Quote:
Originally Posted by Blue Bowtie
That chart is, at best, very confusing, and at worst, incorrect. It also apparently ignores the fact that 70°F is not the highest temperature ever achieved on the surface of Earth. I guess we just run them overinflated and ignore that, as per the table.
Charles' Law and Boyles Law are physical LAWS. They are not theroies, conjectures, musings, nor axioms.
The Standard Temperature and Pressure (STP) for atmospheric air measurement is to have everything equalized to 68°F (20°C), 36% RH, and 14.696 PSIA (1.013 BAR). The linked table seems to make an attempt at that, but fails miserably. As a general guide, it is better than nothing, but also close to nothing of value.
The formula to correct for STP is simple. I can accurately predict the pressure of a given mass of air contained within a fixed volume (tire) at any practical temperaures from near absolute zero to the point of auto-ignition of the tire rubber compounds.
A substantial wildcard is the relative humidity of the air in the tire. That changes with temperature - Thus the term "relative" humidity. This is one reason for the popularity of dry nitrogen being used as tire fill, in addition to some other minor benefits.
Overall, the table is an attempt of someone to partially educate the populus about pressure variations, but doesn't quite meet the need.
|
Could the chart - and the website it comes from - be suggesting that at an ambient temp. of F70° or warmer, it's acceptable to match the recommended cold tire pressure on the door frame decal? That is, assuming the temperature first thing at dawn is F70°.
Below that temperature, say, if the ambient dawn temperature is 60°, then inflate the tires to door frame sticker
plus one psi. 50° outside? add two psi to door frame sticker cold pressure, until you get down to F20°
So to simplify, if you live where the lowest summer temperatures in your part of the world are between 65-75°F, it's ok to inflate to the door sticker value exactly.
IE: Me - 32psi cold.
If your winter lows in your part of the world average 30°F early in the day, then add 4psi to the door placard values, to compensate for differences in air density at the colder temperatures.
IE: Me - 36psi(32 + 4) cold.
That's my understanding, on the face of it.
My aforementioned retired engineer customer at work feels that 36psi cold, in summer or winter, is a bad idea for a car which Honda specifies 32psi cold, "seasonal air density differences be damned!" lol