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Discussion Starter #1
Hi Folks

I know the TPM's on my RT are temperature balanced - but what does that actually mean.

I'd assumed (dangerous I know :wave ) that the balance meant that regardless of the air temp when I fill my tyres the reading on the gauge will be the same when the tyres are warm as when they are cold in comparison to the same pressure when read at 20C?

So if this is the case my readings should be the same at all tyre temperatures (reading lower than actual when the tyres are very hot and higher than actual when they are stone cold).

However my readings increase with speed and temperature and are low when leaving my garage.

OK I'm probably dumb :eek: but I don't get the way these are supposed to work in terms of accuracy????

Anyone got a handle on how thats supposed to work?
 

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I don't know.
Mine seem to change ever so slightly with perhaps altitude, and ambient air temperature.
Maybe also from a bit of riding, just starting out.
So I have no idea what they are doing.
dc
 

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My understanding is the same as Gerhard's. Actual (that is, "absolute") pressure increases as the tire air heats up from flexing (that's called the "ideal gas law" and most scientific-types can recite it rote), and the computer compensates for that so that the pressure you read on your on-board display is irrespective of temperature.

That said, you shouldn't rely upon the TPM system as your only source of tire pressure data. You should "calibrate" that against a good tire gauge with cold tires. I find that my TPM typically shows a reading about 1-2 psig lower than what I measure with a tire gauge. So for example if I fill the tire to 37 psig on the guage the TPM will read 35-36 psig.

The actual number you display on the on board computer may vary a bit on a day to day basis. Tire gauges by their basic construction contain an internal reference to actual atmospheric pressure at the time - the psig means "Pounds per square inch, gauge" and means the actual difference between inside the tire and outside (ambient). I doubt that the TPM system contains a reference to actual ambient pressure - it's measuring an "absolute" pressure inside the tire and converting it to psig relative to what it assumes the outside to be, and (as Gaby notes) it's already assuming 20C (68F). So on a day to day basis, as actual atmospheric pressure rises and falls with weather, temperature and altitude, you get some slight variation in the reading.

BTW, at ambient temperatures the temperature correction is a relatively small part of that inaccuracy (outside air temperature going from 50F to 80F results in a pressure difference of about 1%, or well less than 0.1 psi on the gauge). Atmospheric pressure from weather or altitude is a larger factor.

What I do use the TPM system for is to monitor the tire pressures to make sure I don't have a leak. I use my bike to commute daily and only get down on my knees to check the tire pressure with a gauge on weekends or before a long trip. I use the TPM system to watch for trends, and count on it to warn me of a puncture (which, fortunately, I've never had).

My experience watching the TPM readout daily is that the actual numbers often vary a psi or so over time just from weather, that's not the type of trend that worries me. A drop of 2 psi overnight and another psi by the end of the day - that would worry me and make me break out the tire pressure gauge to find out what's going on.

JayJay
 

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Discussion Starter #6
JayJay

I'm not suggesting I rely on the TPM gauge as a my pressure reading - my question is - if these readings are temperature corrected how come the pressure raises as the tyre gets hotter and drops as the temperature drops?

I noted when I started using the the TPM that the reading was inaccurate as compared to my calibrated gauge.
 

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Dominatio said:
JayJay

I'm not suggesting I rely on the TPM gauge as a my pressure reading - my question is - if these readings are temperature corrected how come the pressure raises as the tyre gets hotter and drops as the temperature drops?

I noted when I started using the the TPM that the reading was inaccurate as compared to my calibrated gauge.
Ian - There shouldn't be much variation in the display with tire temperature - I see maybe a psi as the tire warms up but not much else. If you're seeing significant movement in the pressure reading then maybe there's something horsed up. The TPM has to have two elements to work properly - the pressure sensor and the internal temperature sensor. If you're seeing this anomalous behavior on the readout for only one tire then I'd suspect that wheel's transmitter. If it's both then it sounds like a computer problem to me. Either way, considering the cost of replacement parts, before you go in and change things you might want to consult with your dealer. (If you have a GS-911 you could look for an error code, I don't know off the top of my head if there is one for TPM that the GS-911 can read.)

Then again, TPM is an option. If you're out of warranty you have to now determine if you really need that feature.

But bottom line - no, I don't think you should be seeing a big swing in displayed temperature as the tire warms up.

JayJay
 

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JayJay said:
Then again, TPM is an option. If you're out of warranty you have to now determine if you really need that feature.

But bottom line - no, I don't think you should be seeing a big swing in displayed temperature as the tire warms up.

JayJay
I agree that the difference in indication between cold and hot tyres should be limited.
The reason I opted for the pressure sensors was mainly their alarm function when a tyre goes flat.
 

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There is indeed a battery inside the sensor assembly.
It seems it can't be replaced, so a flat sensor battery may not be cheap!
 

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As far as I'm concerned, the computer's TPM isn't meant to give a precise pressure reading. For this, a hand gauge on the valve stem is the only useful and precise tool. For me, the TPM's only use is to warn me in case of a sudden loss of air pressure, indicating a puncture of some sort. This is the reason why I always leave it showing. If I last inflated at, say, the BMW recommended pressure of 32 psi front, and the TPM shows like 30 or 31, I will not feel concerned a bit.
 
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