>>>> As for Nitrogen in the tyres. More stable than air? BULLSHIT!!
Are you sitting comfortably? This is going to be way long and the
delay in answering this has allowed some to get in a real mess.
There is no such thing as an Expansion Coefficient for a gas in a
sealed closed space, it fills that space fully. Volume in a tyre is
pretty much constant so NO change in volume and NO expansion. OK the
tread grows at speed BUT the sidewalls get closer together, volume
stays near constant. Also after initial stretch on first
pressurisation it doesn't change size enough to bother with. (You do
check pressure next day after having new tyres fitted don't you? They
will top up the N2 for free won't they?)
OK it's not Boyle's law but Charles' law we need. Bottom panel on this
page.
http://hyperphysics.phy-astr.gsu.edu/hbase/Kinetic/idegas.html
P1 x T1 = P2 x T2
Both Nitrogen and Oxygen behave as Ideal gases and follow this law
exactly. In any mix they still follow this law exactly. The atmosphere
with 78%%N2, 21%%O2 and 1%% water vapour + trace other stuff is also an
Ideal gas and obeys the gas law exactly until it's enclosed - see
"water vapour".
Inflate a tyre with any mix of N2, O2 or DRY air at say 15°C to 32psi
and it will always have the same pressure with temperature.
temp °C psi gain/loss
9 31.03 -0.97
14 31.84 -0.16
15 32.00 0.00
16 32.16 0.16
30 34.43 2.43
40 36.05 4.05
60 39.29 7.29
70 40.91 8.91
80 42.53 10.53 (typical running temp)
90 44.16 12.16
100 45.78 13.78 (F1 OK, your car = way too hot)
You filled it during the day and by night time a temp drop of 6°C to
9°C means it "lost" nearly 1psi. (must be that leaky O2 getting out!)
Then there is change in atmospheric pressure from day to day.
http://resource.npl.co.uk/cgi-bin/baromhist.pl?action=dobasepage&startday=1&startmonth...
1042mb 15.11psi
960mb 13.92psi
Over 1psi variation and just over 3 days (10/7/08-14/7/08) during this
week it's changed by 0.3psi. (this link will be stale very quick)
http://resource.npl.co.uk/pressure/pressure.html
14.85psi
14.55psi
As tyre pressures are measured using gauge pressure that 0.3psi
variation has caused a 0.3psi variation in everyone's tyre pressures.
Basically the weather changes the tyre pressure faster that any
detectable loss due to O2 diffusion. You can also see that if you
haven't given a tyre long enough to cool down after use just 1°C rise
in temp will give a pressure reading error of 0.16psi. Likewise if
next day is just that little bit warmer or cooler you won't get the
same pressure.
Tyres are also highly sensitive to filling/measuring temperature.
Inflating to 32psi at different ambient temperatures gives different
running pressures (Dry air / N2 whatever).
temp °C at 60°C at 80°
0 42.26 45.7
5 41.23 44.6
10 40.25 43.5
15 39.29 42.5
20 38.37 41.5
What chance do you have of being able to adjust cold pressure to
always get the same hot running pressure? Every time you check/adjust
tyre pressure you have to measure temp and barometric pressure and
know how to compensate for them. Have the tyre makers/fitter even told
you this? It's way more significant than any O2 loss that takes months
to detect.
Only way to detect O2 loss in the face of daily temperature and
pressure variations every time an anti-cyclone tracks across the UK is
to keep your wheels/tyres in a nice sealed temperature controlled
cabinet and NEVER USE THEM. 2psi/month but most will seep though the
rim/bead.
http://www.safermotoring.co.uk/CheckingTyrePressure.html
++++++++++++++++++++++++
Water Vapour (this is Engineering/Physics grad level work)
Ok, there was that bit "The atmosphere with 78%%N2, 21%%O2 and 1%% water
vapour + trace other stuff is also an Ideal gas and obeys the gas law
exactly until it's enclosed." Well it's enclosed once it's pumped in
to a tyre.
Hands up, who thinks that when water changes to vapour it expands
massively to 1673 times it's volume? You may have paid some attention
to GCSE physics but haven't completed your education.
http://www.simetric.co.uk/si_steam.htm
It's only true for water at 100°C expanding into open air.
Pressure 1.013bar (abs), temp 100.00 °C, specific volume 1.673
m³/Kg. 1L of water becomes 1673L of vapour.
In a tyre it's at 2.2bar and nowhere near the boiling point of
135.88°C, if you boiled 1L water at 2.2bar (gauge) it would only
expand to 568L. (about 1/3.2 the volume as it's at about 3.2x the
pressure)
Below boiling point it's a complex mixture as air can hold water
vapour but the water tends to condense on cooling and then vapourise
during warmup. We need a tool to tell us what's going on, here it is.
http://www.linric.com/webpsysi.htm
Fill tyre with wet 100%%RH (90+%%RH is not uncommon) air at 15°C to
32psi. Air at 100%%RH 15°C 1bar air holds 10.68g water vapour / kg air.
A 195/60R15 tyre contains about 0.14Kg of air so 1.5g of water vapour
is put in the tyre. At 32psi and 15°C it can only hold 3.34g/Kg of
vapour about 0.4g so 1g (1mL) condenses out (to cause rust pits on
steel rims). Now you go drive on it, it warms up to 80°C, the air can
now hold all the water as vapour. The pressure will be 43.2psi a rise
over the DRY air pressure (42.53psi) of 0.67psi. It's still manageable
and isn't going to give wild variations in pressure.
Lets assume the fitter did it outside in the rain (like they do at the
tyre van at race tracks or roadside truck tyre repair) and didn't take
care to dry the tyre when fitting it so there is liquid water in it.
