Nitrogen filled tires - still need to check pressure the normal way? Or can check hot or cold ok?

Maintenance/Repairs
1

With normal compressed air, you’re supposed to check tires cold (and hopefully fill them while cold too?). I’ve always done 2 checks when I need to refill - check once at home/work when ~cold, then when at fill station check again before filling to account for any rise in temperature and take that into account for how much needed to fill up.

Now, with the dry/dryer nitrogen in tire, do you still need to follow that rule as much on checking tires cold or can they be checked warm/hot more ok since they supposedly not having much humidity/water and hence temperature difference as you drive it.

It sure would be nice to be able to just check anytime and not have to bother with “is tire hot or cold” and adjust for rise/fall in temperature difference.

And it sure isn’t fun checking cold tire pressure in early morning when it can be cold.

2

Air is over 70% nitrogen to start with, so checking nitrogen filled tires is no different form what you did before. The reason to check them cold is that air expands when it warms up, and 32 psi when it’s 100 degrees outside will result in a much lower (and maybe unsafe) pressure on a cold morning.

It has nothing to do with the humidity in the air!

3

Pure nitrogen is a stable gas where the pressure doesn’t change much with temperature changes. This is why nitrogen is used in most racing venues.

Regular compressed air not only contains nitrogen, but it also contains other gasses plus moisture. And that’s what effects the tire pressures at different temperatures.

Tester

4

“Pure nitrogen is a stable gas where the pressure doesn’t change much with temperature changes. This is why nitrogen is used in most racing venues.”

This is incorrect. Both nitrogen and oxygen behave identically with changes in temperature. Both expand and contract proportional with changes in temperature, absolutely no difference.

The reasons nitrogen is used in race car tires is to 1. eliminate any possible influence of water vapor; 2. allow precise (0.1 psi) adjustment consistently; and 3. as a safety factor, eliminating the presence of pressurized air in case of a fire.

Treat nitrogen filled tires the same as you would air filled tires. It is a waste of money for regular cars.

This has been discussed many times here, such as:
http://community.cartalk.com/discussion/2281105/nitrogen-filled-tires-pros-and-cons/p1

5

The main purpose of nitrogen fills is to increase the bottom line of the facility providing the nitrogen service.

6

Here’s another discussion by a tire expert:
http://www.barrystiretech.com/nitrogeninflation.html

7

I suggest you check the balance in your checking account instead. Nitrogen, being next to useless in a passenger car, just isn’t worth the expense. Lesson learned. As soon as you do have to add any atmospheric air, you might as well be back to square one. So your only recourse is to have the rip off facility who sold you the nitrogen, check the air for you…for added $$$$$.

8

Tester

9

I’m going to disagree with the PM article. At best, temperature related pressure changes across the the four tires would more consistent since the effect of water vapor would be removed. The effect of temperature on nitrogen or a nitrogen/oxygen mixture pressures would be the same.

Ed B.

10

I think putting nitrogen in tires is done for people who do a lot of night driving.

11

The last time I thought that Tester was wrong, it turned out to be me who was in error, but this time we are in my area of expertise, so I am willing to challenge with some degree of boldness.

That popular mechanics article contains a couple of erroneous, or at least poorly phrased, statements. As Texases stated, both nitrogen and oxygen exhibit ideal gas behavior. They both expand in direct proportion to the change in absolute temperature.

Absolute temperature, by the way, is measured in Kelvins or degrees Rankine. When I say that pressure rises in direction proportion to absolute temperature, that does not mean that heating a tire from 40F to 80F will double its pressure. 40F is 500R absolute, and 80F is 540R absolute.

The temperature increase from 40F to 80F is therefore 540/500 or an 8% increase in absolute temperature, which will result in an 8% increase in absolute pressure. To add yet another twist to this story, the pressure you read on your tire gauge is not the absolute pressure, it is the gauge pressure, or the difference between the measured value and the local atmospheric pressure.

EXAMPLE: At 32 psig gauge pressure, the absolute pressure is 32psig + 1 atmosphere, (~14.7 psia) or 46.7 psia. An 8% increase will result in a pressure of (1.08 * 46.7) = 50.4 psia absolute. Subtracting 14.7 psia for the atmosphere gives us a gauge pressure of 35.7 psig at 80F.
Folks who check their pressure at various temperatures can confirm that this is about the pressure change you would expect when heating from 40F to 80F. This relationship will hold whether the tire is filled with nitrogen, air, oxygen, argon, CO2, or any other gas that behalves as an ideal gas at standard conditions.

