How accurate is your TPMS? – Part 2

RV Tire Safety
with RV tire expert Roger Marble

Last week I posted on the question of “How Accurate is your TPMS?” This week I am continuing my close examination of Tire Pressure Monitoring Systems. We saw that the variations in pressure readings were measurable but that IMO the differences were not meaningful.

I recently traveled from Akron, Ohio, to a large (2700+) RV Convention held by Family Motor Coach Association (FMCA) in Perry, GA. There was a large variation in ambient temperature during the trip, with a low of 26° F to a midday high of 78° F observed. I felt that this trip was a good opportunity to take a first look at the temperature numbers reported by an internal TPMS versus an external TPMS.

To start out, let’s look at the morning temperatures in GA before I started driving home. All temperatures are in degrees Fahrenheit.

Internal System:
RF 61   LF 61   RRO 61   RRI 61   LRI 63   LRO 61

External System:
RF 60   LF 62   RRO 60   RRI 60   LRI 62   LRO 62

I feel it is reasonable to say that for all intents and purposes the temperature readings are the same.

After heading out on my trip north, I stopped a couple of times and recorded the readings. The ambient temperature was 74.0° F.

Internal System:
RF 108   LF 106   RRO 111   RRI 115   LRI 111   LRO 108

External System:
RF 80   LF 84   RRO 84   RRI 84   LRI 88   LRO 85

I made a second stop and this time was able to learn the ambient temperature was 60.9° F.

Internal System:
RF 95   LF 90   RRO 93   RRI 99   LRI 100   LRO 91

External System:
RF 62   LF 68   RRO 66   RRI 66   LRI 73   LRO 71

I consider the above temperature differences to be both measurable and meaningful.

All the above data provides some interesting information. All along I have thought and said that I felt that the temperature reading from external TPM would be affected and cooled by the outside temperature and rapid air movement around the sensor. Thinking about the heat flow, the heat is generated in the outer edges of the belts under the tread in the radial tires. While some of this heat flows out through the tire sidewall, the insulating properties slows this heat flow. Most of the heat energy flows into the air chamber where it is transferred to the metal wheel and then to outside air.

An internal sensor mounted on the wheel will be measuring the temperature of the air on the inside of the tire but since the sensor is in contact with the wheel it will be cooler than the hottest part of the tire.

An external sensor is depending on the heat to transfer from the wheel to the brass valve stem and then to the metal base of the sensor and finally to the thermocouple inside the sensor to report the temperature. But along the way, heat is being lost to the outside air from the wheel outer surface and through the brass valve stem, and from the brass sensor base to the cooler outside air.

BOTTOM LINE
I believe that the default high temperature warning for all or most TPMS is 158° F ( 70°C). With this initial data, I plan on suggesting that the high temperature warning for external sensor TPMS be lowered by 10°F to 15°F. I will be repeating this temperature comparison later this summer when I travel from Akron to Yellowstone. I will be looking for the numbers when ambient temperature is in the 85° F to 95° F range to see if external sensors continue to be cooled as much at these higher ambient temperatures.

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