Nitrogen in tyres

[jamie]

W.T.F.
Come on guys, we are riding small engined scooters around!
Let's live in the real world.
All I was saying is that at £1.50 a wheel it is worth a try.
I think some on here are taking life a bit to seriously, even when we were using it in the race car tyres we didn't exam it to this level if it worked great!
Too much other stuff to worry about!!!

+1

[Data]

Ooooh....living in the real world...scary!

[Bluebottle]

I think most peple won't give a toss (and the main advantage I see for using bottled gas is that its dry).

I'm not trying to convince anybody to put anything in their tyres -do what you want..............but can we have the science right please

Sorry Ferrit, these are mostly about what you and your sources said but its meant in the spirit of banter and tech. accuracy, not as a flaming

why did he say oxygen would get out before the nitrogen? That is daft and not supported by any science.

I'm fairly sure that oxygen will permeate a membrane 3 or 4 times faster than nitrogen (or at least it did when I went to school). This is used in practical applications such as producing Nitrox mixtures for scuba diving (heated air on each side of a membrane, pressurising one side causes oxygen to migrate to low pressure side). The science to back it up is Fick's [2nd?] Law of difusion and Henry's law if anybody cares (technically we are into permeation rather than effusion)

You do realise that air is mostly nitrogen already (and smaller than oxygen

I might be reading that wrong but are you saying oxygen molecules are smaller than nitrogen ones?
Oxygen has the greater molecular weight but I'm reasonably certain that nitrogen has the greater kinetic diameter. I think the difference in electron charge might play a part in the difusion too but I can't substantiate that.

The FAA says

My dear chap, the FAA writes things so that their fellow americans can read them, luckily we are british and have the CAA instead

There was also something about oxydation being a load of rubbish because there was already oxygen on the outside of the tyre - yes there is oxygen on the outside of the tyre .........but the pressure is on the inside and that is what forces the gas into the molecular structure of the tyre exposing its innards to direct contact, not only that it prevents external oxygen penetrating the tyre.

As I said, not sure it changes anything but I like my science to be scientific, I'm more into the passage of wind than trapped wind (and funnily enough the FAA/CAA inspect my wind once Ive produced it to check its right)

[Ferrit]

You do realise that air is mostly nitrogen already (and smaller than oxygen

I might be reading that wrong but are you saying oxygen molecules are smaller than nitrogen ones?
Oxygen has the greater molecular weight but I'm reasonably certain that nitrogen has the greater kinetic diameter. I think the difference in electron charge might play a part in the difusion too but I can't substantiate that.

For nitrogen, the covalent radius is 54 pm (Triple bond)
For oxygen, the covalent radius 57 pm (Double bond).
One would expect diatomic nitrogen (with its triple bond) to be smaller than diatomic oxygen (with its double bond). These numbers are similar to bond lengths for diatomic nitrogen and diatomic oxygen of 110 pm (~2 x 54 pm) and 121 pm (~2 x 57 pm), respectively.

If it is true that oxygen is the cause of loss of pressure in tyres, then each time I top up the pressure I am increasing the level of nitrogen and the rate of tyre pressure loss should slow. Yet this is not seen in the real world as tyre pressure loss occurs more from tyre distortion on the rim than from the leak of oxygen.

Another fact is that CO2 and water vapour are more permeable than either Nitrogen and Oxygen. The reality is however that the pressure difference in a tyre to the atmosphere is not high enough for a high rate of loss.

[jamie]

You do realise that air is mostly nitrogen already (and smaller than oxygen

I might be reading that wrong but are you saying oxygen molecules are smaller than nitrogen ones?
Oxygen has the greater molecular weight but I'm reasonably certain that nitrogen has the greater kinetic diameter. I think the difference in electron charge might play a part in the difusion too but I can't substantiate that.

For nitrogen, the covalent radius is 54 pm (Triple bond)
For oxygen, the covalent radius 57 pm (Double bond).
One would expect diatomic nitrogen (with its triple bond) to be smaller than diatomic oxygen (with its double bond). These numbers are similar to bond lengths for diatomic nitrogen and diatomic oxygen of 110 pm (~2 x 54 pm) and 121 pm (~2 x 57 pm), respectively.

If it is true that oxygen is the cause of loss of pressure in tyres, then each time I top up the pressure I am increasing the level of nitrogen and the rate of tyre pressure loss should slow. Yet this is not seen in the real world as tyre pressure loss occurs more from tyre distortion on the rim than from the leak of oxygen.

Another fact is that CO2 and water vapour are more permeable than either Nitrogen and Oxygen. The reality is however that the pressure difference in a tyre to the atmosphere is not high enough for a high rate of loss.

a quarter to three

[victor]

you two carry on you lost a long time ago

[Bluebottle]

For nitrogen, the covalent radius is 54 pm (Triple bond)
For oxygen, the covalent radius 57 pm (Double bond).... tyre pressure loss occurs more from tyre distortion on the rim than from the leak of .....

Absolutely. Totally agree.

But that gives you the length. "Diatomic" means the molecules have 2 atoms and therefore a length and a width and a waist where the two atoms meet. The smallest measurement (the width at the waist) is what restricts squeezing through gaps. This width is the "kinetic diameter" and that is what i said was smaller; it determines which molecule behaves as the smallest molecule

I'm simplifying, explaining and missing out the maths so other people can follow, I know you can cope with it being a bit more technical.
( there is some debate over the exact kinetic diameters but for oxygen it is around 2.9 angstrom and nitrogen around 3.15)

Either way, oxygen will still migrate quicker than nitrogen. There are whole industries that rely on it doing so.
Water vapour and co2 might be even quicker but they aren't in bottled nitrogen so that is a plus for nitrogen rather than a minus

[anonstarter]

Is a picture worth a thousand words?

[Bluebottle]

.....maybe, but not that picture
As Ferrit already pointed out, oxygen molecules are smaller than nitrogen ones so we would both agree that the picture is wrong - it is the oxygen's kinetic diameter that is smaller. That is what I tried to explain above.

edit: oops, that should read oxygen =larger molecule but smaller kinetic diameter, my typo

[anonstarter]

.....maybe, but not that picture
As Ferrit already pointed out, oxygen molecules are smaller than nitrogen ones so we would both agree that the picture is wrong - it is the oxygen's kinetic diameter that is smaller. That is what I tried to explain above.

And well explained too Mister Bluebottle and Mister Ferrit.



Do the Oxygen molecules in the simplistic picture above appear larger than the Nitrogen molecules to you?