Rain Sucks

I have a TPMS reader, and FWIW it always shows within ~1 psi of my cheap tire pressure gages. I have to tell the reader what car I’m using it on so I suppose the reader could be doing math.

The TPMS programer I have https://www.amazon.com/Autel-Programming-Accessory-Exclusively-MX-Sensor/dp/B07RGZR2M2/ref=sr_1_12?keywords=autel+maxi&qid=1569896236&s=automotive&sr=1-12 can also read the temperature of each wheel. I suspect some cars use that as well as a warning, if one of the wheel temps is out of wack from another. It also tells you the battery voltage of the TPMS itself.

Hmmm. Not following here I’m afraid. I AM at altitude, being in Denver, however.

I can check the car reading again. Car doesn’t report from a standstill, so I have to remember before I drive too far.

Pretty sure standard pressure here is substantially lower than sea level. I know that as a rough rule of thumb one figures 15% less oxygen at 5000 feet above sea level. About 3 % per 1000 feet that is.

It follows that overall pressure would also be lower.

The Leaf is our first car with internal active pressure sensors. All of our other cars monitor indirectly with the ABS wheel speed sensors. Our camper uses external pressure sensors. Both types work well.

Do Leaf's use direct pressure measurements? The fact that they only report after being driven for a while would seem to indicate that the sensors use some sort of indirect measurement based on wheel speed, etc.

TPMS are simple and measure the pressure differential between inside the tire and outside at the stem, the temperatur and acceleration. They have a small transmitter that transmits every so often. They "go to sleep" if they don't detect movement after a given time. Acceleration or spinning them is what wakes them up and they start transmitting again.

For the curious:

U.S. Standard Atmosphere
Properties of US standard atmosphere* ranging -5000 to 250000 ft altitude

Click to expand...

https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html


*US Standard Atmosphere is the same as International Standard Atmosphere (ISA) up to 32km. Temp and humidity will cause variations from "standard", which is never actually experienced in the real world.

GRA said:

For the curious:

U.S. Standard Atmosphere
Properties of US standard atmosphere* ranging -5000 to 250000 ft altitude

Click to expand...

https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html


*US Standard Atmosphere is the same as International Standard Atmosphere (ISA) up to 32km. Temp and humidity will cause variations from "standard", which is never actually experienced in the real world.

Click to expand...

The “standard atmosphere” is just a set of assumptions, it’s useful for keeping airplanes from hitting each other. It doesn’t mean that the pressure is constant from -5000 to 25000 ft.

Kev994 said:

GRA said:

For the curious:

U.S. Standard Atmosphere
Properties of US standard atmosphere* ranging -5000 to 250000 ft altitude

Click to expand...

https://www.engineeringtoolbox.com/standard-atmosphere-d_604.html


*US Standard Atmosphere is the same as International Standard Atmosphere (ISA) up to 32km. Temp and humidity will cause variations from "standard", which is never actually experienced in the real world.

Click to expand...

The “standard atmosphere” is just a set of assumptions, it’s useful for keeping airplanes from hitting each other. It doesn’t mean that the pressure is constant from -5000 to 25000 ft.

Click to expand...

Yup, an artificial, agreed-upon standard. Now to go dig up my old E-6B computer