Oops, Front Tyre Were At 33 Psi

[timberwolf]

I checked the tyre pressures this morning, and the front tyres were only at 33 psi. I normally have the front tyres at 36 or 37 psi and back tyres at 34 or 35 psi.

I haven't checked them for a few weeks, and I can't remember whether I checked them after the dealer service. They haven't varied that much before, could the 5 degree C drop make that much difference to the tyre pressure?

[Chris Dance]

timberwolf: I keep my tyre pressure the same pressures as you have outlined. The drop in pressure is about the same as I have found. I always use the same digital tyre pressure gauge to check the pressure. I have a found a big difference in my electric operated inflator gauge and my foot pump gauge. Hence the use of the digital one but I am not certain that is 100% correct. I find it is easier to use the double barrelled foot pump than run my Battery down using the Michelin electric inflator.

[Sagitar]

I checked the tyre pressures this morning, and the front tyres were only at 33 psi. I normally have the front tyres at 36 or 37 psi and back tyres at 34 or 35 psi.

I haven't checked them for a few weeks, and I can't remember whether I checked them after the dealer service. They haven't varied that much before, could the 5 degree C drop make that much difference to the tyre pressure?

The old rule of thumb was 1 p.s.i for 10 degrees F, but it's a crude measure.

!Removed!-Lussac's law, says that the pressure of a fixed mass and fixed volume of a gas is directly proportional to the gas's temperature.

i.e. Pressure (P) divided by Temperature (T) = constant (k) where:

P is the pressure of the gas.

T is the temperature of the gas (measured in Kelvin).

k is a constant.

So, to do the sums properly, you need to change the temperature to Kelvin and you need to know what the constant is for your particular system. But it doesn't end there. If your tyre is inflated with air, there is almost certainly some water vapour present and water vapour does not behave as an ideal gas so you have to also use the law of partial pressures. And of course the tyre is not entirely rigid, so a change in pressure will alter the contained volume slightly depending also upon the temperature of the tyre - both the bulk modulus and the co-efficient of linear expansion will be altered a little.

It is far easier to check your tyre pressures regularly (once a week is good) and bring them back to the recommended pressure to take account of changes in ambient temperature . . . . . :)

[timberwolf]

Thanks, good to know I am just being paranoid.

As for testing every week, I'm a fair weather/weekend tyre pressure testing kinda of chap i.e. I don't like doing it when it is chucking it down with rain (or hail!).

I use a separate Michelin digital tyre gauge http://www.amazon.co.uk/Michelin-Digital-T...e/dp/B000V5F0XQ but I find that sometimes it is a little tricky to put onto the valve correctly, seems to be the angle that first push it onto the valve, so I am not sure I'd recommend it.

I've never used an electric pump, and I was put off by the reviews saying how noisy they were, plus not wanting to use the Prius' meager 12v supply, and I find my old single cylinder foot pump works okay.

[Sagitar]

I use a separate Michelin digital tyre gauge http://www.amazon.co.uk/Michelin-Digital-T...e/dp/B000V5F0XQ but I find that sometimes it is a little tricky to put onto the valve correctly, seems to be the angle that first push it onto the valve, so I am not sure I'd recommend it.

I know what you mean. I spent my money on one of those and found it a complete pita. I understand it is highly recommended by "Which", but it seems to me to be poorly designed in terms of sealing to the valve. I found myself losing pressure because of the inability to get it to seal properly and went back to using my old analogue guage.

[Chris Dance]

The digital pressure gauge I use has a small brass adaptor which enables the user to get it securely onto the valve. Without the adaptor I would find it imposible to use..

[Sagitar]

The digital pressure gauge I use has a small brass adaptor which enables the user to get it securely onto the valve. Without the adaptor I would find it imposible to use..

Can you tell us more please Chris? Have you got a reference or link to it?

[Chris Dance]

The presure gauge and adaptor were given to me as a gift.

I hope the photo of the gauge and adaptor I have attmpted to attach works. I have used it to check the tyre pressures today no problem except the foot pump packed in. I had to nip down to Halfords and change it for a new one.

[timberwolf]

Sagitar, I like everything else about that gauge (especially the large display that goes in 0.1 psi increments) and it is easy enough for me to hear the hiss of air when it hasn't made a good seal. Last week, of the many readings that I took per tyre, for three corners I had no problem taking readings before and after adding air with the foot pump, but on one wheel the tyre valve happened to be near the top and that was frustrating to find a position for the gauge that would make a good seal.

Chris, thanks for the photo. With my web browser (Opera), I had to reduce the magnification of the page down to 30% before I could view the picture at a sensible size. I am guessing that image was as it was downloaded from the camera? Searching the web, that style of digital gauge seems to have been copied or licensed by a lot of companies although I didn't see the brass adapter. Autoexpress did a comparative review of various digital tyre gauges and they seemed to discover that two otherwise identical looking gauges did not always read the same psi.

[Sagitar]

Sagitar, I like everything else about that gauge (especially the large display that goes in 0.1 psi increments) and it is easy enough for me to hear the hiss of air when it hasn't made a good seal. Last week, of the many readings that I took per tyre, for three corners I had no problem taking readings before and after adding air with the foot pump, but on one wheel the tyre valve happened to be near the top and that was frustrating to find a position for the gauge that would make a good seal.

Chris, thanks for the photo. With my web browser (Opera), I had to reduce the magnification of the page down to 30% before I could view the picture at a sensible size. I am guessing that image was as it was downloaded from the camera? Searching the web, that style of digital gauge seems to have been copied or licensed by a lot of companies although I didn't see the brass adapter. Autoexpress did a comparative review of various digital tyre gauges and they seemed to discover that two otherwise identical looking gauges did not always read the same psi.

Yep, I agree that it would be a brilliant gauge if only it would seal easily.

On my old analogue gauge, there is a sleeve that fits neatly over the valve stem and goes almost to the base of the stem before pin that opens the valve engages. In consequence, the gauge is always square on to the valve stem and needs no fiddling to make it work.

On the Michelin gauge there is virtually no sleeve and it is possible to offer the gauge to the stem at a range of angles. The working bit is at an angle to the body, so it not easy to judge whether the device is square on to the valve stem. The only way to do it is to jiggle it until the air stops escaping, by which time, the pressure may be lower than the required value and I have to use the inflator again. I had four tries at one tyre last week and in the end gave up. Looking at various review sites, it is obviously a common problem.

[timberwolf]

Thanks for the info guys.

The balance of cost to usage frustration has not reached the tipping point where I would like to replace it yet,

but when I do I'll keep these posts in mind.

[Sagitar]

Thanks for the info guys.

The balance of cost to usage frustration has not reached the tipping point where I would like to replace it yet,

but when I do I'll keep these posts in mind.

Your comment made me extract the digit and take it back to Halfords this morning. They were very good about it. Changed it, without a quibble, for a different model at about half the price that seems to work well but doesn't have the super display that the Michelin one did.