Tire pressure deviation

[docj]

Originally Posted by Clay L

That's not what I found when I made a several day test years ago when I thought the ideal gas law calculations would be be fairly accurate. There was a significant difference between measurements and the law.
It is easy enough to test if you have a decent difference between day and night temperatures. You might want to take a few measurements and see if your results match your calculations.

When the air in the tire cools the partial pressure of water vapor will decrease faster than predicted by the gas laws because some of the vapor is condensing out. This will be particularly noticeable if the low temperature you're measuring from is in the range of 30-40F. As the tire warms even slightly the condensed water will once again become a gas and the tire pressure will rise more rapidly than the gas law would predict. I can sometimes see this phenomena on my TPMS readout.

The lack of water vapor is one of the benefits touted by the nitrogen fill people because it eliminates this effect. Another way to minimize the problem would be to fill your tires when the temperatures are very low which will minimize the amount of moisture in the air.

I can assure you that if you account for the effect of water vapor, the ideal gas laws will, in fact, hold.

[jadatis]

And also sunshine on tire can give false effect.
Then tire inside is warmer then outside even when standing still.

But this more water-gas in tire at higher temp, gives lesser deflection so lesser heat-production when driving, wich is never bad for the tire.
Can give a harder ride but higher pressure the tire can stand , and also the valves.

So water in tire gives double advantage, when needed ( hotter inside tire for instance by severe braking) , 1 better cooling down by the water gas then air, and 2 lesser deflection so lesser heatproduction of the tire.

[Ray,IN]

Originally Posted by jadatis

And also sunshine on tire can give false effect.
Then tire inside is warmer then outside even when standing still.

But this more water-gas in tire at higher temp, gives lesser deflection so lesser heat-production when driving, wich is never bad for the tire.
Can give a harder ride but higher pressure the tire can stand , and also the valves.

So water in tire gives double advantage, when needed ( hotter inside tire for instance by severe braking) , 1 better cooling down by the water gas then air, and 2 lesser deflection so lesser heatproduction of the tire.

I see no logic in that. Are you saying to add water to your tires?

[Tony Lee]

The ideal gas law will appear not to hold true anyway because "V" can't be held constant. The volume will increase with pressure and also with temperature because the tyre isn't rigid.

[jadatis]

Once read of Michelin energy savers that they only expanded 8%.
So suppose an average tire expands about 10%.
but what is 8%? is it vollume or hight and width?
I asume volume.
And then in the beginning it most likely expands more then at the end in the higher pressure.

In spreadsheet about Nitrogen filling I made, I make you give expantion in % from zero pressure to AT-pressure ( fi 10%) and a power from zero to 1 to give the expantion a curve .
Then the effect apears to be marginal.

[docj]

Originally Posted by Tony Lee

The ideal gas law will appear not to hold true anyway because "V" can't be held constant. The volume will increase with pressure and also with temperature because the tyre isn't rigid.

That has nothing to do with the validity of the Ideal Gas Law. The law is:

PV=nRT

There is no requirement that V stay constant. Sure, your ability to use the law to calculate pressure change will be made more complicated, but it has nothing to do with the validity of the Law itself.

[Tireman9]

Originally Posted by Clay L

The problem with trying to use the ideal gas law is that the tires are filled with air that contains varying amounts of water depending on where and when it was filled. This increases the PSI variation over temperature a lot.

You are correct re moisture having an affect.
The "Gas Law" applies to all gasses, including Nitrogen, Many times it appears that there will be Magic when they sell you the N2 to inflate your tires.

RE 1 psi or 1.7%. Tire Rack had "1 psi" in the past. Last summer I contacted them and pointed out that that might be reasonable for most of their market i.e. Passenger tires but 2% is easier to calculate and remember. While the math indicates slightly less than 2% it easy to use 2% as this partially compensates for excess moisture. So they changed their web site in Aug I believe.
But Tire Rack revised their web site a few weeks ago and the web master used old information so the old 1 psi showed for a few weeks but I have been told by their Director of Tire Information Services the web site will be corrected shortly back to th 2% figure.

