Wrong kind of tyre pressure

my new Kia Niro can monitor tyre pressures the manual says). Sure enough recently an overhead image of a car appeared on my control panel showing flashing psi values against each wheel indicating 31/32 psi. They should be

36 psi all round.

As soon as the snow cleared, I opened up my cigar-lighter-socket powered electric tyre pimp and went sound all four. it has an analogue gauge but the readings were all 36-38psi. I added a couple of bursts before unplugging.

The graphic still read 31/32 against each wheel.

I posted my bewilderment to a Kia Form and someone came back and said that was normal. The misinterpret when it is cold!

This is 2017, the age of powerful smart-phones. How come Kia cannot compensate for low temperatures?

Makes no sense.

Reply to
Pinnerite
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on 15/12/2017, Pinnerite supposed :

Those analogue gauges on tiny 12v compressors are notorious for being inaccurate. Best check the pressure with a proper gauge.

Reply to
Harry Bloomfield

replying to Pinnerite, Iggy wrote: I think the Kia forum's wrong and you just needed to turn the car off and then back on or to reset the monitoring system after you've filled (read the manual). Cold weather will drop the psi and setoff the monitoring system for any tires that were previously borderline. My 2016 car needs to be turned on or back on and then driven down the block before it will acknowledge the issue has been addressed.

Reply to
Iggy

I don't think its the car, its the manual really. You would expect the pressure to also go up after a drive as well. They should give a range for normal against ambient temperatures and distance travelled. It can only be a rough guide of course.

Brian

Reply to
Brian Gaff

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Reply to
harry

Yes. Although the one on the compressor supplied with my Boxster (no spare wheel) is remarkably accurate. So it can be done.

Reply to
Dave Plowman (News)

Unless you fill your tyres with Nitrogen (with a low coefficient of thermal expansion) temperature is going to affect pressures.

The optimum pressure is given in the owners manual. I wouldn't have thought plus or minus 5% would matter to Joe Average. The given technique is to measure pressure at COLD i.e. before a drive this is sometimes asked in the driving test (minor if you get it wrong).

Additional:- And the tread must be at least 1.6mm over three quarters of the tyres width and in a complete band around the tyre.

Guess who just passed his test so has all this guff in his brain ? :O)

Reply to
soup

Yes, there's no way I would trust the guage on mine as it has such a wide range you really need a magnifying glass to read it!

I fitted some sensors to my Focus and found that the indicated pressure tallied exactly with my normal electronic guage except that it only reads to the nearest pound. Hoever, it did correctly round the tyre I'd measured at

32.5psi up to 33psi.

The indicator in the car only warns of pressure drop, not the actual pressure, but the hand held monitor is a doddle to use as you just hold it against each transducer for a couple of seconds.

Having severe arthritis in both knees, I often used to have to move the car during a check to get some of the valves towards the top of the wheel for ease of access!

Reply to
Terry Casey

I thought the gas constants to slot into pV=nRT were "close enough" for nitrogen, oxygen and air to not bother much?

nitrogen = 297 oxygen = 260 air = (0.78*297)+(0.21*260)=286 ... ignoring the other constituents

Is a TMPS going to care about nitrogen's 3% higher expansion compared to air?

Reply to
Andy Burns

Seems the pressure difference aspect is not really why Nitrogen is used it is more lack of moisture and molecule size which favours N2 use . Two seconds looking on the 'net, shows me the pros/cons of nitrogen V air is NOT a discussion I want to get involved in.

Despite that bum steer (by me) pressures are meant to be taken 'cold'.

Reply to
soup

Yes, KwikFit et al say that nitrogen is better because the oxygen in air degrades rubber, water vapour in air rusts wheels, and nitrogen leaks through the rubber more slowly ...

Likely all scientifically measurable, nevertheless millions of vehicles seem to survive with air in their tyres :-)

Reply to
Andy Burns

Considering the amount of of air, water vapour and water that the outer surfaces of the tyres and wheels get exposed to I don't think I'll worry much about the air inside the tyre.

Reply to
Mike Clarke

The gas laws don't differentiate between different gases so you will find nitrogen changes in pressure by exactly the same amount as oxygen, CO2, etc.

IE.

P1*V1/T1 = P2*V2/T2

as V1=V2 it becomes P1/T1=P2/T2.

AIUI the main reason they use nitrogen is because it leaks out of the tyre slower than oxygen. Which may be true as N2 is a bigger molecule than O2 but I suspect the difference isn't going to matter to anyone other than James Bond who apparently survived underwater by breathing the gas left in a car tyre. This would be mostly N2 if the O2 does leak out faster unless the tyre was recently inflated.

Well the MOT bit is correct in as far as it goes. It also states there must be visible tread over the other quarter.

Reply to
dennis

If nitrogen leaks more slowly than other constituents of air, the gas in the tyre will tend to have a higher proportion of nitrogen than the air used to fill it anyway.

Reply to
Rob Morley

But that could vary by 30C. Running pressure unlikely to be influenced by ambient so much.

Reply to
Dave Plowman (News)

For "ideal" gasses

The O level version of pV=K (easy to remember as our physics master was Peter Victor Kite) became the A level version of pV=nRT where R varies depending on the gas.

Reply to
Andy Burns

But the k is on both sides and cancels out, as does the volume in this case.

Reply to
dennis

The main reason they use it is because they can charge you more.

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says that the effective diameters are 364 vs 346. About 5%, otherwise known as SFA.

I've heard of it being used for aircraft, where it also acts as a handy fire extinguisher. Car tires don't have to carry that sort of weight at that sort of speed.

Andy

Reply to
Vir Campestris

Quite. And as the air inside a tyre doesn't get changed if oxygen is a problem once that oxgen has oxidised the rubber or rim there ain't none left to carry on oxidising whatever, unlike the outside.

Nitrogen filling? At no cost, maybe.

Reply to
Dave Liquorice

Hand held monitor? Is that a specific box for your aftermarket set or a "universal" box?(*) The car has TPMS but I can tell the pressures are low, from the handling or look before it wakes up. I guess TPMS is for those without a clue.

(*) GIMF - Boxes that will read OEM sensors are available at £100 ish. Kits with valve caps around £50. Digital pressure guages with a (manual) memory for each tyre for a tenner.

Reply to
Dave Liquorice

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