Math question, thermal dynamics
Thread Starter
Safety Car
Joined: Feb 2006
Posts: 4,855
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From: Columbia South Carolina
Math question, thermal dynamics
So it was 28 degrees here this morning and I checked my tire pressure because the truck seemed alittle sloppy yesterday. I started thinking 'If I have a total of 28lbs@ 20deg F in the tire what will that pressure be this afternoon when it is 70 deg out?' assuming that the tire is the same temperature as teh air. I know there is a Calc formula or somthing but I am an electrical engineer and cant remember the thermal dynamics stuff. Anyone remember?
Advanced
Joined: Oct 2008
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From: Swansboro NC
Check out this link and if you figure it out let me know!
http://www.tremek.com/forum/technica...er-winter.html
http://www.tremek.com/forum/technica...er-winter.html
Racer
Joined: Jan 2008
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Use the ideal gas law as stated above but like this:
P* / T* = P / T
n, R, and V all cancel out because they are the same on both sides... Also, you can use F and lbs as long as it is the same on both sides.
P* and T* would be at the first temperature and pressure
P and T would be at the second temperature and pressure
P* / T* = P / T
n, R, and V all cancel out because they are the same on both sides... Also, you can use F and lbs as long as it is the same on both sides.
P* and T* would be at the first temperature and pressure
P and T would be at the second temperature and pressure
Team Owner
Joined: Jul 2008
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
Use the ideal gas law as stated above but like this:
P* / T* = P / T
n, R, and V all cancel out because they are the same on both sides... Also, you can use F and lbs as long as it is the same on both sides.
P* and T* would be at the first temperature and pressure
P and T would be at the second temperature and pressure
P* / T* = P / T
n, R, and V all cancel out because they are the same on both sides... Also, you can use F and lbs as long as it is the same on both sides.
P* and T* would be at the first temperature and pressure
P and T would be at the second temperature and pressure
Team Owner
Joined: Jul 2008
Posts: 23,419
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
Check out this link and if you figure it out let me know!
http://www.tremek.com/forum/technica...er-winter.html
http://www.tremek.com/forum/technica...er-winter.html
Secondly, if I am controlling for temperature on a race course, I want to adjust the pressure at operating temperature, not cold temperature. If we make the assumption that the tire will reach the same operating temperature regardless of the ambient temperature, then they are OK (except for the aforementioned use of gauge pressure), but I am not sure I would be comfortable with their methodology if I was going to be screaming around a concrete ringed oval at 200+ mph, but that is just me.
Racer
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You are incorrect about the units... the reason for using absolute temperature (Kelvin or K) and other metric units of measurement is because the constant R uses these units. Since R is canceled from both sides, there is no need to use these units... Calculate the units for youself, they work out. Using F and psi on one side and F on the other side will give you psi for the unknown. This will give you a rough estimate because real gases are not ideal... but what are we, chemistry professors?
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Drifting
Joined: Jun 2005
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From: Chino Hills CA
I was once told by my Michelin rep that the real world relationship is roughly 1psi per 10 degrees temperature change. So 28 psi at 28 degrees would read about 32 psi at 70 degrees. (remember, always check pressures cold, ie. before driving car). Don't know how accurate, but seems about right...
Team Owner
Joined: Jan 2006
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From: Crossville TN
You can always spend $30 and get nitrogen put into your tires... Nitrogen expands less than oxygen. Of course, AIR is 80% nitrogen to begin with.... 
{that 'nitrogen in tires' thing is a great deal if you can find a "patsy" to fall for it}
{that 'nitrogen in tires' thing is a great deal if you can find a "patsy" to fall for it}
Team Owner
Joined: Jul 2008
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
You are incorrect about the units... the reason for using absolute temperature (Kelvin or K) and other metric units of measurement is because the constant R uses these units. Since R is canceled from both sides, there is no need to use these units... Calculate the units for youself, they work out. Using F and psi on one side and F on the other side will give you psi for the unknown. This will give you a rough estimate because real gases are not ideal... but what are we, chemistry professors?
Am I a Chemistry Professor? No. Chemical Engineer?
Team Owner
Joined: Jul 2008
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From: In limbo
St. Jude Donor '13-'15, '17 thru '22
People who put Nitrogen in their tires are suckers. Do the smart thing like I did, fill up with HELIUM!!! You won't believe the performance improvement!
Race Director
Joined: May 2006
Posts: 16,528
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From: Dayton, Ohio
sorta like the car in Flubber?But on the serious side I'm going to bookmark this thread in case i run into a need for one of you to work a formula for me.
That'd be too simple, Peace,,,Moosie 
Le Mans Master
Joined: Oct 2000
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From: Santa Ana CA
Le Mans Master
Joined: Oct 2000
Posts: 8,897
Likes: 1,357
From: Santa Ana CA
No, as was stated, PV=nRT. The Pressure times the Volume is equal to the Number of moles times the gas constant (R) times the Temperature.
Avogardo's number (6.02x10^23) is the number of atoms that is equal to the atomic weight in grams and is defined as one mole.
Avogadro's Law states "Equal volumes of gasses, measured under the same conditions of temperature and pressure, contain equal numbers of molecules."
The type of gas has no bearing on the equation.
Avogardo's number (6.02x10^23) is the number of atoms that is equal to the atomic weight in grams and is defined as one mole.
Avogadro's Law states "Equal volumes of gasses, measured under the same conditions of temperature and pressure, contain equal numbers of molecules."
The type of gas has no bearing on the equation.
Last edited by toddalin; Dec 3, 2008 at 01:19 PM.
Team Owner
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St. Jude Donor '13-'15, '17 thru '22