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Sunday, December 2, 2012

TPMS Accuracy



Should we be concerned when our TPMS pressure readings vary by a few psi from each other?

Measurable vs Meaningful
 
Before we get to the details of Tire Pressure Monitor System accuracy, it is important for us to understand the difference between a “Measurable” difference and a “Meaningful” difference. I think this concept can most easily be explained with a couple of examples. 

Money    It’s easy to measure the exact difference in two amounts of money but when we do so will the results be meaningful? If a group of 50 people were to raise money for a small project and we count the money that the group had,  I am sure we could arrive at a figure we all agree is very accurate. Let’s say we came up with $850.42. Now let’s assume a young boy came into the room and we included the money he had and now have $850.68. Clearly we have a measurable difference of 26 cents. But, is there a meaningful difference between the two sums? The difference is only 0.0306% or ½ cent per person.  Would you feel the group had done a better job in raising money based on the new total? I don’t think so. I don’t think this measurable difference is very meaningful.
Poison   If we study botulinum toxin we learn that 0.035274 oz can kill 80,000 people. If we were to increase the amount of poison by 0.0306% an additional 24 people would die.  Clearly if you were one of the 24 you would think this difference is very meaningful even though accurately measuring 0.0000108 oz is very difficult.

When we look at tire pressure we need to keep the concept of measurable vs. meaningful in mind. Assuming we know our pressure gauge is accurate to within 1.0 psi or less. I think there are different goals for meaningfully different pressure settings. When setting the cold tire pressure on our RV I would be comfortable at +/- 3% of our goal inflation.  This assumes a goal pressure of 65 to 120psi depending load, size and Load Range and would result in a range of about 2 to 3 psi from goal. For passenger car tires with goal pressures near 28 to 36 I would be comfortable at +/- 1 psi. When I was racing and my goal pressures were in the 28 to 36 psi range with each tire being different, I asked my crew to set pressures at +/- 0.5 psi.
Now what about TPMS accuracy? First, we need to remember that the TPMS is primarily a warning device and not a substitute for an accurate hand pressure gauge.

This chart above shows the difference in pressure readings between two different TPM systems run at the same time on my RV. One was an internal system and the other was an external system using the screw on sensors. My cold goal pressure is 65 +/- 1 psi. At the start of the test the the ambient temperature was 68°F  and the two TPM systems gave different readings.  I ran the test for 17 miles with the cruse set at 60 mph with only the first 2.7 mi being non Interstate. You can clearly see the range of pressure readings.  

Bottom Line
This data shows that the pressure readings from TPMS will vary from the actual inflation pressure by a number of psi. BUT since the intent of both systems is to warn when a tire looses pressure. I do not consider these differences MEANINGFUL  as I have run a test on the warning  pressure I have confirmed that both systems will give warning if I lose 12 to 15% in slow leak. Both systems also have a rapid air loss setting if there is a loss of as little as 3 psi over 1 min from the original setting. Since I set my cold pressure using my known accurate hand gauge, I do not worry when I see pressure readings that are different from each other as I drive down the road.

P.S. I have parked my RV for the Winter. I will run a longer test next summer but expect similar results.

2 comments:

  1. Good info. I would be interested in seeing the data from a slightly longer test. The way the data is converging at 17 miles it would be interesting to see what happens at 25-30 miles.
    What are your thoughts about screw on sensors that also monitor temp. Can meaningful data be extracted from a screw on temp sensor?

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  2. Anonymous. I agree but since my MH is parked for the winter you will have to wait. There is a good chance I will have a multi-week trip to give seminars in Gillette, Wy and will use that 2,500 mi trip to collect lots of data.

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