[Healeys] Conclave tire air pressure change trivia

Mon May 24 10:46:04 MDT 2021

I agree that gauge pressure of the atmosphere is always zero.  In 
Portland the tire pressure is 29 psig (gauge) or 43.7 psia (absolute).  
Ignoring temperature effects, I would think the absolute pressure inside 
the tire would still be around 43.7 psia at Conclave. Atmospheric 
pressure at 6,700 ft is 11.47 psia.  So gage pressure would be 43.7 psia 
minus 11.5 psia or 32.2 psig.

It makes sense to me, and that should be my first warning sign...

Cheers,

Bob Haskell
Austin Healey 3000 BN7/BT7 registrar

On 5/24/21 12:21 PM, Bob Spidell wrote:
> 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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