How accurate is your TPMS?

RV Tire Safety
with RV tire expert Roger Marble

I have seen a number of people express some concern about the accuracy of the pressure readings from their tire pressure monitoring system (TPMS). You can read my previous responses here and here

I have also pointed out that the primary purpose or “job” of a TPMS is to warn the driver of a pressure LOSS.

I am inclined to think that some are still concerned with their exact pressure reading. I also have to wonder how some people are comparing various pressure readings reported by their TPMS. If they have external sensors, the simple act of removing and replacing a sensor can allow some air to escape. While I doubt that the small loss of some air will result in a meaningful pressure differential, it does add some uncertainty.

So 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 Truck System Technologies.

The question is: How do I make the test both fair and useful? 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 gauges 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.

It has 6 bolt-in valves for mounting the external sensors plus a valve for me to use with my hand gauge. Also, since safety is always of concern when dealing with a pressure vessel, I added a pop-off valve. One end of the fixture has a cap that can be removed so I can place the internal TPM sensors inside the 4″ tube. It also has a regulated air supply to compensate for the very slow leak around the cap and an analog dial gauge that allows me to constantly, visually monitor the air pressure to ensure the use of my hand gauge does not result in a pressure change that isn’t immediately compensated for by the regulated air supply.

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 psi, 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 68 F

Set B: 4 readings of 69 F, 2 readings of 68 F

I do not consider any of the differences in the readings of pressure or temperature to be significant or meaningful for a TPMS.  I would consider the pressure readings from all 12 sensors to essentially be equivalent.

You can learn a bit more about what a “meaningful” measurement is here.

Read more from Roger Marble on his blog at RVtiresafety.net.

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