The problem with air, as noted previously, is that it tends to be wet. By compressing the air, the shop compressor drives up the dew point and dries the air pretty well, as anyone who has ever had the chore of bleeding the liquid water out of the compressor tank every night can attest. Sometimes, however, water condenses in the hose rather than the tank, and that water will go into your tire.

If you have liquid water in your tire, the pressure becomes much more unstable. Any change in temperature will change the liquid/vapor balance of the water in the tire. Water vapor takes up a LOT more space than liquid water. If even a tiny droplet of liquid water vaporizes, it takes up a lot of space in the tire, increasing the pressure in the tire, disproportionately to the change in absolute temperature. In a race car that is absolutely unacceptable. In your car it is not good.

Also, in cold weather, I would expect that ice inside the tire could cause tire imbalance.

12

@Manolito–The tire retailers’ lobby in my state convinced our state legislature to repeal both Charles’ law and Boyle’s law on the relationship between pressure and volume and temperature and volume of a gas when applied to nitrogen. Therefore, in my state, with a constant volume of nitrogen gas in a tire, a change in temperature does not affect the tire pressure. However, if one takes the car out of the state and into a state where the gas laws have not been repealed, the pressure in the tire will go up as the temperature increases.

13

@Manilito
What are the erroneous statements in the article ? Does not nitrogen contain less moisture and is not nitrogen less apt to migrate through the tire making the readings more stabile ? My problem is, big deal. The pressures will still have to be check and there will be some variation because of temperature and adding atmospheric air immediately compromises the whole exercises. Besides, it’s a small percentage(30) that contributes to the problem with atmospheric air.

But, the article is essentially correct. It just doesn’t point out that it is illogical financially and just not worth the effort for most when their extra funds is better spent on a gauge and air pump.

14

When a sales pitch starts out with a lie, one that is easily proved, the value of the product immediately goes to zero with me. There are a few small advantages for nitrogen, but those aren’t what’s pushed by the salesmen…

15

"there are a few small advantages…"
I agree, but there is not much lying going on. It’s because the average driver doesn’t run Indy cars on a track, not because the advantages do not exist. To me, it’s capitalism. It pales compared the food industry partitioning and getting past a law that says genetically manipulated food doesn’t have to be labeled and banks lieing about home mortgage qualifications. That deserves contempt. This stuff us laughable, like selling warranties and service packages that over charge. Enough truth to rob you,legally if you are too unaware to count or use your H.S chemistry ? Sure, and it doesn’t bother me in the least. I support capitalism…and using nitrogen; by others. As long as tire companies can make enough extra doe off the gullible, maybe we can get more tire sales.

16

Keep four words in the back of your mind.

Tire Pressure Monitoring System.

Tester

17

The web sites I’ve seen claim the ‘tires wear longer’, the ‘pressure doesn’t change like with air’, and that ‘the ride is smoother’. Those are lies. The benefits only come from the elimination of water vapor, a limited benefit in my opinion.

18

If nitrogen has the advantage that it won’t migrate through rubber as quickly as oxygen, it would seem advantageous to fill footballs and basketballs used in collegiate and professional games with nitrogen as opposed to air which contains 21% oxygen.

19

The bank account will be inversely proportional to the amount of nitrogen being used… :slight_smile:

I’m in agreement with texases about those claims being lies; and bald-faced ones at that. Even the one about nitrogen lowering your carbon footprint is total hokum.

Many drivers are oblivious to tire pressures on their cars and don’t care. I can’t see that promotion of nitrogen as being stable, leak free, and pressure worries free is going to do anything other than cause drivers to ignore their tires even more.

Unless the tire store suggests they drop in once a month for a check and refill; which could be thought of as job security…

20

I agree with @Tester.

I always understood that the unstated reason for getting water and oxygen OUT of the tire was to maximize the longevity of the TPMS system…without stating this directly, because it doesn’t pay to tell potential customers how troublesome and failure-prone their car’s sub-systems are…

(Or maybe it’s just a big coincidence N2 and TPMS hit the street simultaneously?)