low pressure in tires with cold weather

[Eduard Nemirovsky]

I have tires on my track for last 23000 miles. One of the tire were repaired summer time. Now every time when temperature fall in the morning -low pressure lights is on. Tires with normal wear thread, close to be replace but not yet, at least by miles rating. Do you think I need to replace all tires and forget for next 25000 miles or this is normal to have lower pressure warning when in the morning temperature going low?

Ed.

[Jerry Bruette]

The tire pressure indicators on my Honda CR-V will indicate low pressure for days when the temps get below 0*.

It's the same phenomenon that the New England Patriots have with their footballs.

[Erik Loza]

Jerry.... Hahahahaha.

Our Mini Cooper does the same thing. I have to reset the warning light every once in a while. Honestly, I find the whole TPMS thing more trouble than it's worth most of the time but unfortunately, there seems to be no way to disable it.

Erik Loza
Minimax

[Dan Hintz]

Honestly, I find the whole TPMS thing more trouble than it's worth most of the time but unfortunately, there seems to be no way to disable it.

Agreed. Though I did read a story of some guy who put all four units in his spare tire (with the obvious loss of high-speed travel on his spare).

[George Bokros]

I have TPMS in both my truck and car and never have this issue. Before the light will come on the pressure has to be at an unsafe level which is more than a couple pounds under recommended pressure. You should check your tires in the late fall to increase the pressure due to the loss caused by the falling temperatures. As Jerry said it is the same issue as the Patriots have with the footballs. On the converse, tire pressure will increase in the spring due to the warmer temperatures. If you get in the habit of checking your tire pressure every month or two you should not have this issue.

[Mike Cutler]

Eduard

No, you don't need to replace the tires. At this point you should fill all your tires to the recommended cold tire inflation pressure stamped on the sidewall. If your getting this warning, one, or some, of your tires are low on pressure, at any temp. Be sure to check your spare tire also. It's gotten many people. ;)


Erik
We have two 'Coopers. An R56, 2009 S, and a 2013 JCW, also R56. I switch tires and rims between summer and winter, and just keep checking psi every now and then. Rarely do I ever get that alarm.
Only way to disable it is to mount all four TPMS sensors in a sealed PVC cylinder, and pressurize them, and put it in the boot. This is how the folks get by the TPMS when they want to use aftermarket rims that aren't TPMS compatible. If you have a Generation 1 Mini, I don't think the TPMS can be faked in this manner though.

[Erik Loza]

Erik
We have two 'Coopers. An R56, 2009 S, and a 2013 JCW, also R56. I switch tires and rims between summer and winter, and just keep checking psi every now and then. Rarely do I ever get that alarm.
Only way to disable it is to mount all four TPMS sensors in a sealed PVC cylinder, and pressurize them, and put it in the boot. This is how the folks get by the TPMS when they want to use aftermarket rims that aren't TPMS compatible. If you have a Generation 1 Mini, I don't think the TPMS can be faked in this manner though.

Mike: Interesting. I suspect that the F56's must be more sensitive about this than the earlier generations. There seems to be some chatter to that effect on the Mini forums. Did the R56's come with run-flats? Ours are and folks generally seem to hate them (myself included). I get the alarm any time the outside temps drop into the 30's.

Erik Loza
Minimax

[Mike Cutler]

I've owned Mini's since 10/08 and they have all come with run flats. I use Blizzaks and Winter Sport 3D's, both Run Flats, in the winter. Summertime I switch to Hankook V12's, non run flat. I'd use Hakkapelita's in the winter, but they don't seem to make a tire compatible with what I have in Rims. I don't think you guys have this problem in Texas.
I pretty much hate the Run Flats also. the ride is very harsh on a 45 series tire.

[Erik Loza]

Ours has the Hankooks. First run-flats I have owned and I don't have an issue with them in terms of peformance or handling, but the road noise is TERRIBLE! Both on the highway and then any time you run over any sort of bump, "Boom!", like you broke something in the suspension. The first time I test drove the car (which I otherwise love...), I remember thinking, "What the heck did I just run over?". Going back to regular tires once these go, for sure.

