free air at filling stations

iolanthe07

thats because thats what tyre pressure is measured by,PSI.

Not for years now. It changed to BAR (ie a proportion of atmospheric pressure), some years ago and now to something new again. My car does not even give the PSI equivalent - I had to work it out for myself.

mrtg0525

As someone above pointed out, the right time to measure tyre pressures is when the tyre is cold. As the tyre warms up, the gas inside expands so you get higher readings. If your tyres are supposed to be inflated to, say, 2 bar when cold, inflating them to 2 bar when they're hot will mean that you're driving around with underinflated tyres. Depending on how hot the tyres are when you take the measurement, they may be considerably underinflated.

goldspanners

iolanthe07 wrote: »

thats because thats what tyre pressure is measured by,PSI.

Not for years now. It changed to BAR (ie a proportion of atmospheric pressure), some years ago and now to something new again. My car does not even give the PSI equivalent - I had to work it out for myself.

thats right,but recently at the new tesco petrol station near me i put in what i thought was 32 psi,the digital machine didnt tell me what units it was measuring,it just said 32,there is no way this was 32 BAR,the tyre would have exploded if it was.

Coveredinbees!!!!

I was in a filling station on the M4 and they were charging for water for the washer bottle as well as air!!! I filled mine up at the lorry pump oddly that was free

toasterman

I'm finding it increasingly hard to get air (free or otherwise) at petrol stations. both of Asda at Cribbs' (cheapest fuel in Bristol afaik) air pumps have been "temporarily out of order" for well over 12 months now.

They seem to have disappeared altogether from a lot of other local petrol stations. That said, I wouldn't want it free if it meant paying more for fuel. Fuel prices are high enough as they are. I'd rather buy my own pump and pressure gauge.

Conor_3

mrtg0525 wrote: »

As someone above pointed out, the right time to measure tyre pressures is when the tyre is cold. As the tyre warms up, the gas inside expands so you get higher readings. If your tyres are supposed to be inflated to, say, 2 bar when cold, inflating them to 2 bar when they're hot will mean that you're driving around with underinflated tyres. Depending on how hot the tyres are when you take the measurement, they may be considerably underinflated.

Rubbish. I've actually tested this and it made absolutely no difference. The tyres were checked near the end point of a 400 mile journey and then checked again the following morning. There was no difference.

tomstickland

Buy a track pump, much quicker than an electric one.
I inflated a car tyre to 30psi in a few minutes. Meanwhile my neighbour was still there messing about with the pathetic electric pump.


Regarding temperature, the equation is:
PV/T = K
so Pressure and Temperature are proportional, with -273 Deg C being the zero point (absolute zero).

Say you increase the air temp from 10 deg to 20deg then that will make a
(273+20)/(273+10) = 4% difference in pressure. Say you pumped the tyres to 30psi and it then cooled by 10 degs then the pressure would be down to 29psi. That temperature swing is larger than anything experience in real life, and even then it'd only make 1 psi difference.

Riq

Add more money to fuel for free air?

You are insane right?

tomstickland

Shhh, the government might hear about it and decide that it's an "air tax" to be paid on all fuel and that it's for "our own safety".

PasturesNew

I was at a Morrisons the other week and wanted to buy a token for my tyres. But the man gave me one free because of the amount of fuel I'd bought, I think it was something like because I'd spent over £10.