Accuracy of onboard tyre pressure measurements?

[uumode]

My tyres should be at 255kPa or 36.99 psi

Digital pump on petrol station forecourt is set to 2.55 bar (=255kPa)

so all 4 tyres should be at 2.55 bar or 255kPa

The car's own electronic reading shows

Front NS = 270
Front OS = 265

Rear NS = 263
Rear OS = 255


Not overly worried about a slight variance, but I take a reading with a cheap Halfords digital meter for the fronts, and

Front NS = 37.5 psi (259kpa vs car reading 270kpa vs forecourt 255)
Front OS = 38.5 psi (265kpa vs car reading 265kpa vs forecourt 255)

I would have thought although the forecourt meter may not be accurate, it should have at least be inconsistently accurate, as both front tyres measure differently with hand held air pressure gauge and onboard car measurement,

BUT the onboard car measurement records the Front NS as higher, but the hand held digital meter records it as being lower than the Front OS

How accurate is the onboard measurement?

 

[PobodY]

The obvious question is "where all the measurements taken at the same tyre temperature?".

Let's assume that air is an ideal gas (i.e. p=nRT/v) pressure is thereby proportional to temperature. - Throw in a nominal tolerance on any of the measurements; perhaps +/- 5% and you've got pretty-much the same number.

 

[rf065]

Tyre pressures should be taken when cold, if you've driven to the garage they will not be accurate. Take the pressures after the car has been parked all night, before it is moved and see what you get.

Russ

 

[brucemillar]

Tyre pressure gauges, especially garage forecourt ones, (sweeping generalisation, I know) are hopeless for accuracy, just on the same axle, never mind the same car. Try testing on one tyre, remove the gauge then try again, probably a different reading on both attempts ;^)

I have found that a "quality" push on, tube gauge, is usually the most accurate and will give the same reading, if tested several times on the same tyre, at the same time ;^)

 

[Headhurts]

I'm not sure of the accuracy myself.

I always take the temps cold and use initially the gauge on the foot pump followed by a decent tyre pressure gauge.

Then set the onboard readout but when checked they rarely show the same albeit without too much variance.

I really think the onboard readout is an aid only for tyre loss problems and always rely on hand gauges for reliable pressures.


Sent from my iPad using Tapatalk

 

[daveenty]

What everyone else has said really.

Mine are done before setting off, so the tyres are cold. They are checked with a PCL inflater (which is still in calibration) so reasonably accurately. As you'll be aware, the TPMS doesn't start monitoring for a mile or so, but the further you go the less accurate it becomes due to natural heat expansion. You're going to get one side giving a slightly different reading at some stage, purely due to heat dispersion; bends, water etc., so nothing to worry about I'd have thought.

Garage gauges are not famed for their accuracy though, if using the same one for all, then you should get a degree of symmetry with it. Either all too high or all too low.

 

[grober]

You might be confusing accuracy and reproducability

http://pyrometry.com/pyrometry-info/accuracy-reproducibility/

If you factor in that the system is primarily designed to detect gross loss of pressure which I assume its reasonably good at ? rather than measuring the absolute pressure of each tyre then it makes more sense. In this respect its perhaps not much better than the cruder standard system that relies on tyre circumference difference to sense tyre deflation. IRRC its introduction was deemed necessary with the introduction of run flat tyres where some drivers might blythly drive on partially/totally deflated tyres with being aware of the fact.

 

[DRIVER200]

As per MB recommended pressures my 19's should be 260/260Kpa minimum and up to 280/340 when fully loaded. I logged onto Comand on-line to remotely check the pressures and the reading was 285/278 for the fronts and 280/275 for the rears but in actual fact each tyre except the front driver side which was 255 the rest were all 260. After increasing the 255 to 260 I checked Comand on-line again and it still shows the high values!! Anyone else had this disparity?

 

[grober]

Sounds as if the system has simply not updated the readings since the pressures were altered. Can you force it to read the TPMS sensors? It may simply be a calibration issue. Without knowing the exact spec of the sensors may be impossible to say. Do they have temperature compensation for example? If they don't you would need to monitor tyre temperature also. Are they calibrated for air or nitrogen. What effect has any water vapour content? Whats the accuracy/calibration of the gauge you are using to check manually. Lots of reasons why you shouldn't worry too much about this.