[Mike Cutler]

You can jump up a series to 50's if you aren't running the 19" rims for a "plusher ride".
Hankook just replaced the V12 with a new series V12. It's a little loud, but not as a bad as the run flats.
Continental is supposed to have a very nice, quiet, non run flat, high performance tire. I'm probably going to switch to those when my V12's need replacement.

[Howard Garner]

Agreed. Though I did read a story of some guy who put all four units in his spare tire (with the obvious loss of high-speed travel on his spare).

This does not work with all cars.
My VW TRS works off rotation differential indicating a lower diameter tire.

Howard

[Erik Loza]

You can jump up a series to 50's if you aren't running the 19" rims for a "plusher ride".
Hankook just replaced the V12 with a new series V12. It's a little loud, but not as a bad as the run flats.
Continental is supposed to have a very nice, quiet, non run flat, high performance tire. I'm probably going to switch to those when my V12's need replacement.

May do that, thanks. We have 17's.

Erik Loza
Minimax

[Art Mann]

The ideal gas law (PV=nRT) says that if you keep the volume constant, as it mostly is with automobile tires, then the pressure goes up and down as the temperature goes up and down, respectively. Over large swings in temperature, this pressure will vary several psi - enough to set off your TPM. If that happens, your tires really do need more air.

[Brian Elfert]

My TPMS will go off at some point every winter without fail. One of these years you would think I would remember to check my tires at the start of winter.

[Jerome Stanek]

I have had them go off even with the proper pressure in them from the battery being weak.

[Brian Henderson]

My car does the exact same thing, if you get in when it's cold, you'll get a low-pressure warning but if you drive for a couple of minutes, it turns off. Last time I had it at the dealership, I had them check out the tires since they were doing a tire rotation anyhow and they said everything was fine. They could offer no solution other than offering to replace the sensors, which wasn't covered by the warranty.

[Art Mann]

The explanation is that as you drive, the tires are heated by friction with the road and flexing. The increase in temperature heats the air in the tires and the pressure goes up and out of the warning range. If your mechanic didn't know that, you need to look for a new mechanic. Perhaps they were just trying to sell some new tire pressure monitor devices.

[Mike Lassiter]

simply fact is if the tires are losing air pressure they have something leaking. Could be the stem leaking at the rim, the valve core, around the rim from debris on the rim or tire when the tires were mounted or a small pin hole it the tire. I have the same issue on our van. ONE tire seeps down and after a few weeks the low tire pressure light comes on. Spec calls for 36 psi, while manual says the indicator light will come on around 45 psi on the high side and 28 psi on the low side. So you could be a could of pounds above the minimum then cold temperature cause the pressure to drop slightly until you drive some - just as mentioned. If you go weeks without having to add air the leak would be hard to locate, and could require dismounting the tire from the rim to cure.

I just started and moved a car that has been parked over 2 years with all the tires very low, but none "flat"; the lowest showed 2.5 psi. Tags expired in March 2013. I think they did good, and have stayed up since. Ready to go to it's new home soon.

[Phil Thien]

When the TPMS idiot light turns on, I check the pressure and adjust as necessary.

If it was happening every day or two I'd suspect I had something stuck in the tire.

[Myk Rian]

I always check tire pressures in late fall.

[Dan Hintz]

This does not work with all cars.
My VW TRS works off rotation differential indicating a lower diameter tire.

Howard

I'm confused... why would a TPMS need rotation (or tire size, for that matter) to determine tire pressure?

[Justin Ludwig]

Sounds to me like the TPMS is working fine. An understanding of gasses in relation to temperature is not. Have your tires filled with nitrogen and rest easy all season, unless you have a leak or a flat.

