Gauge and Absolute Pressure

A tire gauge reads zero in open air, yet a barometer standing beside it says 101 kPa. Both instruments are right, because they use different zeros. A gauge has atmosphere on the back of its diaphragm, so it can only ever report the difference between what it is connected to and the air around it. Absolute pressure is measured from a sealed vacuum reference and counts everything. $P_{abs} = P_{gauge} + P_{atm}$ is the bridge, and neither reading is more correct than the other — they answer different questions.

A tire inflated to 220 kPa gauge holds 321 kPa absolute. The relation matters just as much in the other direction: a "vacuum" rated at −80 kPa gauge is 21 kPa absolute, still holding a fifth of an atmosphere. And there is a floor. The most negative a gauge can ever read is −101.3 kPa at sea level, because a perfect vacuum on one side and one atmosphere on the other is the entire range available. Anyone quoting a −150 kPa vacuum is quoting something that does not exist.

The distinction dates to Torricelli's barometer in 1643, which settled two arguments at once: that the atmosphere has weight, and that a vacuum can exist. Before that there was no absolute zero of pressure to measure from. The unit conventions still carry the split — psia and psig, bara and barg, and in North American vacuum work a third convention entirely, inches of mercury below atmosphere, so that "28 inHg of vacuum" means a gauge reading of −94.8 kPa and an absolute pressure of 6.5 kPa.

Every gas law demands absolute pressure, and forgetting that is the classic error on this site. Boyle's law, Gay-Lussac's law, the combined gas law and $PV = nRT$ all count molecular impacts, and a gauge has quietly subtracted an atmosphere from the count. Compressing a tire from 220 to 440 kPa gauge is not doubling the pressure — in absolute terms it goes from 321 to 541, a factor of 1.69, and a calculation done on the gauge figures is wrong by nearly 20%. The rule is simple enough to make automatic: convert to absolute before any gas law, and convert back only at the end if a gauge reading is what somebody wants.

Then the subtler trap, which is $P_{atm}$ itself. It is not 101.325 kPa where you are. That figure is the standard atmosphere at sea level, and elevation takes it away quickly — Calgary at 1045 m sits near 89 kPa, Denver near 84, and weather moves any of them by ±3 kPa on its own. Converting a gauge reading with a textbook 101.325 in a mountain city introduces a 12 kPa error, which is 12% of an atmosphere. Worse, the barometric pressure a weather service reports is usually sea-level corrected: it has been adjusted upward to what the pressure would be if the station were at sea level, specifically so that maps are comparable. It is not the local absolute pressure and must not be used as one. If you need the real figure, read a station barometer or compute it from elevation. This is not academic — it is why a pump's available NPSH is stated absolute and why a suction lift that works at sea level can cavitate at altitude, with the equation and the elevation together explaining exactly how much margin was lost.