Strange tyre pressure readings

[si tate]

Ok put the new wheels & tyres on the truck the other week & my tyre guy recommended I put them at 33 psi, fine so as soon as I fitted them I thought for piece of mind I'd go check them at my local petrol station to make sure & low & behold they were all at 33 psi.
When I got home I used my digital reader which said they were all at 32.5 psi so allowing a little margin for error thought that was fine.
This morning as I was tinkering with the truck I decided to give them another check with my digital gauge & they were all reading 30.5 so went back to the same petrol station & when I stuck the air line on the vaulve it beeped insantly to say they were all at 33 psi so I had my tester in my pocket & it read 32.5 again on all of them.
So I'm guessing that when the tyres are cold they give a false reading or am I wrong & what is the best way of getting an accurate one.

P.s both times the truck was empty & no passengers.

[Predictable Bob]

1. You're using three different gauges and at least two of them are not calibrated !

2. Tyre pressures will vary dependent on air temperature/pressure

3. If the tyres are hot then the tyre pressure will vary

[puddlesurfer]

The air in your tyres expands with heat after a little drive - You should do you tyres when cold.

[legendgamesmaster]

pressure varies with temperature.
as temp increases, so does the pressure.
so yes, hot tyres, higher pressure.

I believe 'they' tell you to check pressure on cold tyres????

[si tate]

1. You're using three different gauges and at least two of them are not calibrated !

2. Tyre pressures will vary dependent on air temperature/pressure

3. If the tyres are hot then the tyre pressure will vary

Two gauges bob, sorry mean't to say it was the same digital tester on both occasions.

[Albannach]

If you use air, the pressure will vary with temperature. Cold pressure, using your digital thing as it's probably more accurite than something that spends its day being thrown around a garage forecourt, is the correct one. If you want the pressure to be constant, use Nitrogen.

[si tate]

If you use air, the pressure will vary with temperature. Cold pressure, using your digital thing as it's probably more accurite than something that spends its day being thrown around a garage forecourt, is the correct one. If you want the pressure to be constant, use Nitrogen.

Don't think I'll bother with nitrogen although I've heard alot do use it now.

No I'll stick with checking them cold & adjusting the pressure accordingly.
Cheers.

[Albannach]

Don't think I'll bother with nitrogen although I've heard alot do use it now.

It's easy to have N2 put in, the problem is topping it up if needed.

[si tate]

It's easy to have N2 put in, the problem is topping it up if needed.

That's the only put off for me really.

[Rustinho]

It doesn't matter what gas you put in the tyres. Pressure will increase directly proportionally to temperature (other factors can reasonably be assumed to be constant values in a closed system such as an inflated tyre) according to the following equation:

pV=nRT

Where p = pressure in pascals, V = volume in cubic metres (can be considered pretty much as a constant within a tyre), n = amount of substance in moles (again, a constant within a sealed system), R = gas constant (8.314 J/K.mole) and T = temperature (in K)

The friction of the tyres against the road as you drive will cause heat input into the system, thus increasing the pressure.

If you want a more complete explanation of the behaviour of gases please feel free to attend one of my A level physics or chemistry classes when I'm covering the subject. Time tables are available from reception. Don't be late or I'll have to start issuing detentions.

[Albannach]

Air isn't a pure, dry, gas, N2 is. The expansion characteristics of N2 are known and stable, they are not known or stable in air.

[si tate]

I'll leave you guys to it cos your getting too deep for me.

[Rustinho]

Air isn't a pure, dry, gas, N2 is. The expansion characteristics of N2 are known and stable, they are not known or stable in air.

See HERE.

[Albannach]

See HERE.

No need to click that, I fully understand why N2 is used in preference to compressed air, in tyres.

You know full well too, so stop trying to be a smart arse.

[shokenore]

and just to add my two pennies surely you would be using N and not N2?