On 1/5/2016 10:06 PM, Don Y wrote:
On 1/5/2016 6:43 PM, Ed Pawlowski wrote:
Newer cars have a TPMS and warn you when the pressure gets low. Now
that cold
weather is here,check to see that your tire is at least the recommended
pressure. I forgot about it and this morning it was 7 degrees. One
tire was 1
pound under and set off the warning. It would not reset after driving
as it had
to come up even more than driving did. Filled it up when I got home.

Not a major deal as I knew the pressure was adequate to drive, but it
annoys me
to have that yellow light on when driving. I understand this is a common
happening for the first really cold snap.

We're back at Costco every week or two having them add nitrogen to
the tires. On a cool day (30-ish in the AM), tire pressure (all
around) will be low. However, on a warm/normal day (80-ish in the PM),
pressures will be high -- TOO high if we'd added nitrogen on one of
those colder mornings!

[I think it's 1 psi per 10 degrees F?]

And, the Costco tire droids want to "overfill" by ~3 psi claiming
the tires are "hot" now that you've driven on them. So, instead
of 35/33 psi, they'll fill to 38/36 psi. Then, the ambient
temperature climbs 40 or 50 degrees and the tires are considerably
overinflated.

So, bleed out some nitrogen to bring them down to ~40/38 ("hot")
and hope we don't get another cold day to bring them *down*, too far.

From the perfect gas law, pressure varies with temperatu PV=nRT

Temperature is measured in degrees Kelvin so you can calculate potential
pressure drops or increases by the ratio of temperatures:

PV/T=P'V'/T'

The constants drop out and if you assume no volume change:

P/T=P'/T'