Today I had to pay a scary amount of money for new car tyres. And, in yet more evidence that chemistry permeates everywhere, I found this amazing sign in the garage.
I studied it at some length. The diagram on the right particularly fascinated me. They’ve helpfully included a key, which seems to suggest that the peculiarly square-shaped ‘tyre’, labelled as being filled with compressed air, only contains particles of oxygen, water (and water vapour, because ‘water’ isn’t a broad enough label apparently). The other, nitrogen-filled, one appears to contain oxygen, water (and water vapour) and nitrogen. As well as some mysterious green and red circling arrows.
I can’t quite get my head around it. Someone drew this, and sent it to printers, and presumably it’s been displayed in more than one reception area (I’m deliberately not naming the specific garage, since the staff there were nice and helpful and gave me a good price really, and I’m sure they had nothing whatsoever to do with the sign beyond being told by Head Office to put it on the wall).
Did no one think to check it with, well, anyone? It’s almost as bonkers wrong as the American school sign advertising ‘leteracy night’.
Ok, so I’m not a car mechanic. My experience in that area is limited to occasionally topping up my own screenwash and once watching my Dad change some spark plugs. But I’m pretty sure that compressed air is, well, compressed air. As the sign itself makes clear, air is about 79% nitrogen and 21% oxygen (the numbers vary, but that bit, at least, is more or less right). Therefore, first problem, if you fill a tyre with compressed air you are by definition filling it with nearly 80% nitrogen.
In fact, I don’t think I’d want to drive a car with tyres which had been filled with pure oxygen and a bit of water, as the key suggests. Oxygen is a jolly effective oxidising agent. Tyres may not be the most flammable things in the world, but I reckon there’s a significant chance that your hot wheels would become a little more literal than you might like.
Moving on, in the round-ish tyre diagram there appears to be water, oxygen and nitrogen. Call me naive, but if you tell me you’re filling my tyres with nitrogen I’m going to assume it’s pure nitrogen. Whereas what you have there is (I’m so sad I counted and worked it out) 40% nitrogen, 27% water 33% oxygen. I dunno what that particular mixture is, but it’s not air and it’s definitely not pure nitrogen.
And then there are the captions underneath: “Undesirable components of the air are removed when the tyre is filled with nitrogen”. Well not according to that diagram, because there’s still water and oxygen in it…
And, “nitrogen is the only inflation medium developed solely for the use of pneumatic tyre inflation”. What about compressed air then? Doesn’t that count as an ‘inflation medium’?
And, “maintains the correct tyre pressure for longer”. Well, actually I’m not sure about this one (there’s an interesting article here expressing some skepticism though). The Formula One website says that they do indeed fill racing tyres with “a special nitrogen-rich air mixture, designed to minimise variations in tyre pressure with temperature. The mixture also retains the pressure longer than normal air would.” The internet tells me Red Bull alone invested over $100 million in 2012, a significant chunk of which would necessarily have to be research and development. If anyone knows about the best stuff to fill tyres with, it’s Formula One.
But, there is a bit of a difference between cars that are designed to routinely get up to 200 mph and tyres that are designed to cope with temperatures comfortably over 100 oC, and your household runaround that averages 35-ish miles per hour. Let’s say I’m not convinced we’re comparing apples with apples here.
I also don’t understand those blue arrows coming out of the squarish tyre (why IS it that shape anyway?) They seem to suggest that water is escaping. But if so, why is it escaping from one tyre and not the other? And how permeable is rubber to water anyway? (Answer: not very, otherwise the welly boot would have been a bit pointless really.)
The one thing I do accept is that water might, possibly, have a small effect on tyre pressure. Water has an annoying habit of changing state at everyday temperatures. Just 1 millilitre of liquid water occupies over 1300 millilitres when it turns into a gas at atmospheric pressure and 25 oC. If it’s hotter (which it probably would be, if it had suddenly turned into gaseous water) it occupies even more space. Of course it’s more complicated than this because tyres aren’t at atmospheric pressure, but the point stands: if there’s water in your tyres the pressure would fluctuate a bit as they warm up. This’ll happen anyway, since in general gases expand as they warm up, but water could make the difference more significant.
But I checked, when compressed air is produced they take out most of the water. Most of it condenses when the air is compressed, and the condensate is simply removed. Then they use an air dryer and a filter as well. So I dispute the idea that there’s a lot of water in standard compressed air in the first place. (It has since been pointed out – see comments – that a lot of garages have their own air compressors, and that although they’re supposed to dry the air they may not do it very effectively, so there could be a fair bit of water in there, although there shouldn’t be…)
Anyway, my musings over gases were interrupted by having to pay the terribly big bill. It did seem like a lot for about 50 kg of rubber, but I’m assured that tyres with the correct depth of ridgy bits are quite important. They told me they filled them up with nitrogen for free.
Although the diagram is nonsense there are some sound reasons for N2 vs compressed air. N2 has a smaller molecular size than O2 so permeates through rubber more slower so tires should stay inflated longer and the rubber less prone to oxidative degradation. Also! industrially produced compressed air that comes delivered in a cylinder with be near anhydrous but the compressors I’ve seen at garages (or labs) are not concerned with being anhydrous so will contain water which can have a profound effect, particularly if it gets below freezing.
I take your second point, I have no idea if garages are ‘making’ compressed air on site or not, or how it works, so that may well be true.
Your first though, not so much. The atomic radius of oxygen is 60 pm and O2 has a covalent bond length of about 120 pm, and nitrogen (N2) is 65 pm with a bond length of about 110 pm. In other words, they’re almost exactly the same size. There may be something in oxidative degradation. However air is still only 20% oxygen, and 80% nitrogen. I have yet to get to the bottom of whether the ‘nitrogen’ being used to fill tyres is actually pure nitrogen.
Permeation rates (one measure of molecular size) through polymers are generally greater for oxygen see http://www.researchgate.net/publication/42088497_Permeation_of_atmospheric_gases_through_polymer_O-rings_used_in_flasks_for_air_sampling/file/79e41506f2e3a54c9c.pdf
I’m sure that’s true but it’s (I imagine) more to do with the electronic structure of oxygen than its size.
Yeah…. an air compressor that’s working correctly and with a functional dryer will produce dry air. Your average gas station compressor, not necessarily. A mechanic’s shop air compressor, maybe more likely, depends on whether or not they have a dryer or just a condensate drain, and how well they keep up on maintenance of the dryer.
I worked at a gas station when I was in high school and that air line regularly froze in the winter. It sure wasn’t the air that solidified…
Compressed nitrogen from a cylinder is guaranteed dry.
I am also baffled by that sign. I suspect nobody who knows what molecules are was asked for input.
We-ell, if there was water vapour in the air it sort of WAS the air that solidified 😉 But yes, I take your point. We have no way of knowing how much water is in garage compressed air.
I enjoyed that.
I heard second hand that a transport company was getting 10% more distance from tyres filled with Nitrogen rather than air. I was told that they run cooler have no idea why that would be or how they would measure it
Interesting. If it’s true I’d be curious to know why myself. Oxygen is a reactive gas, and truck tyres are massive, so maybe there’s something going on.