twohoos

Actually we can estimate the internal air temp using PV=nRT. The cold pressure is set at ambient temp (nominally 20degC = 293K), and is about 30+14.7=45psi absolute. Say the hot pressure is 10 psi higher, or 55psi absolute. That's a 22% pressure increase, which means a 22% temperature increase, to 358K, which is 85C. That's well below the boiling point of water.

marrenr813

I work at costco and i use this stuff everyday. Guess what, it works. With regular air, the moisture in the lines and compressor can thow off air pressure by an avg of 2 psi. Furthermore, pressure does not need to be checked as often because of the larger molecules. Also, the miosture can cause damage to the finish of wheels and cause corrosion around the bead.

Oh, and at the end of thenight I have to drain the air tanks and you would not believe the amount of water that comes out.

raise your hand if you believe that water expands when heated.

marrenr813

pretty much sums it up

macr88

Cool
Thank you..
So what's expanding with heat? The gasses in air.

What race cars actually use nitrogen?
I can see how it might be better for dragsters since they only run about 5 psi.

mikegarrison

It's not quite that simple. There's a fair amount of water in the air I am breathing right now, and I assure you it is not at 100C.

Water is absorbed into the air depending on a number of factors, but as long as it is at <100% relative humidity the air can still absorb more water. It does not have to reach 100C.

In a closed container like a tire, the water will continue to evaporate until it reaches saturation. Since saturation of hot air is significantly higher than cold air, as the air in the tire heats up, more and more liquid (if there is any at all) will evaporate -- even if the temperature is below 100C.

twohoos

I think we're saying the same thing? The water is vaporized, yes, but it is NOT gaseous (which implies steam, which implies >100C temp).

My argument was that since the vapor is not gaseous, it doesn't obey PVnRT, which makes it harder to predict pressure changes. Then someone suggested the water vapor might turn to steam when the tire gets hot, but my back-of-the-envelope calculation says that won't happen.

twohoos

A side thought:

When my tire guys mount a tire on a rim, they first swab the bead of the tire with soapy water to help it pop over the edge of the rim. That simple action probably introduces as much or more water inside the tire than any water in the air used to inflate the tire. Or, put another way, even if they used N2 to inflate the tire, there'd still be a bunch of water in there!

So perhaps the way to get the inside as "dry" as possible is to heat the tire (vaporizing as much water as possible), then completely deflate, then reinflate with N2, and repeat...

macr88

It think I covered this on the botom of the first page. But was thinking maybe the water
doesn't matter because it's not yet a vapor but didn't think about air carrying some
moisture in the form of vapor wich will still expand as the temperature rises..

mikegarrison

No, we are not saying the same thing. Vaporized and gaseous are synonyms.

Water is always evaporating. After it rains, there are puddles on the concrete. Where do they go? They don't boil. It never gets above 100C. But if the vapor pressure at the surface of the water is higher than the partial pressure of the water in the atmosphere, the water will slowly evaporate off the ground.

There's not really any difference between the water that evaporates that way or the water that evaporates when you boil water -- it's the same water vapor either way. It's just hotter.

In a closed container, water will evaporate until the humidity inside the container reaches saturation (100% relative humidity). Now if you heat up that air, it can hold more water. So more water will evaporate. The water does not have to boil.

Where boiling differs from evaporation is that the vapor pressure gets so high that the water vaporizes in bubbles inside the water itself, as opposed to just at the surface of the water.

Anyway, for this topic what is important is that vaporized water does obey the gas law, but liquid water does not. So the liquid water does not add anything to the air pressure inside the tire, but when it vaporizes then suddenly it does. In the pv=nrt equation, you suddenly just added to "n", and so of course "p" gets bigger.

mikegarrison

In one of Carrol Smith's books he says his race teams always used to mount the tires (ideally with dry nitrogen, but sometimes that was out of their control), then use a vacuum to suck as much of the gas out of the tires as possible, then reinflate them with dry nitrogen.