How accurate is TPMS?

[Alex L]

Just aligned all my wheels whilst cold with my new digital air compressor. I have verified the pressures also with a hand-held digital gauge. This is how I set them up (in psi):

34 - | - 34

40 - | - 40

Interestingly though when I now look at my TPMS readout in the car it shows:

31 - | - 31

37 - | - 37

So -3psi down on the real reading.

Even more interesting is that when you check the 'info pressure' in the TPMS menu it shows:

+2 - | - +2

+2 - | - +2

Meaning the tyres are over-inflated by 2psi each, which doesn't match with the recommended 34/40 as indicated inside the door.

So the question is: is this common, or should I take my car back to the dealership to have the TPMS adjusted?

 

[snarf]

Alex - I have not gone to the trouble to compare readings to same degree of accuracy as you but I have a subjective feeling that the TPMS on mine is at variance to my Race X Pro tyre gauge (as recommended by Autoexpress http://www.autoexpress.co.uk/products/products/66830/tyre_pressure_gauges.html). I have put this down to a difference in temperature between the air in the tyres, which are sitting on my garage's cold concrete floor, and that seen by the sensor which is a little more insulated. I rely on my gauge to set the pressures and just use the TPMS to check that pressures do not vary by more than 0.2 bar from one side to the other while driving.

BTW the manual says to adjust the pressure by 0.1 bar for every 10C change in temperature, which means that at 0C the pressures should be 2.1 and 2.5 bar. Also the bar/psi conversion used by Porsche is incorret. The recommended pressure at 20C is 2.3 and 2.7 bar (which I presume is Porsche's unit of choice), the conversion factor is 14.5038 and to the first decimal place this equates to 33.4 and 39.2 psi.

 

[Alex L]

Actually at the back of the drivers manual it says 33 and 39 psi. Only on the side of the door does it state 34/40.

If you take Boyle's law into account then at nearly freezing (which it is today) pressures would be 3psi lower than at 20 degrees C. So perhaps the TPMS are calibrated to 20 degrees C values, thus are 3psi out compared to external digital measurements at ambient temperature.

The TPMS "Info Pressure" menu screen in the car though is meant to be sensitive to temp deltas so this may be a better reading it follow.

What I am trying to understand though is: are you meant to have pressures that at 20 degrees C would be correct, or are set to the recommended pressures no matter what the current ambient temperature is?

ie. are they MEANT to be lower pressure when it gets cold, or are you supposed to top the pressures up during winter?!

 

[snarf]

From what I remember reading you are not supposed to top up pressure. That is you set the pressure to the temperature adjusted level of 2.1 and 2.5 bar when the ambient temperature is 0C, such that if the temperature rose to 20C the pressure would be the required 2.3 and 2.7 bar.

 

[lowndes]

I have wondered about this issue from time to time and with TPM there is that bit more information available in "realtime"

Correlation between TPM readings and those from a dial gauge is I would say on average +/- 0.1bar. (Although brought up in the era of feet and inches to say nothing of rods, poles, perches, bushels and pecks, I tend to work in bar for tyre pressures as I am not sure that many gauges are accurate to 1 psi.)

I have also noted that whilst driving the TPM reading of one wheel may "hunt" for no particularly obvious reason save that the pressure sender is sending a digital not analogue signal and is between steps on its calibration.

My take on adjusting pressures to compensate for temperature differences is that once the tyres are at a steady state temperature the pressures should be (in my case) 2.3 and 2.7. So if the temperature when you set those pressures was 20C then by the time you had driven 100 miles the pressures would be likely to be 2.5 and 2.9. Stop for coffee and as the tyres cool the pressures will drop back toward 2.3 and 2.7. And as I would prefer to have the tyres slightly overinflated rather than under inflated when actually driving, I tend to leave things as they are. Otherwise you would be constantly adjusting pressures to compensate for change in air temperature and driving conditions.

If I remember O level physics correctly this inverse relationship between pressure and temperature for a constant volume (assuming the tyre doesn't stretch under pressure) is Charles Law.

The most obvious illustration is on the track. After the first couple of sessions you have to let out some air to keep the pressures correct and probably during the course of the day will let out sufficient that when you let everything cool down to drive home you have to add back 6psi per tyre.

By the same token at this time of year when air temperture is low, and there is often the cooling effect of cold rain and standing water on the tyres, you have to add some air that you will let out once Spring comes.

 

[snarf]

Physics revision:

Boyle's law PV = K
Charles Law V/T = K
Gay-Lussac's law = P/T=K
Ideal gas law PV/T = K
Avogadro's law PV = nRT

R is the ideal gas constant
n is the number of moles of gas
(T is in Kelvin, -273 abs zero)

 

[yan]

ORIGINAL: Alex L

What I am trying to understand though is: are you meant to have pressures that at 20 degrees C would be correct, or are set to the recommended pressures no matter what the current ambient temperature is?

