ARB deflator guage.

[Heath]

ARB guage with the valve removing thingee built in and the sliding collar to dump the air out.

Anyone got one of these that's accurate? Mine is about 7psi out. Dropped the pressures yesterday to 34psi from 37psi. The guage showed a high pressure (around 40psi), but I figured it may just be the ambient temp making a difference.

Checked them today and they were 27psi.

How do I adjust the ARB one so its at least close to right? 7psi out is way too much of an error.

[meatc]

I've got one that appears to be reasonably accurate. What are you checking it against? I would try taking it to a tyre shop and comparinmg it to the high end ones they use, then decide on accuracy.

[Heath]

The place I got it has a tyre bay so I'll pop in and get it checked I guess. Maybe it was just the temp of the air in the tyres and gave a high reading and today in the morning cold it was low (really low).

[meatc]

The place I got it has a tyre bay so I'll pop in and get it checked I guess. Maybe it was just the temp of the air in the tyres and gave a high reading and today in the morning cold it was low (really low).

Ah that will be it. You should always check your tyres cold. running onthem heats them up and increases the pressure. (that the cold tyre thing the race car drives talk about) 15-20psi between cold and hot (15+min drive) would not be unusual.

[Sadam_Husain]

yeah the one I've got it about 5 psi out, cant remember which way its out, I just use it to keep my tyres all at even pressures when I pump them back up ?

[suzukisport]

We got one and works fine. Have seen another one tho that was out by a bit.

[flyingbrick]

15-20psi between cold and hot (15+min drive) would not be unusual.

I was told by a tire shop that this is complete nonsense. I asked because they pumped up one cold tire to the same as my 3 warm tires. (ever seen them check the temp of your tires and compensate?)

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi

Today Hamilton had a high of 16°C. We took our car for a drive to auckland (much longer than 15 minutes driving) so your telling me that our tires increased t0 20psi (an increase of 200 degrees Fahrenheit or 93°C so 109°C total

Before we left I actually set all four tires at a gas station to 36psi.. so when we got home they must have been at 56psi.... i think thats probably over max recommended inflation even!!!

Race car drivers warm their tires up to make the compound softer and they weave to scrub their tires in. Not to increase PSI.

[Nivapulledout]

ah but the pressure does increase on race cars because you will always see them under inflate them on a hot day when the tyres are cold due to high track temp.
However due to the heat generated in racing I doubt that you will find the same temp change in road use.
But a 15deg C temp change could result in around 5psi change in pressure.

we have a Zodiac inflatable rescue boat on the yacht I work on and I blew it up on a sunny day to check it for leaks overnight, came back off the piss about 2am and all the tubes were flat as, thought here we go hole day patching leaks, low and behold by the time I got around to looking at it in the morning, (10 am and temp up around 28Deg ) it was all pumped up again.

[DesmondD]

I just bought an ARB tyre deflator. I find it leaks air (so probably gives inaccurate pressure readings) between the tyre's valve stem and the brass adaptor that screws onto the valve stem. Should there be an O-ring in that adaptor to stop that leaking from happening? I can't see otherwise how the valve stem can seal properly against the bottom of the inside of the brass adaptor.

[DesmondD]

Two quotes have been posted:

15-20psi between cold and hot (15+min drive) would not be unusual.

The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi

Here is why the second quote is reasonable.

Remember Boyle's Gas Law from the 4th form (year 10 I think). It says PV=kT where P=pressure, V=volume, T=temperature, and k= a constant that even includes the number of gas molecules!

[This is named after Robert Boyle (1627-1691), Irish philosopher, chemist, physicist, inventor, not Susan Boyle, the Scottish singer!]

Because the volume change is negligible over the change of pressure (delta P) and change of temperature (delta T) that we are talking about, and we are not changing the number of molecules (eg by pumping the tyre up), this means (delta P)/P = (delta T)/T.

First, let's suppose your pressure is 34 psi. This is actually pressure above atmospheric pressure so the real pressure is 34+14=48 psi (nb the air in an "empty" tyre, say with no valve in it, is still at 14 psi!).

Second, temperature here is Absolute, measured in degrees Kelvin, so let's suppose the air is 16°C = 272+16=288°K (Absolute).

Then delta P = (P/T)*(delta T) = (48/288)*(delta T)
i.e. delta P = (1/6)*delta T
i.e. 1 psi pressure change for 6°C temp change

[nb: I picked that above pressure and temperature examples to make this come to exactly 6!].

This equates to 1 psi = (9/5)*6 °F = 11°F temp change.

I hope this is not too geeky a posting for this thread (I only joined two days ago) but if my sums are right I have great trouble believing the first quote above.