[Healeys] Conclave tire air pressure change trivia

[Bob Spidell]

OK, looks like I'm wrong here:

https://www.me.psu.edu/cimbala/Learning/Fluid/Pressure/pressure_basics.htm

BUT:

"Example: Does gage pressure change with elevation?

Solution: No. Since gage pressure is always relative to the /local/ 
value of atmospheric pressure, the gage pressure of the atmosphere is 
always zero, regardless of elevation. Note that /absolute/ pressure 
decreases with elevation, just as water pressure increases with depth. "

So, the difference can only be accounted for by temperature delta.



On 5/24/2021 9:16 AM, Bob Spidell wrote:
> Interesting point. I'm familiar with three types of tire pressure 
> gauges: 1) the 'pencil' type with a sliding 'ruler', 2) the analog 
> type which usually use a Bourdon tube (used in our 'safety' gauges) 
> and 3) digital.
>
> I can't imagine ambient pressure affecting the pencil/slider type; a 
> Bourdon tube could be slightly affected by temperature I suppose, and 
> the digital gauges usually use a pressure/load sensor (like digital 
> scales) and I've not noticed a port to compare with ambient. So, I'm 
> gonna go out on a limb and say gauges measure absolute--not 
> differential--pressure so the ambient/tire delta is a non-factor (but 
> I have no proof).
>
>
> On 5/24/2021 9:05 AM, Bob Haskell wrote:
>> Atmospheric pressure at sea level, 59 degF is 14.7 psia.  At 6,700 ft 
>> and 59 degF, the pressure is 11.47 psia.  If the tire gage is 
>> measuring gauge pressure, the delta between the pressure in the tire 
>> and atmospheric pressure, the change in atmospheric would account for 
>> most of the difference seen.  But I have no idea if the tire gauge 
>> used works that way or not.
>>
>> Cheers,
>>
>> Bob Haskell
>> Austin Healey 3000 BN7/BT7 registrar
>>
>> On 5/24/21 11:36 AM, Bob Spidell via Healeys wrote:
>>> I for one can't think of an explanation for this startling 
>>> phenomenon. Obviously, the ambient air pressure is less at 
>>> altitude--known as 'adiabatic lapse rate' to nerds and 
>>> pilots--which, theoretically could cause the tires to expand, 
>>> increasing their volume and lowering the pressure (according to the 
>>> Ideal Gas Law). But, I doubt steel-belted radial tires expand or 
>>> contract much, but air pressure in fixed volume goes up about 1psi 
>>> per 10degF IIRC.
>>>
>>> Was there a 40degF difference in temperature between Portland and 
>>> Big Bear (California has been pretty warm the last few weeks, which 
>>> I can attest to since our A/C is on the fritz)? Also, temps vary 
>>> when tires are warmed by the sun or friction with the road, so if I 
>>> can't check pressure completely in the shade I'll under-fill the 
>>> tires in the sun by half a degree. So, my guess is a significant 
>>> difference in air temps, and asphalt absorbs a lot of heat which 
>>> gets transferred to the tires (Portland = (often) cloudy and cool; 
>>> SoCal = (often) sunny and hot).
>>>
>>>
>>> On 5/24/2021 8:20 AM, Jean Caron via Healeys wrote:
>>>>
>>>> Sounds like you have one of those rare wheels with self-adjusting 
>>>> pressure, I wonder when these became an option and on what model 
>>>> !!!!!! LOL!
>>>>
>>>> Jean
>>>>
>>>> Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for 
>>>> Windows 10
>>>>
>>>> *From: *richard mayor <mailto:boyracer466 at gmail.com>
>>>> *Sent: *May 24, 2021 9:28 AM
>>>> *To: *Healeys <mailto:healeys at autox.team.net>
>>>> *Subject: *[Healeys] Conclave tire air pressure change trivia
>>>>
>>>> My Healey tire pressures went from 29 psi here in Portland, Oregon, 
>>>> to 33 psi at Big Bear Resort in California - 6,700 feet above sea 
>>>> level.  I then lowered my tire pressures to 29 psi.
>>>>
>>>> Checked my tires today back in Portland - 70 feet above sea level 
>>>> and all tires at 25 psi.
>>>>
>>>>
>>>
>>>
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>>
>
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