Alex

I have found TPMS to be pretty accurate and reliable, despite only using an analogue gauge.

In winter you need to factor in lower pressures which would be corroborated by TPMS telling you that you have over-filled by +2PSI. However I would expect your screen to display 34 / 40 for observed pressures over-inflated by +2 so I can not see where the 31/37 comes from.

 

[bobfair]

Alex the TPMS on my 997 TT gives very similar reading to yours that is the TPMS says the pressures are lower than the tyre gauge does. I will be interested to know the outcome when you get to the bottom of this as it is a little confusing

 

[channella]

... and to think before TPMS you would never have known... How do the rest of us manage []

I suggest using it as a relative measure to look for excessive or sudden loss of pressure whilst driving, and trust my guage to set pressures when static and cold.

 

[garyw]

ORIGINAL: boxster27

... and to think before TPMS you would never have known... How do the rest of us manage []

I suggest using it as a relative measure to look for excessive or sudden loss of pressure whilst driving, and trust my guage to set pressures when static and cold.

As I use it Andrew... a monitoring system []
however I am aware we should check all pressures and levels before using the car [] as I'm sure you do []

Its nice to know how it works incase it does go slighly over the edge.... Mine has only ever warned once that the pressure was low, which it was for some unknown reason..[&:]

garyw

 

[channella]

ORIGINAL: garyw

Mine has only ever warned once that the pressure was low, which it was for some unknown reason..[&:]

Gary, so when you stopped and checked with a gauge all was okay then?

(Check every time? No, not every time - but on a sensible frequency, yes of course - although I can't but help wonder whether there will be the same bars of oil each time).

 

[garyw]

A rear had dropped a 0.2bar or the likes... enough to trigger a warning and easy to correct..
It had been fine going down the A14 but when I stopped for fuel it registered a drop in pressure..??

Been fine ever since..

garyw

 

[Alex L]

UPDATE:

As someone who replaced a TPM sensor yesterday...

I found that after some playing in the TPM menu, if you re-select the tyre type (summer tyres) then TPM goes back into 'learning' mode. It then takes about 10 minutes of driving to pick up the sensors again and then system kicks back in.

Whether or not it actually recalibates the sensors whilst it is learning I don't know. However, I set the tyre pressures to be 33/39 front/rear from cold at home this morning, then reset the TPM, then went for a drive. When the readings came back up they were 33/39. They were not aligned before I did the reset!!

 

[robert997]

Alex, been meaning to ask you about your errant front wheel. Was it losing pressure or just a sensor fault do you think ? Things certainly seem to be better now ?

 

[Alex L]

I had 2 issues:

1) Front drivers wheel was losing pressure slowly - fixed by resealing the wheel and tyre with some rubber sealant a couple of weeks ago.
2) Same tyre had a broken TPM sensor in it which must have happened when the OEM wheels were changed for HRE ones - fixed at the weekend with a new sensor.

All seems well now thanks for asking []

 

[fireblade]

Has anyone worked out a definitive answer yet on whether tyres should be inflated adjusted for temperature in winter rather than just to recommended cold temperatures?

If we are meant to adjust what is the formula please, I saw somewhere that 0.1 bar per 5 degrees C below 20 should be deducted but I cannot remember where.

I have emailed Pirelli as I am running PZero Corsa's but have not had a reply.

My sensors underread according to my tested guage.

Last week the red light and tyre sign came on (frightened the life out of me!) the tpm showed no pressure in either of the left tyres but on checking nothing was wrong, so I drove off and after about five miles the lights went out.

 

[snarf]

The answer to this question is in earlier postings in this thread, however, Porsche 997 handbook recommends adjusting pressure by 0.1 bar for every 10 deg C above or below 20 deg C. So if the ambient tempertature is 10 deg C set your cold tyre pressures to the rcommended pressure less 0.1bar.

 

[snarf]

I keep a copy of this table with my pressure gauge.

 

[garyw]

After having had another warning recently and Rob noticing that my tyres looked low, I checked all my pressures recently...
I did the same as alex and checked via a digital gauge, set at the correct pressures the TPMS indicated that the pressures were low, however after a decent run the numbers read exactly where they were supposed to be..
So I think the temperature has alot more to do with this...

garyw

 

[fireblade]

ORIGINAL: snarf

I keep a copy of this table with my pressure gauge.

Thanks for the table, makes sense now.

I am going on a track day at Easter and want to make sure that at least I start from home with the correct pressures