Temperature Change. All gases expand and contract with temperature. If you live in an area that experiences dramatic temperature changes, you will have to adjust your tire pressure accordingly. Typically you will only have to adjust your tire pressure "up", adding pressure as ambient temperatures decrease. Count on losing about 2% of your total tire pressure for every 10 degrees in temperature reduction. -http://www.nitrofill.com/nitrogen-filled-tires-faqs.aspx

[Jerome Stanek]

I have had them go off even with the proper pressure in them from the battery being weak.

I had to replace the sensor

[George Bokros]

There are some tire pressure sensors that require tire rotation (driving the car, called indirect TPMS) to determine pressure. There are also some cars TPMS systems that need to be reset when you rotate your tires. I had a friend that had those on his Cadillac. The one on his Cadillac actually tells him what tire is low.

[Rod Sheridan]

Eduard, if the weather is cold you need to add air to your tires to inflate them to the value specified on the sticker on the door.

The air pressure in your tire falls with lower temperatures and the tires will be under-inflated.

regards, Rod.

[Phil Thien]

I'm confused... why would a TPMS need rotation (or tire size, for that matter) to determine tire pressure?

They shouldn't. But they likely aren't doing anything when they aren't rotating, in order to preserve battery life.

[Dan Hintz]

They shouldn't. But they likely aren't doing anything when they aren't rotating, in order to preserve battery life.

You're right, I'm an idiot for not remembering that...

[Steve Peterson]

Sounds to me like the TPMS is working fine. An understanding of gasses in relation to temperature is not. Have your tires filled with nitrogen and rest easy all season, unless you have a leak or a flat.

I let the tire store fill my tires with nitrogen. I top them up with a 78% nitrogen, 21% oxygen, 1% carbon dioxide blend (i.e. normal air). Pressure changes relative to temperature are the same for all types of gasses.

Steve

[Dan Hintz]

Sounds to me like the TPMS is working fine. An understanding of gasses in relation to temperature is not. Have your tires filled with nitrogen and rest easy all season, unless you have a leak or a flat.

Temperature Change. All gases expand and contract with temperature. If you live in an area that experiences dramatic temperature changes, you will have to adjust your tire pressure accordingly. Typically you will only have to adjust your tire pressure "up", adding pressure as ambient temperatures decrease. Count on losing about 2% of your total tire pressure for every 10 degrees in temperature reduction. -http://www.nitrofill.com/nitrogen-filled-tires-faqs.aspx

I've never understood this suggestion (in theory or realistic terms). Air is already 78% nitrogen. The remaining 22% has negligible effect at the pressure we run our daily drivers.

Reminds of my wife's trip to the local tire shop to top up a tire originally installed by Costco (they use Nitrogen). Shop initially refused to top up with shop air (they didn't have nitrogen) because it would "contaminate" the nitrogen in the tires. After informing them of the above percentage, I was still mystified by their reluctance to add 3-4psi of shop air, adding a whopping 2-3% of "contaminants". :rolleyes:

[Jim Matthews]

We have two 'Coopers. An R56, 2009 S, and a 2013 JCW, also R56.

Well, isn't that special.
* you suck *

305347

[Art Mann]

I've never understood this suggestion (in theory or realistic terms). Air is already 78% nitrogen. The remaining 22% has negligible effect at the pressure we run our daily drivers.

Reminds of my wife's trip to the local tire shop to top up a tire originally installed by Costco (they use Nitrogen). Shop initially refused to top up with shop air (they didn't have nitrogen) because it would "contaminate" the nitrogen in the tires. After informing them of the above percentage, I was still mystified by their reluctance to add 3-4psi of shop air, adding a whopping 2-3% of "contaminants". :rolleyes:


The truth is that all gasses and mixtures of gasses respond in exactly the same way by the exact same amount until you start to approach the condensation temperature of one of the gasses. There is absolutely no truth to the idea that nitrogen filled tires have a more stable pressure.

[Justin Ludwig]

The truth is that all gasses and mixtures of gasses respond in exactly the same way by the exact same amount until you start to approach the condensation temperature of one of the gasses. There is absolutely no truth to the idea that nitrogen filled tires have a more stable pressure.


