated pressure turns off the alert.
What puzzles me is, why has this happened two winters in a row? After the first time, I didn’t bother to let any air out when summer came, so shouldn’t it have been high over the summer and then, when winter came again, come back down to where I inflated them last winter? Do they lose that much air over the summer?
The general rule of thumb is 1 psi loss (or gain) for every 10 degrees Fahrenheit drop (or rise) in outside air temperature and as much as 1psi loss a month through diffusion and slow valve stem/bead leaks. While the six months of increasing temperature from winter to summer may have cancelled out (depending on your leak down rate and your summer/winter temperature difference) the other six months were all continuous pressure losses, resulting in you driving on tires that were underinflated by a few psi (or several psi).
This causes more rolling resistance (lower gas mileage), worse handling and braking (reduced safety) and more heat and sidewall flex (which shortens tire life and increases your chances of a blowout - bad news anytime and potentially fatal at freeway speeds).
Weekly checks (or at least semi-monthly, it only takes a few minutes and a $10 gauge) on cold tires with pressure top-offs as needed (preferably to the spec on your factory sticker, that’s what the engineers tuned your suspension to work best with - but never exceeding the max pressure rating written on your sidewall) will not only save you gas and tire costs and increase your safety, but it allows you to spot a tire that starts leaking faster (from a screw/nail or bad valve stem) so you can get it fixed before the tire is ruined or it causes you an accident.
I have a monitoring system, so I don’t need weekly checks with a gauge, right? Otherwise, what’s the point of the monitoring system?
The monitoring system is to alert you when a tire gets dangerously low, not when it is effecting fuel mileage.
Mithras, CurtC (or anyone with the formal education to weigh in):
I finally re-located the physicist’s clarification that I’d read explaining why N2 molecules are physically larger than O2 molecules, why Fick’s Law of Diffusion and Henry’s Law of Solubilities (not Graham’s Law of Effusion) govern O2 permeation through rubber and why partial pressure is a driving factor in greater oxygen migration through the tire body.
Is Dr. Murphy talking solid science here or is he just trying to mislead?
I’m not a physicist or a chemist. I read that PDF, and checked his statements with other sources, and it seems to be correct - although you may expect (I did) that a nitrogen molecule would be smaller than an oxygen molecule, it’s actually the other way around.
Actual tests conducted by Consumer Reports show that a nitrogen-filled tire leaks more slowly than one filled with regular air (cite). The nitrogen-only tires lost 2.2 psi after one year (starting at 30 psi), while the air tires lost 3.5 psi. That 1.3 psi of difference tells me that all things being equal, I’d opt for nitrogen. On the other hand, if it’s an extra dollar for nitrogen, I may not think it’s worth it.
penumbrage, the issue I had with your earlier cite was the bit about partial pressures. That link said that the ideal mix was not 100% nitrogen, but a small amount of oxygen, to equalize the partial pressures of oxygen outside and inside the tire. I think they were wanting to avoid oxygen molecules traversing the tire wall. That’s just wrong, though. The deal with equalizing the partial pressures would be that the number of oxygen molecules going IN would be the same as the number going OUT, so a net of zero. It does NOT mean that there would be fewer molecules going across the barrier, just that the “in” would equal the “out.”
On the other hand, my gut feel tells me that the amount of leakage you get through the tire itself would be close to zero, and the main source of leakage would be where the tire bead didn’t seal well at the rim. I realize gut feels are not science, so if I’m wrong about that I’d appreciate being corrected.
By the way, I’m a bicycle rider, and my mountain bike has no tubes. This is pretty common for high-end mountain biking. Without tubes, the tire bead would leak, so mountain bikers use a latex (or latex-like?) liquid sealant in the tire. A big advantage is that small punctures in the tire get sealed up immediately. Why don’t cars use this same kind of sealant?
They do, and they market them as run-flat tires. Not all run-flats are like this, but some are and more are coming online. Tirerack.com
Following up on my own cite, it would have been interesting had Consumer Reports tested the mix of air in the tire after it lost 3.5 psi of pressure. If the increased loss in the regular air tire was due to the oxygen leaking out faster, it would be easy to calculate the new mix of oxygen and nitrogen in the tire at the end of the test. IOW, after the test, the “air” tire would have more nitrogen, proportionally, than the 80% it started with, because the oxygen leaked out more.