You can make your own air dryer if you want.

Don't get your shorts in a bunch over a few psi. Remember the Load Inflation tables are in 5 psi increments. There is a difference between Measurable and meaningful.

[flaggship1]

Originally Posted by Tireman9

Don't get your shorts in a bunch over a few psi. Remember the Load Inflation tables are in 5 psi increments. There is a difference between Measurable and meaningful.

Probably the wisest words yet.

[jadatis]

Originally Posted by Ray,IN

I see no logic in that. Are you saying to add water to your tires?

In fact I do say that. When going from freesing point of water is zero degrees C/32degrees F to boiling point of water 100degr C/212degr F the humidity water 100% is rising partial pressure of water from 0.00611 bar = 0.09 psi to 1.0134 bar=14.7 psi , so rising of pressure only by the more watergas in tire=14.61 psi, this added to the exact calculated pressure rising of dry gas with my spreadsheet when no expansion assumed ( already stated effect marginal).
Asuming a tire has about 3 moles of gas when pressure 1 bar/14.5 psi, this 14.61 psi means about 3 moles of liquid water needed in tire at 32degr F , so this can all go over to watergas at 212degr F.
this would mean 3 x 18 gr= 54gr of water =0.12 lbs of water ( calculate yourselfes gallons) .
I think this little liquid water wont give problems of disbalance .

With this filling if 212degrF the tire will have about 14.6 psi higher pressure then dry air would have , not depending on the pressure the tire is filled with.
For instance tire filled at 32 degr at 20 psi would give at 212 degr F dry 32.7 psi and with enaug water 32.7 + 14.6= 47.3 psi.
A large percentage higher.
For a 100 psi filled tire at that same 32 degr would give at 212degrF dry 142 psi and wet 142psi + 14.6 psi= 156.6 psi . A good 10% higher pressure then dry.

If there is less water then 0.12 lbs, for instance half so 0.06 lbs in that same sise tire , at 212 degrees F the humidity will be 50% , and the extra rising of pressure because of the water will be "only " 7.3 psi.

This higher pressure will give lesser deflection so lesser heatproduction of the tire at same speed. this is positive for the tire.

That 212 degr F it will only incidentially get by for instance the heat of the brakes , transported trough the rimms to the gas in the tire. Then the tire cant transport that much heat to the outside because the temperature differences are smaller between critical tire spots and inside gas.
But because then the tire produces lesser heat by the lesser deflection, those critical sport most likely still wont get to hot , that they damage.

But what I wanted to state is that the eventual water in the tire will sooner have a positive then a negative effect for the tire.

Corosion is for the todays rimm not a problemm, tire-fitters will comfirm that if they take a tire of the rimm , the part of rimm inside the tire is not coroded.

Only disbalance of the liquid water could be a problemm, as someone already reacted, but then howmuch is allowed ?

[Cooperhawk]

I suspect there is a school of thought here that spends vast amounts of time thinking about and monitoring tire pressures. It's a good hobby I guess. As someone who has driven millions of mile my advice is, KISS!

Keep It Simple Stupid!

Tire pressures vary with temps and tires warm up when being used. Set your pressures, get a TPMS and monitor them, relax.

[docj]

Originally Posted by jadatis

In fact I do say that. When going from freesing point of water is zero degrees C/32degrees F to boiling point of water 100degr C/212degr F the humidity water 100% is rising partial pressure of water from 0.00611 bar = 0.09 psi to 1.0134 bar=14.7 psi , so rising of pressure only by the more watergas in tire=14.61 psi, this added to the exact calculated pressure rising of dry gas with my spreadsheet when no expansion assumed ( already stated effect marginal).
Asuming a tire has about 3 moles of gas when pressure 1 bar/14.5 psi, this 14.61 psi means about 3 moles of liquid water needed in tire at 32degr F , so this can all go over to watergas at 212degr F.
this would mean 3 x 18 gr= 54gr of water =0.12 lbs of water ( calculate yourselfes gallons) .
I think this little liquid water wont give problems of disbalance .