You couldn't be farther from the truth. Why are aviation tires required to be filled with Nitrogen? In extreme temperature changes it is LESS reactive. Nitrogen is inert, oxygen is not. "Everyone is entitled to his own opinion, but not his own facts." - Daniel Patrick Moynihan

I won't pay extra for N to be put in my truck when I can top them off in the winter in about 5 seconds each.

[Pat Barry]

The explanation is that as you drive, the tires are heated by friction with the road and flexing. The increase in temperature heats the air in the tires and the pressure goes up and out of the warning range. If your mechanic didn't know that, you need to look for a new mechanic. Perhaps they were just trying to sell some new tire pressure monitor devices.
This is exactly correct. In NASCAR they purposely start with the tires with low air pressure and in a couple laps the tires warm up and the pressure goes up, in fact that is one reason you see the guys swerving back and forth on the track before the green flag drops - puts more heat in the tires. F1 racing rules allow the teams to warm up their tires before they put them on the car - they actually have electric heater blankets to keep the tires hot before using them

[Phil Thien]

You couldn't be farther from the truth. Why are aviation tires required to be filled with Nitrogen? In extreme temperature changes it is LESS reactive. Nitrogen is inert, oxygen is not. "Everyone is entitled to his own opinion, but not his own facts." - Daniel Patrick Moynihan

I won't pay extra for N to be put in my truck when I can top them off in the winter in about 5 seconds each.

The likely explanation for requiring nitrogen for airliner tires would be the chance of compressed atmosphere to carry enough water that ice could form in the tire during flight. You do not want to land with 100+ souls on board and a block of ice in one or more of the tires.

[Justin Ludwig]

This is exactly correct. In NASCAR they purposely start with the tires with low air pressure and in a couple laps the tires warm up and the pressure goes up, in fact that is one reason you see the guys swerving back and forth on the track before the green flag drops - puts more heat in the tires. F1 racing rules allow the teams to warm up their tires before they put them on the car - they actually have electric heater blankets to keep the tires hot before using them

You're half correct. NASCAR uses Nitrogen filled tires because they can predict the heat expansion.


Back to the main argument. Check your spare tire. Some have sensors in them.

[Anthony Whitesell]

The likely explanation for requiring nitrogen for airliner tires would be the chance of compressed atmosphere to carry enough water that ice could form in the tire during flight. You do not want to land with 100+ souls on board and a block of ice in one or more of the tires.


I can't speak to airplane tires, but I can for automotive (both street and performance) tires and Phil has hit the nail on the head. Water vapor. We are all vaguely aware the volumetric expansion rate of water is in credible (water to ice and water to steam). We live and work within it's solid, liquid, and vapor temperature range. Realistically any gas or gas combination that is free of water/water vapor is comparable to using nitrogen. The use of nitrogen is a matter convenience and expense. In some aspects, filtering out or minimizing oxygen content would be helpful as well.

I read this factoid years ago when filling street tires with nitrogen was all the rage. If you fill a tire to 29.4 psi with nitrogen, what percentage of nitrogen is in the tire? 93%. 29.4 psi is picked for a reason. Technically it is 29.4 psig (psi gauge). When the tire is first mounted there is 14.7 psia (psi absolute) in the tire. Tire pressure gauges read psig not psia. To increase the tire pressure so the gauge reads 29.4, you must add two additional volumes of air to the tire. First volume of nitrogen brings the pressure to 14.7, the second volume to 29.4. Since the tire started full of a volume of air, it initially contained 79% nitrogen. Adding two volumes of nitrogen to the mix, increase the percentage to 93%, or diluting the other gasses to just 7%. What was gained? Not much, since the primary cause of temperature instability is water vapor. To really reduce the water vapor content (or increase the nitrogen content), there is a simple solution. Let out all the air and refill it. When the tire is deflated and refilled a second time, the nitrogen percentage is increase to 97.7%. Deflate and refill a second time and the percentage climbs to 99.2%. Now the tire is filled with nitrogen. But really all you need is an air compressor with a really good air drier.

