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Old 05-15-2013, 02:50 PM   #1
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TPMS Pressure Differentials

Ok first thing, I don't intend this to be another question that starts a debate concerning the need for or the best choice for a TPMS. I already have one because I need it and it is the best one of course. I simply am curious about what you have experienced while using one if you do.
I bought one a couple months ago and have been a little surprised at the info it provides.
I have noticed that the pressure starts to increase almost immediately as I take off and the temp lags just slightly and then sort of catches up after several miles. I didn't know what to expect as I have never used one until now. I would have thought that the temp would go up first then the pressure would follow. I believe there is some physics rule concerning the relationship between a gas pressure and temp, but I don't recall it as we speak. I grageeated a long time ago.
I have my tire pressures set at 80psi cold. I have the low pressure alarm set at 72psi (10% decrease based on manf. recommendations) and the high pressure set at 96psi 20% increase based on the same. I have seen pressures and temps average in the upper 80's with one particular tire getting a little higher 91psi and 93F.
It has been relatively cool here and the highest ambient while towing so far has been around 80F. it will be interesting to see the differentials when full blown summer gets here. I am also wondering about whether I will need to increase the high alarm setting when it gets hotter. 98-99 degrees and even higher ambient is common here. Obviously max cold tire pressure takes into account the increase for temps but I wonder what the excess factor might be? In other words if a tire max cold pressure is 80psi, at what pressure would it fail under operating conditions?
Anyway I would be interested in your experience, findings, and feedback. Do you see significant differentials and are your limits set differently for varying circumstances?

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Old 05-15-2013, 03:25 PM   #2
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I found the pressures start increasing immediately and the temp gradually. I've found that pressures will increase 10-15# and temps will approach 125-135 on the tires in the sun with ambient temps around 85-90. Haven't towed in temps above 90 yet with the sensors. Will see what happens on the way to Goshen the last of July.

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Old 05-15-2013, 03:41 PM   #3
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I see the same thing. Actually, I was (and maybe still am to some degree) doubtful that those sensors could really measure temperature. After all, there's no air flowing into them, so it's just a matter of the temperature generated in the rubber being conducted thru the air and into the tiny amount of air that is actually in the sensor. Balance that against the ambient temp that the majority of the sensor is sitting it. Given all of this, I'm convinced that there is probably a pretty good delay on how long it takes for the sensor to show a temp increase. I think that's why pressure goes up first. (And there's no doubt that they show a temperature much higher than ambient, so they are clearly measuring temperature to some degree.)

I also observe that the side of the RV in the sun shows several degree higher temperatures than the side in the shade (and a higher pressure as well).

Here's an interesting "problem" that I went thru. We had a morning "cold pressure" temp of about 70 degrees, set them at 80psi. Drove a few hours into a cold front where the temperature dropped into the 30's. Now the "hot" pressure shows up as about 80, and when I come back after a meal stop, the alarms are going off due to out-of-range on the low side. Should I have aired them up? (I didn't.)

On the way back, set at 80psi with a "cold" temperature about 40. When we return to the warmth, we're getting an over-pressure warning at 97psi! Should I have let air out? (I didn't; just raised the limit (and slowed down!).)
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Old 05-15-2013, 04:25 PM   #4
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My temps vary quite a bit on the right side due to the exhaust from my truck. When traveling an uphill grade, the temps slowly increase on right side, then decline once I've reached the summit and start to descend. I keep my temp alarm about 20-30 degrees higher than expected high ambient temp. I would not adjust air pressure when ambient temps drop considerably, but would monitor them closely once moving again. I find they rebound rather quickly. Same goes for the hot days when the sun is beating down on one side, which may see a 20 degree difference from the other. The sensor is more sensitive to temp change than tire.
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Old 05-15-2013, 04:30 PM   #5
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I have not towed any distance yet with my TPMS, but I find that the pressure reported varies some between sensors. That is to say, I put a particular sensor in the tire and read 50 PSI. Put another sensor on the same tire, and it reads 52 PSI. There is a variance between all 10 of the sensors that I have.

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Old 05-15-2013, 05:14 PM   #6
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I have had to adjust my cold tire pressures. We left Minnesota for the Gulf Coast for the month of April. I had 55 cold when I left MN. By the time we got to the Gulf Coast, my cold temps were 57 to 58. I run my alarms pretty close; 10 pounds under and 10 pounds over. So, my upper alarm was close to going off, once underway, with the warmer temperatures.

I notice a big difference in tire temperature depending on the sunny side of the trailer. Normally my temps are about 8 to 10 degrees over the outside temperature.
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Old 05-15-2013, 05:15 PM   #7
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Does anyone have the TST in tire sensors?
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Old 05-15-2013, 05:29 PM   #8
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When GY installed my new tires, the ambient temp was around 65, so I will the maintain the 65# they put in them. If the ambient cold temp is less when I prepare to leave, I will increase the pressure accordingly.
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Old 05-16-2013, 11:25 AM   #9
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Wow, good info. I appreciate the responses. It looks like what I am experiencing is pretty consistant with all of you.
I understand variances between tires, and sensors, and even sunshine have to be considered. And I agree that temps shown are subjective but at least I have some indication of whats going on back on the wheels and can use that info for a baseline anyway.
I will learn a lot this summer and will revisit this thread with an update.
Thanks again.
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Old 05-16-2013, 10:38 PM   #10
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I have yet to connect my TPTS. I am having the valve stems replaced next week from rubber to metal. This is good info so I know what I should expect and what kind of variances I might see but the main reason for me to have one is for the time that one tire starts to overheat extremely or start loosing pressure. One tire out of sync with the others means be wary and check it out, now.

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