Re: [Healeys] Conclave tire air pressure change trivia

[Bob Spidell <bspidell@comcast.net>]

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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
>>>>
>>>> *Sent: *May 24, 2021 9:28 AM
>>>> *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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    OK, looks like I'm wrong here:<br>
    <br>
<a class="moz-txt-link-freetext" 
href="https://www.me.psu.edu/cimbala/Learning/Fluid/Pressure/pressure_basics.htm";>https://www.me.psu.edu/cimbala/Learning/Fluid/Pressure/pressure_basics.htm</a><br>
    <br>
    BUT: <br>
    <br>
    "Example: Does gage pressure change with elevation? <br>
    <br>
    Solution: No. Since gage pressure is always relative to the <i>local</i>
    value
    of atmospheric pressure, the gage pressure of the atmosphere is
    always zero,
    regardless of elevation.
    Note that <i>absolute</i> pressure decreases with elevation, just
    as water
    pressure increases with depth. "<br>
    <br>
    So, the difference can only be accounted for by temperature delta.<br>
    <br>
    <br>
    <br>
    <div class="moz-cite-prefix">On 5/24/2021 9:16 AM, Bob Spidell
      wrote:<br>
    </div>
    <blockquote type="cite"
      cite="mid:3fe801aa-1279-75be-2757-9f1adf0e7427@comcast.net">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.
      <br>
      <br>
      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).
      <br>
      <br>
      <br>
      On 5/24/2021 9:05 AM, Bob Haskell wrote:
      <br>
      <blockquote type="cite">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.
        <br>
        <br>
        Cheers,
        <br>
        <br>
        Bob Haskell
        <br>
        Austin Healey 3000 BN7/BT7 registrar
        <br>
        <br>
        On 5/24/21 11:36 AM, Bob Spidell via Healeys wrote:
        <br>
        <blockquote type="cite">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.
          <br>
          <br>
          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).
          <br>
          <br>
          <br>
          On 5/24/2021 8:20 AM, Jean Caron via Healeys wrote:
          <br>
          <blockquote type="cite">
            <br>
            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!
            <br>
            <br>
            Jean
            <br>
            <br>
            Sent from Mail
            <a class="moz-txt-link-rfc2396E" 
href="https://go.microsoft.com/fwlink/?LinkId=550986";>&lt;https://go.microsoft.com/fwlink/?LinkId=550986&gt;</a>
 for
            Windows 10
            <br>
            <br>
            *From: *richard mayor <a class="moz-txt-link-rfc2396E" 
href="mailto:boyracer466@gmail.com";>&lt;mailto:boyracer466@gmail.com&gt;</a>
            <br>
            *Sent: *May 24, 2021 9:28 AM
            <br>
            *To: *Healeys <a class="moz-txt-link-rfc2396E" 
href="mailto:healeys@autox.team.net";>&lt;mailto:healeys@autox.team.net&gt;</a>
            <br>
            *Subject: *[Healeys] Conclave tire air pressure change
            trivia
            <br>
            <br>
            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.
            <br>
            <br>
            Checked my tires today back in Portland - 70 feet above sea
            level and all tires at 25 psi.
            <br>
            <br>
            <br>
          </blockquote>
          <br>
          <br>
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