[Mike Cutler]

Well, isn't that special.
* you suck *

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Jim
That's just mean. Funny too.;) I love that photo.

We started with a brand new 2007 CoopeS that got wrecked in 2011. So we replaced it with an '09 CooperS. This past summer my wife found a baseline model '13 JCW on a Subaru lot in New Jersey. It had 6000miles on it and was selling for well below Blue Book.
I told her that she could buy leather salvage seats to replace the basic seats that came with it. She found and bought an entire Carbon sport trim line leather interior from a boneyard in Maryland cheaper than it would have been to buy just one seat. Especially the passenger seat, it's really expensive. So I became an interior car renovator and changed all the seats. changed the door handles to a different trim line and carbon fiber handles. I'd had a carbon fiber JCW shift handle for a few years, so that went in. She's done with her Mod's for now, but she was asking how hard it would be to change the dash.:eek:
My truck has 315,000 miles on it, so I kinda needed a new car anyway. I already had a garage full of CooperS stuff, so the JCW made sense.

I like the Mini Coopers. They're a complete riot to drive.

[Art Mann]

The idea of water changing from liquid to gas inside the tire has merit, but I have dismounted and mounted quite a few tires in my life and watched other people do a lot more. I can't recall seeing a lot of free water in the tire. The very act of compressing air causes a majority of the moisture to condense and settle in the tank or lines. That is why air compressors and well designed distribution systems include drain valves. I am on my soapbox about this because I hate to see people duped by someone selling an unnecessary service at a premium price.

[Art Mann]

You couldn't be farther from the truth. Why are aviation tires required to be filled with Nitrogen? In extreme temperature changes it is LESS reactive. Nitrogen is inert, oxygen is not. "Everyone is entitled to his own opinion, but not his own facts." - Daniel Patrick Moynihan

I won't pay extra for N to be put in my truck when I can top them off in the winter in about 5 seconds each.

Pardon me if I don't accept your "facts" but I acquired my knowledge in an engineering thermodynamics class in college, which dealt with gasses and pressure vessels. I consider that source a little more reliable. Sometimes, explanations can be perfectly obvious to the casual observer and perfectly wrong at the same time. Nitrogen is not an inert gas in the sense that helium or argon is, but whether it is chemically active has no bearing on whether it obeys the ideal gas law.

[Myk Rian]

Nitrogen, schmitrogen. Just check the tire pressure a couple times a year.
This group has to argue about everything.

[Eduard Nemirovsky]

Thank you, Rod.
It is funny how people can modify posted question to absolutely different area. I learn a lot about Mini Cooper, which my wife really likes, but after test drive absolutely refuse to buy.
But this is why I really like this forum - a lot of very knowledgeable people, some very opinionated. And most of the time very fast and good answer on posted questions.

Thank you everybody, keep warm.
Ed.

[Dan Hintz]

This is exactly correct. In NASCAR they purposely start with the tires with low air pressure and in a couple laps the tires warm up and the pressure goes up, in fact that is one reason you see the guys swerving back and forth on the track before the green flag drops - puts more heat in the tires. F1 racing rules allow the teams to warm up their tires before they put them on the car - they actually have electric heater blankets to keep the tires hot before using them

While the air pressure does rise due to the heat increase, they swerve back and forth to heat up the tread (the pressure increase is just a side-effect they take into account). You want those treads nice and sticky, and swerving gets them to temp more quickly.


Phil hit on the main reason why nitrogen is used in airliners (that, and less combustible gasses if/when things go wrong... seeing airliner brake rotors turn cherry red from heavy braking is a sight to see, that's for sure).

[Pat Barry]

While the air pressure does rise due to the heat increase, they swerve back and forth to heat up the tread (the pressure increase is just a side-effect they take into account). You want those treads nice and sticky, and swerving gets them to temp more quickly.


Phil hit on the main reason why nitrogen is used in airliners (that, and less combustible gasses if/when things go wrong... seeing airliner brake rotors turn cherry red from heavy braking is a sight to see, that's for sure).
So, scrub the tires, sure the tread gets hot and sticky, doesn't that heat then go into the body of the tires and therefore the air (nitrogen) and that heat translates to increasing tire pressure. Voila~!

