Originally Posted by
Kapusta
No.
Yes.
A tire pressure gauge is not calibrated to be zero at sea level atmospheric pressure. It is calibrated for zero when the pressure on the inside of the tire is equal to the pressure outside the tire (atmospheric pressure). That could be at sea level or at 20,000 feet. A flat tire reads zero in either situation.
Pressure gauges are calibrated to read zero at sea level. “Gauge” in psig means standard pressure. Yes, it will read zero at any altitude but it is calibrated to read zero at sea level. We use it as a differential but it is designed to read compared to sea level.
By the way, thank you for saying that “flat” means zero pressure.
All tire pressure gauges are designed to read the pressure inside the tire ABOVE the actual atmospheric pressure. This is why measured pressure in a tire increase with altitude, even thought the absolute pressure does not. The pressure in the tire remains the same, but the atmospheric pressure drops. Thus the difference increases, thus the tire measures (and behaves) higher in pressure.
It’s more complicated than just due to an increase in altitude, especially where tires are concerned. A tire is at least partly elastic. The pressure differential between altitudes is most likely to be due to that elastic nature than any change in external pressure. A rigid metal cyclinder will not measure a decrease in pressure with a change in altitude because the cylinder is rigid. A tire expands slightly which means that the volume increases. Increased volume with the same amount of gas will cause the pressure to decrease.
They aren’t taking into account the decrease in pressure on the outside of the elastic tire, however. That could easily account for the pressure difference. It’s not a simple problem.