Tire Pressure Monitoring

Yes, I have noticed the accuracy may be less than the precision would imply. I carefully filled my tires to 39/37 (+/- 0.5 not cold, not hot), and reset the TPWS to "current pressures".

The next morning the TPWS showed:

I was definitely surprised to see a morning reading higher than the carefully filled pressure the night before.

The next day the readings were different but the wheel to wheel differences remained constant.

 

I haven't noticed any difference, but I'm too busy with my "2 to 4 mph pulse and glide", and trying to anticipate all the crackpots playing a driving video game around me.

I watched a video about using an infrared thermometer to find the proper pressure for tires - thinking of getting one and see what it comes up with. After watching the Consumer Reports youtube on low profile tires and pot-holes I'm hoping the temperature method will justify running them higher than the car door pressures.

 

I saw a statement that tire pressure will vary 1psi for every 10 degF change. I don't know how accurate that is, but the temperature in my garage does get as low as 80F and usually 90F or more when I get in the car each day, so perhaps could explain some of the variation.

That same article claimed a loss of 1-2 psi per month.

Today after driving home in the rain at 79 degrees F, it was showing:

If you use the app, make sure you check the "Tire Status Updated: Today/Yesterday/six weeks ago at x:xx PM" because it was once two days old and saying good when the right rear tire was flat.

 

Glad to see I'm not the only one whose monitor reads the right side different from the left, except my right side is always higher. Anyone know why that might be? Is it just a poor measuring system?

 

I find that when parked outside, the side that is facing the sun may register a pound or two higher. If it's constant or always parked under cover, you may want to check it with a gauge or stop by a tire shop to have them check it.

 

duplicate delete

 

duplicate delete

 

Some serial overthinking happening here..

 

It is a reasonable-enough approximation, and much easier to remember and mentally apply than the more accurate Ideal Gas Law.

The sensors don't have perfect accuracy, but rather a noticeable error tolerance. And each sensor likely has a different error, so they don't match. That is why they may read different than a tire gauge (which has its own error), and each one different from the gauge by a different amount. These differing errors are small compared to the degree of underinflation that the system is meant to warn about.

Both right side tires higher than the left? That could how the different sensor errors were distributed by random chance. Or it could be real, caused by the sunlight being on that side now. Or on the other side when someone aired up the tires.

 

Those little pressure measuring devices are pretty complicated. Believe they're usually a thin silicon film over a vacuum cavity, the base of which is conductive and forms a capacitive element, for which the resonant frequency can be calculated, and the pressure from the gas modifies the frequency. It then has a calibration curve that combines the resonant frequency with the temperature, to arrive at a pressure. It will try to account for temperature, but the calibration is not exact. You might also have a temperature difference between the sensor, and the temperature of the gas in the tire -- if it's mounted behind the stem, maybe the sensor is colder than the air in the tire. So bottom line, it's all ball-park, but good enough to spot and gauge the severity of a leak. Is it slightly low and I can ignore it for a while? Or is it really flat and I need to stop?

 

https://www.avnet.com/wps/portal/abacus/solutions/technologies/sensors/pressure-sensors/core-technologies/mems/

 

Ken, do you know if the Prius sensors have a battery or are they powered by movement as mentioned in your attachment?

Thanks
J

 

Battery. Gave it a quick glance before I had them installed. Looked like a watch type battery glued into the sensor.

 

Those batteries are designed to last many years. They should outlast the tires on the car (if driven regularly).