 

[camerafodder]

grober's point about not worrying too much is spot on. Just buy a decent gauge and pump them up yourself. Interestingly all this unnecessary tech has already caused me grief recently. At the beginning of December I returned a rental Ford Explorer to Kansas City airport. The weather had been unseasonably warm all week but on the day I was due to fly back to Denver, the temps had dropped below freezing. This caused the tyre pressure warning alarm to activate even though all they needed was a couple of puffs of wind here and there to compensate. The young and inexperienced inspector at the rental place couldn't get his head around this and proceeded to spend 20 minutes examining the tyre in question and filling out an 'incident' report. I explained that it was plain to see (as the nose on your face) that all tyres were fine and that it was simply a warning due to temperature drop causing pressure drop. He really didn't get it...computer says no... They walk amongst us.

 

[190]

I can't be doing with converting kPa to PSI.

In simple terms a proper TPMS system with sensors should be accurate to within 1 PSI and a hand held pressure gauge should be at least as good as that but you could reasonably expect to see a spread of as much as 2 PSI between them. Add to that the change in pressure with temperature which is approx 1.5 PSI for 8 Deg C and it's easy to account for the differences.

If anyone wants to do sums with the gas laws then it has to be in absolute temperature units. So to calculate the temperature difference between 2.0 bar and 2.1 bar the equation looks like this:

(-1+273)*(2.1+1)/(2+1)-273 = 8.07 Deg C

 

[DRIVER200]

Well I checked/adjusted all four tyres yesterday morning when cold to 260Kpa. I reset the Tyre Pressure Monitor in Comand but when I checked again both the on-board/on-line reading it was 278 for 3 tyres and 273 for the remaining tyre. I use a pretty accurate pressure gauge so surprised that the in-car/on-board system isn't closer to actual pressure. Having said that I don't need to rely on the car based system as prefer to check for myself anyway. Strange though....

 

[grober]

How to Calibrate a Pressure Gauge With a Dead Weight Tester ~ Learning Instrumentation And Control Engineering
As an example every pressure gauge on an oil rig will have an accompanying calibration certificate with an effective expiry date. Because they are calibrated in an instrumentation workshop against a known international standard using an approved methodology you can use the term accuracy. Other gauges may capable of extremely reproducable results but without constant calibration against a standard you cannot technically call them accurate. :d
ps what you can say is their relative measurement of 4 different tyre pressures will be valid since the calibration error is going to be constant for all. But you cannot compare readings with another pressure measuring device unless they were both calibrated against the same standard within the allowable time frame.

 

[190]

...when I checked again both the on-board/on-line reading it was 278 for 3 tyres and 273 for the remaining tyre. I use a pretty accurate pressure gauge so surprised that the in-car/on-board system isn't closer to actual pressure. Having said that I don't need to rely on the car based system as prefer to check for myself anyway. Strange though....

That's a difference of 1.8% It could have been just a little better but it's in the ballpark of what you can expect and not worth worrying about. In the end it's another example of complexity in modern cars. Like you I prefer to check them manually.

Pencil type pressure gauges can be calibrated very easily by unscrewing the top and adjusting the internal spring. Just need a good reference to compare it against.

Talk of dead weight testers takes me back 45 years. I was trained in instrumentation and spent many hours calibrating pressure gauges on a dead weight tester. 1% accuracy across 80% of the full scale was a very good result.

 

[PobodY]

I agree +/- 1% (relative) is pretty good accuracy. - If the precision is about the same, the uncertainty will be very low.

 

[DRIVER200]

Yep know what you mean so will just take not of current Computerised measurements eg 278/278/278/273 and assume that is the equivalent to 260Kpa which my gauge says is the pressure and use that as a benchmark for any variances in the future. But either way will manually check tyres routinely anyway. Assume the same discussion could be had for the fuel gauge readings but don't want to go there this time around, LOL Cheers all.