With this filling if 212degrF the tire will have about 14.6 psi higher pressure then dry air would have , not depending on the pressure the tire is filled with.
For instance tire filled at 32 degr at 20 psi would give at 212 degr F dry 32.7 psi and with enaug water 32.7 + 14.6= 47.3 psi.
A large percentage higher.
For a 100 psi filled tire at that same 32 degr would give at 212degrF dry 142 psi and wet 142psi + 14.6 psi= 156.6 psi . A good 10% higher pressure then dry.

If there is less water then 0.12 lbs, for instance half so 0.06 lbs in that same sise tire , at 212 degrees F the humidity will be 50% , and the extra rising of pressure because of the water will be "only " 7.3 psi.

This higher pressure will give lesser deflection so lesser heatproduction of the tire at same speed. this is positive for the tire.

That 212 degr F it will only incidentially get by for instance the heat of the brakes , transported trough the rimms to the gas in the tire. Then the tire cant transport that much heat to the outside because the temperature differences are smaller between critical tire spots and inside gas.
But because then the tire produces lesser heat by the lesser deflection, those critical sport most likely still wont get to hot , that they damage.

I agree with your logic, but I don't know why you are assuming that the temperature of the air/water vapor mixture in the tires is anywhere near 100C (212F). Yes, the temperature of the gas in the tires does increase when they are in motion but it shouldn't get anywhere near this hot in normal usge.

[flaggship1]

Originally Posted by Cooperhawk

I suspect there is a school of thought here that spends vast amounts of time thinking about and monitoring tire pressures. It's a good hobby I guess. As someone who has driven millions of mile my advice is, KISS!

Keep It Simple Stupid!

Tire pressures vary with temps and tires warm up when being used. Set your pressures, get a TPMS and monitor them, relax.

and of course another cup of coffee while the sun warms one side - then turn the coach around and have another cup. Viola - within 2 lbs... Temps are rising as summer nears - watch the 507s.

I carry the equipment - but won't be a

[jadatis]

@ docj
Your reaction was that soon , that you did not read my edit about that 212 degr F.
It was written to me by a man from a valve-importer in Holland , when I had contact with him for information about pressure of valves .
When descending from the mountains , thoug its adviced to use the brakes as less as possible, they are used and this heat is transported trough the rimms, to the tire inside gas, and can only incidentially get that 212 degr F = boiling point of water at 1kg/cm2= 1013mb= 1.013 bar= 14.7 psi.

but the tire and valves must be able to stand this pressure rising, thoug in normal use it maybe even never happens for most.

Normal warm inside tire temp when driving about 100km/62m/h is when outside temp 18 degrC/65 degrF, about 45 degr C/ 112 degr F.
For that the partial pressure of water gas for 100% humidity is 0.096 bar/1.4 psi.

[aauummm]

Originally Posted by Gary RVRoamer

I gotta agree with Tony Lee. There seems to be a lot of hysteria over such a simple thing.

The RV manufacturers placard gives a recommended pressure for those who haven't weighed the coach before using (the vast majority). Since the RV maker doesn't know your actual loaded weight, the recommendation has to assume max load or something very close to it. They (or their lawyers) all learned from the Ford Explorer tire fiasco that low-balling tire pressures is a no-no and borders on financial suicide.

But if you make the effort to weigh the rig, you can better tune the tire pressures for a more pleasant ride while still remaining safe. It's not something you must do, but you will likely get a benefit (softer ride) if you do. Of course, if the scale shows you are near max load anyway, nothing is going to change much.

The other advantage of a weigh-in is that it detects an overloaded tire or axle. RV Safety Foundation (rvsafety.com) has found evidence that a large percentage of RVs are overweight in at least one tire position. Yours could be one of them.

Wheel Position Weighing

Well said.