The max amount of water that air can hold as vapour at 80°C running
temp is 75.15g/kg = 8.8g (8.8mL) of water, any more than that and the
rest will still be sloshing around. Pressure is now 49psi a 6.5psi
rise over the norm for DRY air. Easy to see why racers have had
problems with tyre pressures.
Now get a compressor with an air tank, fill it to 100psi and let it
stand for a day. The air in tank can only hold 1.34g/Kg of vapour, the
rest condenses into the receiver tank and should be purged by opening
the drain cock. Now fill the tyre in a nice dry room to 32psi. Only
0.2g of water vapour goes into the tyre, it stays as vapour unless
temp drops near freezing. Pressure at 80°C is 42.56psi a HUGE 0.03psi
increase on dry air or N2 pressure. So always let your Clarke
compressor from machine mart stand a day before filling those tyres.
++++++++++++++++++++++++.
Those with a profit to make from selling N2 have had a report written.
http://www.getnitrogen.org/pdf/graham.pdf
Statement: Diffusion rate for N2 is 3, O2 it's 10. (3rd para from end)
Air contains 78%% N2
Total diffusion rate of air = 0.78 x 3 + 0.22 x 10
= 2.34 + 2.2 = 4.54.
The N2 is exiting the tyre 6%% faster than the O2!
100%% N2 diffusion rate = 1 x 3.
ratio = 4.54/3 ~ 1.5
A tyre with N2 will have same pressure after 3 weeks as an air filled
tyre has after 2 weeks. All it can possibly do is allow you increase
the pressure check intervals by 1.5.
++++++++++++++++++++++++
Some claims made
:1. Better tyre pressure retention
:“Nitrogen molecules are significantly larger than oxygen molecules
:which reduces the rate at which compressed gas escapes through
:the tyre walls; which helps in tyres maintaining their original
:intended pressure for longer.”
"Nitrogen molecules are significantly larger than oxygen molecules".
Outright lie, N2 is only 3%% bigger than 02. (see the getnitrogen
report)
There is always some loss of pressure in doing a weekly [1] tyre
pressure check (as recommeded in safermoting link above) and there is
always one that goes hiss when you don't get the gauge on square. That
loss will need topping up before effect of diffusion of O2/N2 from
tyre could have been detected.
Taken from Michelin pdf document link below.
::“Inflation using nitrogen does not dispense with the need
::to frequently verify the tyre pressure.”
http://www.michelin.co.uk/uk/document.DocumentRepositoryServlet?codeDocument=472&codeRubrique...
OK you may stretch the checks to every 1.5 weeks.
:2. Improved fuel economy
You will get that just by doing frequent checks of pressure and
setting correct pressure for use - should be adjusted for every trip
M-way or town, laden/un-laden. You can set pressure easily with an
electric pump anywhere, have to run to your N2 supplier before every
change of usage pattern if using N2. If you fail to set the correct
pressure for use, then N2 wonÂ’t give any benefit.
:3. Cooler running tires
As above.
:4. Removal of oxidation
IÂ’ve had some alloy rims out on the yard without tyres in all weather
for the last year, I canÂ’t see any deterioration. Alloy has an oxide
film that prevents corrosion, if you remove it, it re-forms nearly as
fast as you can abrade it away. Steel wheels should be painted and the
tyre fitter should touch up any scratches they make (yeah as if). Mag
wheels have to be painted, rim protectors are vital when fitting
tyres.
:5. Improved retreadability
?? what. Who uses re-treads on cars? The tyre makers have taught the
public that they are not safe and historically have to have lower
speed ratings than vehicles top speed. Used lots on trucks (see later
link to truck tyres).
:6. Elimination of rim rust
Teach tyre fitters not to gouge and abrade the paint off the rim.
:7. On-the-road reliability
Stops you getting nails in the tyre? I think not.
++++++++++++++++++++++++
Truck tyres
http://expert.fleetowner.com/tires/2007/07/23/nitrogen-filled-truck-tires/
++++++++++++++++++++++++
Getting it RIGHT.
Hard use like race and track days. Stuff the pressure. If you can't
read the tyre wear pattern or feel what it needs, then you need to
measure the tread temp and get it even all the way across. If the
middle is warmer let air out, middle cooler put air in, one side
hotter than other, change the camber or toe-in! If it's even temp
across tread but too hot/cool then suspension is wrong. Next morning
when it's cooled off you can take a pressure reading and use that to
set cold pressure next time you're at that track. It will be different
for different tracks and you may get different camber/toe/suspension
setting from one track to another as well. At top level it will be
different driver to driver. Test, test and test again.
http://www.elephantracing.com/techtopic/tiretemp.htm
http://www.coleparmer.com/techinfo/techinfo.asp?htmlfile=Fluke-track.htm
++++++++++++++++++++++++
Still concerned about diffusion of gas from the tyre? Fill it with
CO2. O2 and N2 are linear molecules they slide though small holes
endways. Watch them line up and rush out of that micro pore.
---------------
N=N O=O
---------------
there they go, N2 is only 3%% bigger that way on than O2.
CO2 is "bent"
O -------------
\ C-O
--------------
It jams in the hole, so needs a much bigger hole to get though or a
lot more speed to be straightened out and rammed though a hole that N2
or O2 would slip though with ease. It's very inert and won't give up
an O atom below 1500°C. C02 is readily available from welding or keg
beer suppliers.
++++++++++++++++++++++++
I don't give a flying F about tyre pressure, I do care about the
F'wittted claims dressed up as science that charlatans make to dupe
the public (and it seems motorsport techs). Next thing you know it's
law that tyres have to be filled with N2 and only qualified tyre
tech's can check your tyres and adjust the pressure with their magical
source of N2.
[1] Lets just say I'm a bit lax.