[Mike Cutler]

Thank you, Rod.
It is funny how people can modify posted question to absolutely different area. I learn a lot about Mini Cooper, which my wife really likes, but after test drive absolutely refuse to buy.
But this is why I really like this forum - a lot of very knowledgeable people, some very opinionated. And most of the time very fast and good answer on posted questions.

Thank you everybody, keep warm.
Ed.

Sorry about the Mini Cooper tangent.:o
I think we've beat air, or nitrogen pressure, in tires pretty good. Air is fine in your truck tires. You won't be doing 200mph, I hope. ;)
Good luck with your truck.

[Dan Hintz]

So, scrub the tires, sure the tread gets hot and sticky, doesn't that heat then go into the body of the tires and therefore the air (nitrogen) and that heat translates to increasing tire pressure. Voila~!

I never said the pressure didn't increase, just that the increase is a side-effect, not the main purpose...

[Curt Harms]

I've never understood this suggestion (in theory or realistic terms). Air is already 78% nitrogen. The remaining 22% has negligible effect at the pressure we run our daily drivers.

Reminds of my wife's trip to the local tire shop to top up a tire originally installed by Costco (they use Nitrogen). Shop initially refused to top up with shop air (they didn't have nitrogen) because it would "contaminate" the nitrogen in the tires. After informing them of the above percentage, I was still mystified by their reluctance to add 3-4psi of shop air, adding a whopping 2-3% of "contaminants". :rolleyes:

I'm not sure how relevant this is to cars but most aircraft manufacturers recommend nitrogen in tires. Not because nitrogen is good for the tires but because compressed air can contain moisture, nitrogen from a bottle does not. Get enough moisture in a tire then leave it outside in freezing weather (or cold soaked for a few hours at -60oF at altitude) and you have a frozen puddle, resulting in an unbalanced tire. Also sorta related, there is sometimes a table to be found for high pressure accumulators. I noticed the same phenomon, pressure drops when it gets cold and I'd get the nitrogen added. Later I discovered a table that gave differing "full" values depending on temperature.

[Pat Barry]

This whole discussion prompted me to dust off my memory related to the old PV=nRT formula. I did the calculations and here is a resulting table showing the pressure versus temperature. Note: It assumes a constant gas (air/nitrogen) volume (probably not true for a tire or a football that can inflate), also assumes that the gas is always a gas and not a condensing material. None the less the table probably is a pretty good first order estimate:


305434
Also, for you NFL fans, I also did the calculations for footballs

305435
.

[Jason Roehl]

One of the reasons for nitrogen in airplane tires is that if one bursts during braking, you have a non-flammable gas flooding the area as bits of hot rubber come into contact with even hotter brake parts. Pointless in car tires.

[Jason Roehl]

This whole discussion prompted me to dust off my memory related to the old PV=nRT formula. I did the calculations and here is a resulting table showing the pressure versus temperature. Note: It assumes a constant gas (air/nitrogen) volume (probably not true for a tire or a football that can inflate), also assumes that the gas is always a gas and not a condensing material. None the less the table probably is a pretty good first order estimate:


305434
Also, for you NFL fans, I also did the calculations for footballs

305435
.

I had to double check with a friend who is a nuclear chemist to make sure I was using Charles' Law correctly, but I found that if a football (assuming no change in volume) is inflated to 12.5 PSI at 70ºF, and the temp drops to 50ºF, the pressure will drop about 1.1 PSI--more than double the drops your chart shows for 80ºF to 60ºF to 40ºF, interestingly enough (but your chart is right). Supposedly the Patriots' balls were 2 PSI low...

[Art Mann]

A friend of mine who is an electrical engineer at Boeing came over last night. I asked him why airliner tires are inflated with nitrogen. He didn't know for sure but he knows some people who would know the correct answer and he said he would ask. If I hear back from him, I will post what he said.

