Nitrogen becomes a liquid at around -322F.
My math teacher called me average. How mean!
Everyone is average around here in lake woebegone. My chemistry teacher would have just said it is in the book. Look it up. Nobody would have bothered though. So if air is mostly nitrogen and it turns to liquid in the tire, what about the rest of the air? I’m not going to Alaska though so doesn’t really matter.
It’s really quite simple. If you cut the temperature in half, you cut the pressure in half. But you need to go by a temperature scale that begins at absolute zero, not Fahrenheit zero or Celsius zero. The Celsius absolute scale is called the Kelvin scale. The Fahrenheit absolute scale is called the Rankine scale. Since absolute zero is minus 459.67F, convert F to Rankine by adding 459.67 to the Fahrenheit reading.
Zero F = 459.67 R
-20F = 439.67 R
439.67/459.67 = 0.95649, The new ABSOLUTE pressure will be 95.649% of the original ABSOLUTE pressure. Don’t forget to convert gauge pressure to absolute pressure before doing the multiplication. Zero psia is a perfect vacuum, not atmospheric pressure.
Absolute zero is the temperature equivalent of a perfect vacuum. Absolutely no heat whatsoever.
Yes, the old rule of thumb is fairly close, provided you stay close to normal automobile tire pressures and normal temperatures. The absolute temperature ratio method works even in oven or arctic temperatures. Even on Boeing 747 tires, inflated to 200 psi and Top Fuel dragster tires inflated to 4 psi. It even works for Tom Brady’s football pressures.
It does break down as you approach minus 322F, because nitrogen becomes a liquid at that temperature.
Clear as mud, thanks!
I’ll just check my early morning tire pressures, and if they don’t match the value on the door frame placard, just reset!
But all the children are above average in Lake Woebegone.
Later there will be coffee and cake in the church basement.
Well, I regret not having grown up in Lake Wobegon.
Proverbially, you could say I was 'born in a bar, and raised in ‘Nam’.
Add a few choice cuss words and a flying ashtray, and a four year old boy who began to believe his name really was “dummy!”, and you get the picture.
But at least I know that if a cold tire pressure isn’t where it’s supposed to be, I know how to get there, be it 10F out or 95.
My comment was actually a play on words. Mean is another word for average.
One of my other favorite math jokes:
There are 10 kinds of people, those who understand binary and those who don’t.
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That math might a have been a fun exercise but it “ain’t gonna happen…” Close, but no cigar… There is no way temperature can go lower than “absolute zero” or −459.67 °F (−273.15 °C).
From what I’ve read, things will get smaller when cooled (though contraction), but only to a point. Granted, the statement of zero air pressure at -460°F is relativistically correct, but in reality, at that temp, or rather at that no temp, there is nothing, no movement, no energy, not even enough electricity to move your EV out of the garage so you can get your snowmobile out…
And if the wife says the tires looks low, I ain’t getting out of bed to check…
Oh, now I get it. 
How about a correction factor for sea level? I just look at the dashboard.
Speaking of bleeding moisture from compressors:
At a gas station just up the street from me, just for the heckuvit, (yes that’s a word in these parts!) I pressed the Free Air button on the air machine outside. It fired up. I counted to ten, then pressed the middle thingy in the chuck that release air, for a count of five. A stream of moisture in bubble form came out.
Another count to ten, another press for a count to five. More moisture.
I repeated this several times, and each time some moisture came out along with the air.
I’m not sure I’d recommend anyone using this particular machine, as it will probably need several such ‘sessions’ until only air comes out when it’s chuck is depressed.
At that temperature, you would have to thaw out and boil some air so you would have something to breathe in your space suit. The atmospheric pressure would be a hard vacuum because the atmosphere would all be a layer of air-snow lying on the ground. Schools would close in Minnesota; Canadians would lower the ear flaps on their caps.
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In Minnesota most of those outside air hoses are frozen solid in the winter and unusable. Just my observation and no reliable statistics. I have my own air source and don’t understand why most guys don’t spend a couple hundred dollars for a compressor. They’ll buy a phone but not a compressor.
Spending a couple of hundred dollars is not needed . I have a 110/12V tire inflator that is quite accurate . It takes up very little space so I one in each vehicle. Many people live in apartments or just do not need a full blown compressor.
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I expect most car owners would prefer to own one, very useful for a reasonable expense, but they don’t have any space to store it.
I expect most car owners prefer that some else checks their tire pressure. As for storage a small pancake compressor has about the foot print of 2 pairs of shoes. Easily stored in a closet, if they don’t have a garage.
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Or the handheld rechargeable or power point compressors.
I’m too self-reliant to let someone else check my tire pressure. They would probably adjust it to what ‘they thought’ was correct.
That’s the problem in this country - even tire pressure has become as sensitive as politics!
Well good for you.
Why the long face, my man?
No long face, just have more important things to worry about than if my air gauge is off +/- 1 psi. Or if my tires are exactly at the manufacturers spec.
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