I think all of us are savvy enough to keep our tire perssure up and keep an eye on the battery. I'm going for a short jaunt this weekend, and checked the tires, and, as usual, they all were the same pressure but not the pressure I inflated last time, when the temperature was cooler. We all know that temp & pressure are directly proportional, but to what degree? Does anyone know the difference 5 deg F will make on tire pressure? At what temperature is the max inflation pressure accurate? On the other hand, we also know that battery voltage will drop with temperature, but, again, at what temperature is the rated voltage 'correct'?? How much should it drop when the ambient drops 10 deg??
If anyone has real scientific sources for the answers, I'd love to see them.
Opinions are like noses, everyone has one and no two are alike, right??
Joe
By the Perfect Gas laws, specifically Gay-Lussac's Law, pressure is proportional to the Absolute temperature, which is 459 degrees higher than the Fahrenheit temperature. Thus a 5 degree F rise from say, 80 to 85 degrees, will make a pressure change of 544 / 539, or just over 1%. This assumes that the tire is rigid and doesn't expand to give the air in it more volume, which is valid for small changes to a well inflated tire.
I leave the rest of your questions to others, or until I get around to them.
sounds like you're over-thinking the inflation issue. Fill your tires, per instructions from the manufacturer, to the rated pressure when they're ambient temp (not driven on) and forget it. If you start trying to second guess temperature changes you'll be pulling over every time you go around a curve and take the sunlight from a different angle. Tire makers are smart enough to KNOW that the temp will effect them and they've got you covered.
I am a tire guy. We see low pressure each fall and winter for that very reason. Industry wisdom says you lose approximately 1 lb for each 10 degree drop in temperature. And most tires (not inflated with nitrogen) lose around a pound a month.
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casydon gave you a good answer and Sam (Hurricaner) as always did also, but in direct answer to your question about the formula:
The formula for pressure increase in a constant volume container (as caseydon said tires are pretty close to a constant volume) is:
P1/T1= P2/T2
Where P1 is the initial pressure, T1 is the initial temperature.
Where P2 is the new pressure, T2 is the new temperature.
The temp must be in degrees kelvin.
Or P2 = P1 X T2/T1
For P1 = 85 psi, T1 = 294.26' kelvin, and T2 = 305.37' kelvin. (T1 = 70'F, T2 = 90'F)
P2 = 85 X 305.37/294.26 P2 = 85 X 1.038 = 88.21
So for a 20' F increase in temp the pressure should increase by about 3 psi.
The bad news is that apparently water vapor in the air in the tire can cause the numbers to be somewhat larger than those predicted by the formula.
( Kelvin: A temperature scale whose zero point is absolute zero, the temperature of 0 entropy at which all molecular motion stops, -273.15° C. The size of a degree Kelvin is the same as the size of a degree Celsius.
K = 'C + 273.15)
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Wow - this has got to be the smartest bunch of RV guys in the US - I am impressed! The data helps me (and everyone else who reads them) to keep things in shape, so to speak. I interpret that I should just fill 'em to the req'd 80PSI unless we're going out between Thanksgiving & Valentines day, in which case we should have out heads examined. 'Specially if there's 8-10 inches of snow.
Thanks to all for the timely and clear responses.
nbounder wrote: On the other hand, we also know that battery voltage will drop with temperature, but, again, at what temperature is the rated voltage 'correct'?? How much should it drop when the ambient drops 10 deg??Joe
nbounder wrote: On the other hand, we also know that battery voltage will drop with temperature, but, again, at what temperature is the rated voltage 'correct'?? How much should it drop when the ambient drops 10 deg??Joe
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