[Pat Barry]

I had to double check with a friend who is a nuclear chemist to make sure I was using Charles' Law correctly, but I found that if a football (assuming no change in volume) is inflated to 12.5 PSI at 70ºF, and the temp drops to 50ºF, the pressure will drop about 1.1 PSI--more than double the drops your chart shows for 80ºF to 60ºF to 40ºF, interestingly enough (but your chart is right). Supposedly the Patriots' balls were 2 PSI low...
Jason, thanks for the sanity check. I now see that I used PSIG for the calculations, not absolute pressure. Using absolute pressure (correct application) the pressure drops will be more than what I calculated in the tables above. I did a quick check and got 1 PSI for the football example at the temperatures you identified.

[Jason Roehl]

Jason, thanks for the sanity check. I now see that I used PSIG for the calculations, not absolute pressure. Using absolute pressure (correct application) the pressure drops will be more than what I calculated in the tables above. I did a quick check and got 1 PSI for the football example at the temperatures you identified.

LOL! I heard back from my nuclear chemist friend, and he went back on what he originally said--that we SHOULD use PSIG, not absolute pressure. The atmospheric pressure wouldn't change. However it just occurred to me that what's tripping everything up is that the volume of the football is not constant. Footballs are neither perfectly rigid where the volume never changes, nor are they "perfect balloons" that can collapse completely. With an increase in pressure, their volume probably follows some sort of inverse exponential curve that approaches some theoretical limit until they burst.

I may need to do an experiment...if I can find a needle...

[Anthony Whitesell]

Both are incorrect. The ideal gas law and Boyle's law both use temperature in Kelvin and pressure in Pascals. When using 70 deg F (294K) and 40 deg F (277K) and a starting pressure of 13 psig (89631 pascal) all converted to Kelvin and pascals, then perform the Boyle's law calculation (T2/T1*P1) and convert the resulting pressure back to psig, the footballs are at 12.2 psig (84448 pascal). As soon as you calculate the ratio of temperatures in Kelvin, you already there is going a 6% drop in pressure. Also remember that this is for an IDEAL gas, which there aren't any and void of water vapor. One aspect I have not heard mention of is the source to fill the balls. A bicycle pump, air compressor, or concentrated gas cylinder?

As for the car tires, the same 6% drop applies so a 30 psig tire at 70 deg F drops to 28.2 psig at 40 deg F. The TPMS in my car goes off just under 28 psig.

[Jason Roehl]

Both are incorrect. The ideal gas law and Boyle's law both use temperature in Kelvin and pressure in Pascals. When using 70 deg F (294K) and 40 deg F (277K) and a starting pressure of 13 psig (89631 pascal) all converted to Kelvin and pascals, then perform the Boyle's law calculation (T2/T1*P1) and convert the resulting pressure back to psig, the footballs are at 12.2 psig (84448 pascal). As soon as you calculate the ratio of temperatures in Kelvin, you already there is going a 6% drop in pressure. Also remember that this is for an IDEAL gas, which there aren't any and void of water vapor. One aspect I have not heard mention of is the source to fill the balls. A bicycle pump, air compressor, or concentrated gas cylinder?

As for the car tires, the same 6% drop applies so a 30 psig tire at 70 deg F drops to 28.2 psig at 40 deg F. The TPMS in my car goes off just under 28 psig.

We both did that. As long as the reference point is correct (absolute zero), units don't matter since the equation is T2/T1 = P2/P1. Try it. You can do it in Fahrenheit gradations above absolute zero and the difference will still be the same percentage. But neither of us did 70ºF to 40ºF. I calculated 70ºF and 12.5 PSIG (minimum pressure for the NFL) starting conditions dropped to 50ºF, which yields a pressure of about 11.4 PSIG.

