[acerboo]

tyrepal have an offer on their tyre monitoring system at the moment of £99 + postage. ive been using one of these for a year and they are very useful. it detects a loss of pressure in each tyre individually to about 2lbs psi. )runflat indicator is 10lbs) if you save one tyre from being destroyed due to slow deflation it will more than pay for the system.

[creepy coupe]

My tyres can vary 3 PSI due to temperature changes, how does it deal with that?

[doughboy]

It must use temperature compensated sensors.

3psi is a fairly conservative increase, i've tried to check hot tyres before and found them around 10 or more psi over cold.

[creepy coupe]

Originally Posted by doughboy

It must use temperature compensated sensors.

3psi is a fairly conservative increase, i've tried to check hot tyres before and found them around 10 or more psi over cold.

Yes-but we don't all drive like you

[doughboy]

Never tried checking pressures after hooning! Just motorway on a summers day....

Back OT - temperature compensated pressure measurement is effectively measuring density (i.e. the mass of air in the tyre) so regardless of pressure, if the mass of air in the tyre is the same then non has leaked out....

But the compensation has to accurately reflect expansion coefficient of the substance the sensor is meant to measure of course.. ahem..

[creepy coupe]

Originally Posted by doughboy

Never tried checking pressures after hooning! Just motorway on a summers day....

Back OT - temperature compensated pressure measurement is effectively measuring density (i.e. the mass of air in the tyre) so regardless of pressure, if the mass of air in the tyre is the same then non has leaked out....

I suppose from a cool garage to a motorway run in the summer, you'd get a fair change in temp. But 10 degrees?

[doughboy]

I just remember often trying to check hot rear pressures on the 330i (having forgotten to check them cold) and found them at 40psi or so instead of the usual 32.

The tyres must be at around 50degC at least (as opposed to less than 10deg in the morning), and the wheel rims hotter still, so the actual air temp could be even higher!

[acerboo]

Originally Posted by creepy coupe

My tyres can vary 3 PSI due to temperature changes, how does it deal with that?

you can vary the rate plus or minus but is useful as although the Rf system lets you know when youve lost 10 lbs running a tyre that down on pressure for any length of time could damage it. the system also gives a temp reading

[doughboy]

http://www.tyrepal.co.uk/products/tp...-specification

It looks pretty nifty, but i'd be interested to know how it gives you 'tyre' temperature when it's simply attached to the outside end of the valve stem!!

Surely it will just give you 'outside' temperature? And the sensors look big enough to upset wheel balance - any issues with that acerboo?

Could be useful for the missus, stop her asking me to check her tyres all the time!

[acerboo]

Originally Posted by doughboy

http://www.tyrepal.co.uk/products/tp...-specification

It looks pretty nifty, but i'd be interested to know how it gives you 'tyre' temperature when it's simply attached to the outside end of the valve stem!!

Surely it will just give you 'outside' temperature? And the sensors look big enough to upset wheel balance - any issues with that acerboo?

Could be useful for the missus, stop her asking me to check her tyres all the time!

the sensors are ungainly but quite light so have not had any balancing issues, the unit is in contact with the air inside the tyre so i imagine this is how it measures the temp

[creepy coupe]

Could be useful on the blow up doll collection, save any dissapointing evenings

[acerboo]

Originally Posted by creepy coupe

Could be useful on the blow up doll collection, save any dissapointing evenings

never thought of that brilliant!! even measure her temperature in case shes getting overheated. theres 4 in a set thats one for each doll.

[Giff]

Originally Posted by doughboy

temperature compensated pressure measurement is effectively measuring density..

Any more info on that? Air density changes with temperature.

[creepy coupe]

So do the Dodabs that screw onto the valve have batteries in them?

[acerboo]

Originally Posted by creepy coupe

So do the Dodabs that screw onto the valve have batteries in them?

correct

[doughboy]

Originally Posted by Giff

Any more info on that? Air density changes with temperature.

Yes it does, in open space where it can change its volume, but if the volume of the container (i.e. the tyre) doesn't change then the density can't change either so the pressure changes instead as the heated gas tries to expand.

I.e. if you have 100 grams of air in the tyre and say the tyre volume is 0.5m3 (density = kg/m3 = 0.2kg/m3) and there is no leak, then the density will always be the same regardless of temperature and or pressure.

In reality the tyre expands a little, so the density will fall a little too, but the pressure will rise more so as the tyre resists the expansion.

If you consider a rigid pressure vessel (i.e a gas bottle), then no expansion will occur, so the density will never change.

[Giff]

Originally Posted by doughboy

Yes it does, in open space where it can change its volume, but if the volume of the container (i.e. the tyre) doesn't change then the density can't change either so the pressure changes instead as the heated gas tries to expand.

I.e. if you have 100 grams of air in the tyre and say the tyre volume is 0.5m3 (density = kg/m3 = 0.2kg/m3) and there is no leak, then the density will always be the same regardless of temperature and or pressure.

In reality the tyre expands a little, so the density will fall a little too, but the pressure will rise more so as the tyre resists the expansion.

If you consider a rigid pressure vessel (i.e a gas bottle), then no expansion will occur, so the density will never change.

Theres no way these little devices can actually weigh the wheel and I strongly doubt theres any sensor or algorithm to take in to account temperature variation either.

[doughboy]

Originally Posted by Giff

Theres no way these little devices can actually weigh the wheel and I strongly doubt theres any sensor or algorithm to take in to account temperature variation either.

They wouldn't don't need to weigh the wheel, that was just explaining the theory behind it.

Just measure pressure, but have it compensated for temperature.

Anyway they aren't compensated anyway AFAIK.

Better to keep it simple