Which leads me to another realization - if the oxygen leaks out faster, and over the years you top it off with regular air, then your tires will gradually have a higher and higher concentration of nitrogen, and less of oxygen.
How long do tires normally go without a total loss of air? Is there any time during normal servicing when tires are removed from the rim (or totally deflated for some other reason) and then re-inflated?
There’s no standard reason to deflate tires as part of routine servicing of the car. If you have a flat and they need to repair it, then it will be removed and reseated. If you have a leak around the rim caused by separating chrome, they will remove the tire to rework the rim.
Otherwise, the tire should remain seated and inflated. Getting a bit low and then repressurized does not change the seating on the rim.
I read the pdf file. It makes sense and matches my understanding of chemistry from college and high school. I’m no expert, but I can’t fault the information provided. It seems right.
The important “size” for permeation purposes is the size of the O2 or N2 molecule, not the size of the atom. Being polar molecules, they have an angle. Couple with that the extra proton of Oxygen causing a sligthly stronger pull between the nucleus and electrons, and the overall size of the molecules makes Nitrogen gas molecules larger.
I asked if Dr. Murphy was right because his permeation equation Ji = [ Pij x A x (pi inside - pi outside) ] / L says if the interior partial pressure (pi inside) exactly equals the exterior partial pressure (pi outside) then you multiply by zero in the numerator and get zero for overall permeability (Ji) regardless of the values for the permeability coefficient of the material (Pij), the area (A) or the material thickness (L) - in fact he says “the driving force for transport across the wall, which is the difference in concentration of gas (i) across the tire wall - for convenience with gases, a nearly exactly correct measure of this is the difference in partial pressures (pi) of that gas (i) on the two sides of the tire wall…”.
Yes, the actual N2/O2 molecular diameters are very counterintuitive, as is the fact (if I understand partial pressures correctly) that a 32psi, 100% nitrogen fill will slowly gain oxygen molecules inside the tire as it leaks down due to the far greater external partial pressure of O2.
Both the this example (with O2 leaking in) and an air inflated tire (with O2 leaking out faster than N2) should eventually balance out after many years of topping off and diffusion at the 94.4% partial pressure balance point (assuming a valve stem failure didn’t require dismounting that 500,000 mile rated tire).
Sure, you’d get zero net oxygen permeating through the wall. In other words, the O[sub]2[/sub] molecules going in would be equal to the O[sub]2[/sub] molecules going out.
But so what? Your earlier cite was trying to make the point that they were trying to minimize the number of O[sub]2[/sub] molecules making the trip through the material. In that case, you should inflate with 100% nitrogen. The argument that you should put in air with 6% oxygen because of the partial pressures explanation was bogus.
All the sources I’ve found that address the issue say that pure nitrogen inflation (or any concentration above about 94%) permits oxygen migration back into the tire.
Are you saying Dr. Murphy’s permeability formula is wrong?
Or is my algebra just rusty - if the tire is filled with 100% N2, reducing the interior O2 partial pressure {pi inside} to zero, doesn’t that make the term (pi inside - pi outside) negative and reverse the sign of the permeability (Pij), reflecting O2 migration into the tire?
No, your conception is wrong.
It’s like having a box of apples. There are 27 apples in the box. I now find 5 apples and put them in the box, then take 3 apples out and sell them, then am given 7 apples and put them in, then take 9 apples out and give them away. How many apples are in the box? 27. The net number of apples in the box is the same. The number of apples put into the box equals the number of apples removed from the box. But apples still went in and out of the box.
Dr. Murphy’s formula is looking at the number of molecules in the tire, not the number of molecules that pass through the tire.
If you split his formula into two parts
Rate of migration out of tire - Rate of migration into tire =
[( Pij x A x pi inside) / L ] - [ (Pij x A x pi outside) / L ]
You see then that he made an algebraic simplification to look at the net result.
Yes, if the partial pressure of oxygen is lower inside the tire than outside the tire, the net result will be an increase of O2 molecules in the tire. That’s basically osmosis.
So if you wish to limit the number of molecules going through the membrane, then fill with 100% dry nitrogen, and thus slightly reduce the number of molecules that go from inside to outside. Because all molecules that get into the tire come from outside first, then begin passing in and out, vs passing in and out at an equal rate from the get go. I’m not certain how much of a reduction in molecules passing through the tire that will actually be.