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Thursday, September 12, 2013

TPMS Accuracy & I had an air leak!

I sometimes see people worrying about temperature or pressure readings they see on their TPMS monitor. Based on the reported numbers, I have never been concerned with the minor variation but I felt I needed data to see if my opinion was justified.

As you can imagine I can get pretty focused on tire inflation pressure so it may not surprise you that in addition to two digital pressure gauges,
which have been checked and found accurate to +/-0.5 psi against ISO laboratory certified pressure gauges, I also have two TPMS on my RV. I am probably the only individual in the US and possibly the world to have an RV so equipped. The reason for having two systems is so I can give you this test report as I have relatively high confidence that with my frequent inflation checks with my digital gauges and adjustments to inflation I feel I would identify any problems with my TPMS. More on that later.

I installed my internal TPM about two weeks after buying my RV in 2008. At the time I was quite familiar with internal systems as I had been spending a significant portion of my work time involved with the OE versions of TPMS sensors. Some like this one had been damaged by improper mounting practice.


 My internal system is actually a car based system as it has an upper pressure range of 75psi but since I set my tires to 65 psi which is about 10psi above the minimum needed based on actual tire load, I felt I would not be seeing a big rise in inflation pressure. I was of course more concerned with the potential of a puncture and loss of air pressure. I subsequently learned that I would occasionally get a "high press" warning on one or two tires but have confirmed that my tires only get up to about 77 psi max.

I started this blog in 2011. After that I also increased my reading of various RV forums where I started to see more and more comments and questions on TPMS accuracy, I decided I needed to do some testing.

Here are the results from a number of tests.
 This data compares the two TPMS with my accurate hand gauge with the tires all set to 65.0 psi cold
 



 As you can see that while there is measurable difference in the readings I do not consider these meaningful and would consider the +/- 3 psi acceptible as I consider a TPM to really be a warning system based on a change in pressure and not a substitute for an accurate pressure gauge.

Running pressure shows a simmilar range of variation as seen above.
Running temperature is a slightly different story.Since each tire has a slightly different load and there is a significant difference in air-flow around the fronts vs the dual rears as well as a difference between the inner and outer dual, I would expect more variation and we see this in the test results shown here.
I note that the internal temperature is consistently higher than the external temperature reading which is to be expected with the external sensor both further from the heat source and being cooled by the external air flowing over both the sensor and in the rears over my extender hoses.

THE REST OF THE STORY

I previously mentioned that I felt I would be able to see a failing TPM sensor and I can report that I was able to discover a leak of  about 8 psi a month because I pay attention to the range of readings of my TPMS. I do not write down the numbers every time but I do pay attention to the range of numbers observed. I can also confirm that my external system gave me a warning as it is set tighter than the internal system. The leak was at the TPM sensor itself but since my system has a three year warranty I have been able to get a free replacement sensor.

BOTTOM LINE:
You can expect some variation in both pressure and temperature between different TPM systems. If you use a good hand gauge to set your pressures and just make a mental note of the range of pressure readings right after setting the tire pressures and on your first couple of trips, I think you will be able to recognize when there is a leak or an unusual reading that warrants additional investigation.

3 comments:

  1. I use exactly the same thought process in my experience with readings...when I first got the tpms system I was a bit taken back by the fact that it was not as accurate as my gauge. I called the manufacturer and they said the tolerances where good....basically learn to deal!! They did not say that, I did. It has warned me on occasion of a tire going down to 65 psi which is my warning range and if you where to look at the tire would have never seen it. They are a warning system but I must say at first a distraction in the differences of the readings to actual pressure gauge readings. Sometimes I worry when on a long trip in hot areas the temps and pressure go up into the 90s or more on a tire rated for max pressure of 80...but have never had a problem. Does teach you how much cause and affect driving and weight can affect the tires. I like yourself have always found the average person in everyday life totally ignoring their car tires and unless they visually look bad never do a thing with them. I am the pain in the rump to my family about checking the pressure but not sure they all pay attention. I have my own compressor at home and a portable one in the RV as it is tough to find one that works at a Gas Station.

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  2. I am sure your digital gauges provide more accuracy than my dial gauge but how do I fit the digital gauge into the small wheel opening on my class A and get a reading from a valve stem which points toward the inside of the wheel?

    Jerry

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    1. Jerry Sorry for slow reply. I got distracted and forgot I need to check for comments.
      You should be able to use a standard digital on the fronts. You can use a dual foot "stick" or "Dial" for the rears. I would get the readings on both fronts with the digital. Then use the dual foot gauge to also check both fronts. This will confirm the accuracy of the dual foot gauge. If it is good you can then use it to check the rear duals. If it is bad you can still "adjust" for that day but you need to get e new dual foot gauge if it is off by more than a few gauges.

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