Stupid Question - Air Pressure Readings??

[kcjaz]

My in-deflate kit gauge reads against ambient right? Right? Gotta control my impulse shopping.

any tire gauge you are holding in your hand is PSIG so yes. One side of the diaphragm sees the tire pressure and the other is open to ambient. You probably need a better one though...

 

[TCH2FLY]

No not exactly.

TPMS is an absolute pressure transducer. You can not actually measure pressure directly. You can only measure pressure difference between two things by the deflection of a diaphragm. In an absolute pressure transducer one side of the diaphragm is exposed to the pressure you are trying to measure and the other side connected to a sealed chamber that has been evacuated to zero pressure.

What your TPMS is really measuring then is the pressure in your tire plus the atmospheric pressure on the outside. I agree that the TPMS is in a "sealed" tire but that tire is still being squeezed by the atmosphere. So when your tire is filled to 37 psig (hand held gauge) and the barometric pressure is 14.7, your TPMS would measure 51.7 psia. No matter where you take your tire to, Ouray or outer space, the TPMS would measure 51.7 psia (as long no air leaks and the temperature in the tire doesn't change).

The ECU simply subtracts a constant 14.7 from the measured TPMS value and it will display 37 psi the whole time (as long no air leaks and the temperature in the tire doesn't change). Its the constant 14.7 that causes the error here.

You should inflate your tire to RCTIP at whatever elevation you are at. The reality though is that 3 or 4 psi isn't that big of a deal in most cases. When you are down to 15 psi accuracy starts to matter.

If I'm still not making any sense, just google, lots of info out there probably explaining better what I'm trying to say here.

How Do You Adjust Your Tire Pressure for High Altitude?

There's an easy way to set your vehicle's tire psi while road tripping across elevations.

www.motorbiscuit.com

Temperature is by far the bigger deal. RCTIPs are "cold" pressures. After I wheel all day and go to air up my tires are not cold. My RCTIP is 42 psi. I've just learned to go to ~45 when I air up and then in the morning, I'm pretty close to 42. Conversely, if you air down, with your tires hot, and don't air up at the end of the day, you will be lower than you think in the morning.

Even more rabbit hole:
I believe it is possible to program a PID in OBDFusion or possible other OBDII apps, to take the TPMS readings, add back the constant 14.7 and then subtract the OBD barometric pressure. This could potentially give you accurate TPMS data.

I think we are trying to say the same thing, yes it is always measuring the pressure differential but it is no longer an “accurate” reading when atmospheric pressure varies from 14.7 since there is no direct measurement of ambient atmospheric pressure.

The internal pressure changes that occur due to temp changes in the tire as shown on the TPMS reading should be accurate (hot=increase, cold=decrease). eg a 3psi increase on TMPS will also be 3psi on gauge

 

[AnyMal]

any tire gauge you are holding in your hand is PSIG so yes. One side of the diaphragm sees the tire pressure and the other is open to ambient. You probably need a better one though...

It was grey anodized though... How could i not?

 

[GordJ]

Thanks guys, you learn something almost every day.