[Anthony Whitesell]

I'm sorry Jason, I don't follow. I made the mistake of using Deg F first time I thought of using the gas law. Then I made the mistake of using Deg C. Neither compute the same as Kelvin. T2/T1 = 40/70 deg F = 0.57, 4.4/21.1 deg C = 0.21; and in Kelvin 277/294 =0.94. To reproduce the calculation using 70 and 50 deg F and 12.5 psig starting pressure. 12.5 psig=86184 pascal; 283K (50degF)/294K(70degF) = 0.96 (4% loss) * 86184 pascal (12.5 psig) = 82959 pascal or 12.0psig.

[Jason Roehl]

Anthony, you're right that you can't use 70ºF/50ºF for the equation. However, if you add 460ºF to each of those, it will work, since absolute zero is -459.67ºF. That sets absolute zero as the reference point. The temperature units cancel. The same would be true for Celsius if you add 273.15 to the appropriate Celsius temperatures (which is how you get the Kelvin temperatures).

The reason I had come up with a different number is that I used absolute pressure (PSIG + PSIA, which is the gauge reading plus atmospheric pressure). However, since footballs aren't rigid, atmospheric pressure has an effect that changes the volume of the football. Probably not a large effect, but it does complicate the calculation, and we don't (can't--it's not ever measured by the NFL) know what that volume change was on the field. Anyways, I was multiplying that 96% by 27.2 PSI (absolute, or 12.5 PSIG + 14.7 PSIatmos).

[Pat Barry]

I used deg K for the calaculations I did both times (PSIG and abs pressure). The pressure unit of measure doesn't matter since gong pascal to PSI is a simple proportion. I suppose PSIG is actually OK to use since a deflated football would read 0 psig and go up from there to whatever. In absolute you just need to add 14.7 psi. Anyway, the pressure goes down with temperature as we all know. I'm looking forward to actual test data if anyone can produce that.

[Pat Barry]

I found someone doing the air pressure vs temperature football test (https://www.youtube.com/watch?v=_Gd0kGhIcF4)
13.3 psig to 11.9 psig (76 deg vs 52 deg)

[Frank Trinkle]

I have nitrogen in all four of my new large Michelins on my Armada SUV. I drive in Hot areas, Cold areas, and Freezing areas. What I have found with the nitrogen fill is that my tire pressure never strays more than 1-2 psi no matter what conditions I'm driving in. This is a far cry from my prior air filled tires that would give me the tire alerts regularly when the temperature outside dropped.
+1 all the way for Nitrogen. Oh, I am also a pilot, and all of our tires on our Helicopters are nitrogen filled, much for the same reason as my SUV.

Cheers,
Frank

[Jason Roehl]

Okay, so here's the thing with nitrogen. Air is 78% nitrogen, almost 21% oxygen and about 1% all kinds of other stuff. Just for round numbers, let's say it's 80/20 nitrogen/oxygen. You fill your tires with air to 35 PSI. By the law of partial pressures (Dalton's Law, if you need to look it up), that means that 1/5 of that is oxygen (7 PSI). Assuming the pro-nitrogen-fill guys are correct, then the oxygen will leak out, but the nitrogen won't.

So, in a little while, the tires will be at 28 PSI, all nitrogen. Then then get topped off with more air (80/20 nitrogen/oxygen). Now the tires are at 35 PSI again, but 80% of that 7 PSI addition was nitrogen, or about 5.6 PSI. This means the tires are now 33.6 PSI nitrogen and 1.4 PSI oxygen.

The tires leak down again (only the oxygen, to 33.6 PSI), and get topped off again (to 35 PSI). The nitrogen is then 34.8 PSI...

[Anthony Whitesell]

BINGO! The only other factor is the water vapor content which is highly variable and exaggerated in compressed air systems. Water and water vapor will make the tire pressures change with temperature even more so than the gas. Worse is if you follow the pro-nitrogen guys logic, the water vapor won't leak out because it is an even larger molecule than nitrogen and oxygen.

On the other hand this could be a benefit. Once you have refilled the tires the third time to 35 psi (99.4% N2) plus water vapor, as the tire, air, and water vapor heat while you drive, the air pressure will rise thus decreasing the rolling friction and increasing your gas mileage.