SWCDuke

What's your point?

Water vapor is a gas. So is superheated steam - two different names for H2O in the gas state, and both exhibit ideal gas behavior.

Vapor pressure of a liquid has nothing to do with ideal gas behavior. All liquids have a vapor pressure uniquely associated with temperature, and it tells you the saturation pressure of water vapor in air at a given temperature, but so what. All you need to know is the dew point of the air in your tires.

If it's lower than the lowest anticipated temperature you expect to see, then there will be no condensation and no loss of air pressure due to condensation, just a pressure variance based on temperature, which can be computed from Boyle's Law.

Duke

rgs

The point is that the temperature range we expect auto tires to experience approaches the transition of water from liquid to gas. Once it becomes gas, it will then behave per the Idea Gas Law, but in a tire under pressure, it remains a liquid. A liquid in suspension, or commonly referred to as vapor, but still a liquid, so it does not expand linearly as the tables clearly show.

rgs

The point is that the temperature range we expect auto tires to experience approaches the transition of water from liquid to gas. Once it becomes gas, it will then behave per the Idea Gas Law, but in a tire under pressure, it remains a liquid. A liquid in suspension, or commonly referred to as vapor, but still a liquid, so it does not expand linearly as the tables clearly show.

John McGraw

Well, I fill my tires from my dry air in the shop, so the moisture advantage of N2 is not an issue in my case. I came out of the aircraft industry where we used N2 exclusively, but never have on my cars. I figure that the -40 dewpoint air that I use in my tires ought to be dry enough!

Regards, John McGraw

Dave McDufford

I thought racing teams used nitrogen rather than compressed air because in a pit fire a damaged compressed air hose can become a blow torch. The advantages of nitrogen over dry compressed air is minimal but makes for good marketing.

Dave

SWCDuke

What??? "Water vapor" is a gas. There is no condensation unless the air temperature drops below the dew point. At the dew point air is "saturated" with water vapor and liquid droplets will form.

Duke

rgs

This is where we are disagreeing. My experience is that vapor is not an Ideal Gas. It only becomes an Ideal Gas when it changes state. In the case of water, at 1 atmosphere this occurs around 212F. More atmospheres, higher temp. In a road race tire, we reach temperatures near 200F, well below the point where water becomes gas (or steam as we call it). Once it is steam, then I will agree that it reacts per the Ideal Gas Law, but as a vapor, as the Handbook of Chemistry and Physics tables show, it has not reached the gasseous state and it's V/T relationship is not linear.
All liquids demonstrate a vapor and resulting vapor pressure. Not till they reach a high enough temperature, do they change state to gas. I use liquid Oxygen in the production of Ozone for the treastment of drinking water. We go through the steps of expanding and raising the temperature of the vapor to convert it to a gas. This is precisely controlled so the Oxygen reaching the generator is completely in the gasseous state and no vapor or condensation occurs (shorts the generator cells).

SWCDuke

Suggest you read Pauling's "College Chemistry" or other introductory chemistry text.

My copy of Pauling is 42 years old, but the basics haven't changed.

Duke

AZDoug

Water vapor at any temperature is, and acts like a gas.

Water boiling, in this example 212 F at 14.7 PSIA, is merely the the point where pressure equilibrium is reached and the liquid state wants to turn gaseous throughout the mass of liquid, and isn't just surface evaporation anymore. In a total vacuum, this temp is 32 F where water ice sublimes to water vapor, bypassign the liquid stage. In a tire at 32 PSIG, water boiling would occur at at about 255 F.

Doug

rgs

I believe the problem people are missing is "gas" versus "Ideal Gas". They are different things. Vapor is a gas, I agree. It displays some properties of a gas. As I have said before, it is not an Ideal Gas corresponding to the Ideal Gas Law. Only after it has transitioned to the gasseous state, will it.

