So myself and a friend of mine are both getting tyre pressure warnings. The pressure fluctuates from 2.0-1.6. It changes everyday. Anyone else having this issue? Ford Focus St-Line X Edition 21 plate. 

20 minutes ago, ES51MCD said:

So myself and a friend of mine are both getting tyre pressure warnings. The pressure fluctuates from 2.0-1.6. It changes everyday. Anyone else having this issue? Ford Focus St-Line X Edition 21 plate. 

Pressure (from Boyles Law) is directly related to temperature. The hotter it gets, the higher the pressure gets. In the last few days the ambient temperature has ranged from -2c to 10c here in Northern Ireland. In addition, when driving, your tire will get warmer hence increasing the pressure. A fluctuation is to be expected...

Yes, but surely not from 23psi to 29psi due to a 12 degree change in temp????

The temp change you mention is in the ratio 283/271 - so about 1.044, so would elevate a pressure of 1.6 bar to 1.67.

In July, I could leave home with 32psi, and after 10 miles at 70MPH, it could go to over 40psi. Does your TPMS also tell you the individual tire temperatures?

How Tyre Pressure Varies with Temperature

https://www.arden.org/misc/pressure.html

Good article (though I've only skim-read it). To correct for absolute pressure (I used gauge pressure) my calc above should have said 1.6 bar would elevate to 1.71.

BUT ....

I'm basing that on the temp rise of the 12 deg that Stephen mentioned in his first post. But as he rightly suggests in his second post, driving at speed will push the temperature of the air in the tyres way above ambient temperature. So I now think my initial expression of surprise was wrong, and a variation of 1.6 - 2.0 is is not unexpected.  In thinking about my own experience, I realise that I've only ever measured tyre pressures "when cold" - because that's what we're always told to do!

All very interesting but I suspect not much help to the OP 🙂

Surely the TMPS system takes all the above into account, maybe by just comparing the difference between each tyre so that if one drops, or does not increase by as much as the others then it alerts?  Or maybe just if one drops a certain amount below a minimum threshold value?  My alert came up once and one tyre was just 0.1bar below the other three, but they did all fluctuate between cold at rest and hot after a run (can't remember the numbers, maybe 2 bar at rest and 2.2bar after driving a while).  I just topped up the lowest one and reset the system.

52 minutes ago, south_bound said:

Surely the TMPS system takes all the above into account, maybe by just comparing the difference between each tyre so that if one drops, or does not increase by as much as the others then it alerts?

My TPMS system measures each tire individually. It also shows me the temperature of each tire.  Of course, it's a 'top of the range' system, and now on it's 3rd car with original sensor batteries installed. It cost a whopping  £22, that was for 4 x sensors, display panel, and P&P 🤣 (Oh, and it also throws in a voltage reading too for good measure!)

 

Looks like a great little system.  Are you spinning the wheels every time the lights go green, judging by the front wheels being 3 degrees warmer than the back?

2 minutes ago, south_bound said:

Looks like a great little system.  Are you spinning the wheels every time the lights go green, judging by the front wheels being 3 degrees warmer than the back?

Simpler explanation! The sun was over front half the car parked in my driveway, the other in shade, it's a very minor difference. Years ago, I use to be really anal about looking at the temp & pressures all the time. Now, I just glance at it once in a while, and just wait for a 'beeping alarm' to let me know that a tire needs attention... (which it did a couple of months ago, really handy!)

On 3/3/2022 at 8:39 AM, StephenFord said:

My TPMS system measures each tire individually.

I had no idea these things existed as after-market gadgets. Brilliant!