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Friday, April 29, 2022

Is the temperature reading of your TPMS correct? Probably Yes, BUT...

 IMO TPMS are of little value when it comes to temperature warning for tires. Same for IR guns.

I have written a number of times on TPMS Temperature reading really being a reading of the metal wheel and not the rubber tire as metal conducts heat while rubber is an excellent insulator. So your TPMS can be used to provide warning of potential problems with wheel bearings, breaks and other metal mechanical parts.

These posts use data from my ongoing, direct comparison of internal vs external sensor TPMS. My data suggests that there is no meaningful difference in pressure reading based on the test of 12 sensors.
When seeing people claim the TPMS temperature reading were useful for predicting impending tire failure, the engineering DNA in me kicked in and I devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from TireTraker and one set of 6 internal sensors is from TechnoRV who provided the internal TST  system. My thanks to both companies for their support in my efforts to help educate the RV community about tires.

The question is: How do I make the test both fair and useful? For the pressure test, I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauge that I have checked against an ISO certified laboratory gauge.
Note: my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.
6 Internal sensors are inside the tube and 6 external sensors are on the outside along with a pressure regulator, a safety pop-off, and a reference dial gauge that allowed a visual check plus the test port for my hand held digital gauge that I have confirmed accurate to +/- 0.5 psi against an ISO Certified master gauge.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I have recorded the internal vs external pressure difference in reported running pressure with the following results.

I do not consider any of the differences in the readings of the internal vs external sensors pressure to be significant or meaningful for a TPMS, remembering that at Ambient I observed 3 to 5 psi differences when all 12 sensors were measuring an identical pressure, which is not too far off the claimed accuracy for TPMS.

The problem with TPMS temperature readings is that they are not able to read the hot spot of a tire. Here is a graphic representation to tire temperature differences.

Since heat can kill a tire, and tires simply do not fail based on their average temperature but they can fail if a single spot exceeds the ability of the rubber to maintain its strength. With this large temperature spread and the fact that the hottest spot in a radial tire is about 3/8" to 1/2" away from the air chamber and the internal TPM sensor is reading the average air temperature in the air chamber, I would ask why anyone would believe that a TPMS "High Temperature" reading is a reliable method of warning of an impending tire failure.

I started this post by saying I did not think that TPMS or IR guns were reliable tools for predicting an impending tire failure based on the reported temperature.  I will cover IR guns next week.


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