Duke, lets keep our debate on a professional level. If you have evidence that water vapor, (not water gas, steam) behaves per the Ideal Gas Law, by all means present it. I have presented evidence, by the link to the table, that it does not. Also, my experience dealing with liquid Oxygen confirms my ascertion to me, but as with any personal experience, others are open to challenge it. If you have credible references you can pinpoint, as I have, please present it. I don't insult your intelligence, please don't insult mine.

rgs

I believe you are thinking of damaged oxy/acetelene hoses. Compressed air by itself is not a fire hazard any more than nitrogen. If anything, a typical nitrogen bottle is under many times more pressure than typical compressed air system and poses a greater hazard.

rgs

It acts like a gas because the water is suspended in gas. Water still remains in the liquid state. This is called humidity.

panchop

Well most of this discussion is above my head. I used to use nitrogen in my drag bikes tires but only because I had a tank of it around to charge the airshifter. Seemed to be the real reason most of the guys used it if I recall. On the other hand bracket racers where always looking for an edge and the thought that it would heat up less and give you some tiny little advantage was bandied about. I wouldn't worry about it or pay much attention to it in a street car.

65nassau

AHHHHHHHHHH ... we haven't had one of these in a while ...

As my outspoken sister-in-law commented when both of my brother's in law were arguing about something.

Allright guys, just whip 'em out and measure and get it over with....

JUST KIDDING!

Dave McDufford

No, I am thinking of compressed air. If you have a fire, a damaged air hose will add oxygen and intensify the fire. Nitrogen will not. I cannot imagine that a welding outfit would be allowed any where near a pit.

I do not have specific knowledge but I remember Clem talking about the use of nitrogen rather than compressed air in the pits and why.

Dave

ctjackster

I am so pleased to see that, after all these years (26) since my AP Chem class in my junior year of High School, the one formula I still remember (PV = nRT) is relevant in a discussion about the car I have always loved. There is now balance and closure in my world; I can die in peace . . . .

as for actually getting "hot" about anything having to do with this stuff, well ya lost me there, I'll fight with you all about KOs and restamping and sidepipes, but even I stop at heated debates about the gaseous state of H20 . . . . .

SWCDuke

Your vapor pressure table has nothing to do with this discussion.

Consider the following example - a bright sunny day, 75 F with a dew point of 50F, so the H2O vapor pressure is a fraction of a mm of Hg.

Is this H2O in the air in the liquid or gas state?

Now, the sun sets and the temperature drops while the partial pressure of H2O remains the same. How does the relative humidity change as the temperature drops? The temperature continues to drop to 45F just before sunrise. What events happen during this process?

I have given creditable reference - Pauling. You need to review a basic chemsitry text because you don't understand the difference between a gas (which is the same as "vapor") and a liquid.

Duke

rgs

This is where you're missing the boat. Prior to going to the gasseous state, all we have is vapor.

Liquid State

Water condenses going from vapor to liquid. It never made it to the gasseous state. It was only suspended in the gasses of the air.

Lets see what we do agree on.

Given 1 atmosphere of pressure, water below 0C (32F) is in the solid state. Agree/disagree

Given 1 atmosphere of pressure, water above 100C (212F) is in the gasseous state. agree/disagree

Given 1 atmosphere of pressure, water between 0C (32F) and 100C (212F) is in the liquid state. agree/disagree

Given 1 atmosphere of pressure, and the example of fog, water can exist in air in the liquid state. agree/disagree

Increase the pressure of a vessel and you raise the point of transition to the gasseous state. agree/disagree

water in a tire, under pressure will transition to the gasseous state at a temperature greater than 100c (212F). agree/disagree

Based upon the above, water in a tire, in the typical operating temperature of driving will remain in the liquid state. agree/disagree

From an online reference:
Vapor pressure
From Wikipedia, the free encyclopedia
Vapor pressure is the pressure of a vapor in equilibrium with its non-vapor phases. Most often the term is used to describe a liquid's tendency to evaporate. It is the tendency of molecules and atoms to escape from a liquid or a solid. At any given temperature, for a particular substance, there is a pressure at which the vapor of that substance is in equilibrium with its liquid or solid forms. This is the equilibrium vapor pressure or saturation vapor pressure of that substance at that temperature. The term vapor pressure is often understood to mean the saturation vapor pressure. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The higher the vapor pressure of a material at a given temperature, the lower the boiling point.

Back to the original question concerning using nitrogen.
A tire with only an ideal gas will have no vapor pressure in it since it only exists when liquid or solid is present. It's pressure is solely the result of the closeness of the moving gas atoms. A tire with water suspended in a mixture of gasses (air) will have part of it's pressure based upon the vapor pressure of the liquid water, even if it is in suspension, per the table I previously posted. As the table shows, this vapor pressure is not linear and does not react as an Ideal Gas.


Come on Duke, quit the mud slinging. I refuse to stoop to this level. Give me something credible instead. I have.