TPMS Caution:  Absolute vs Differential Air Pressure

 

Standard tire gauges measure the pressure DIFFERENCE between atmospheric pressure and internal tire pressure.  Technically, this is called "differential air pressure":  one side of the sensor diaphragm (or mechanism) is pressed on by atmospheric pressure, while the other side is pressed on by internal tire pressure.  This is reliable at any elevation because it parallels "how hard" the rubber will be pushing out away from the wheel (against atmospheric pressure) to support the load on that wheel.

 

Absolute air pressure, on the other hand, is measured against a theoretical vacuum, or against a fixed air pressure.

 

Be advised:  some TPMSes measure absolute pressure.  You will know if you have one of these if the technical specifications say the device has been calibrated to be accurate at a certain elevation.  Other TPMSes measure differential pressure like standard tire gauges, and thus (assuming they are working correctly) can be expected to be accurate across all elevations.

 

If indeed you have a TPMS that measures absolute pressure, it will be up to you to know the difference between an accurate tire gauge and what your TPMS will read.  Hint:  assuming it is accurate and working correctly, it will be the same at the calibrated elevation, and will be different at other elevations.

 

The bottom line:  I use an accurate standard tire gauge to measure my tire pressure, and only use my TPMS measurement:

 

1. to know the changes in tire pressure as I am driving (e.g. as a signal to know when a tire is losing air pressure);

 

   OR

 

2. after I know how it behaves compared to my tire gauge, in which case I can use it to know the real pressure without having to lose some air by checking it using a tire pressure gauge.

 

If in doubt, trust a tire gauge before a TPMS:  batteries run down, electronics fail—you know the story.