Innertube Porosity?

Joe Riel wrote:
I notice the opposite effect, which I find curious. That is, a
completely deflated inner tube, with its valve (presta) tightly
closed, partially reinflates after several days if it is not tightly
compressed. That is, when I attempt to roll it back up I have to
again loosen the valve to let out some air. What causes this? Is it
leakage at the valve? This occurs on more than one brand of butyl
inner tube.

Paul Hobson wrote:
Tubes lose air because they are under high pressure relative to the
atmosphere, which causes diffusion from the tube back to its
surroundings. If you push out all of the tube and seal it, now the
atmosphere is at a higher pressure than the tube. So it would make
sense that air would diffuse back into the tube, right?

Agree. I think the unspoken bit here is that some tubes are
'flatter' or 'rounder' than others. If you cut out the
valve, what cross section does it display? Tubes which lie
flat in that test would be less likely to expand when left
laying around I think than tubes which 'want' to be round in
cross section.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

A Muzi writes:

Joe Riel wrote:
I notice the opposite effect, which I find curious. That is, a
completely deflated inner tube, with its valve (presta) tightly
closed, partially reinflates after several days if it is not tightly
compressed. That is, when I attempt to roll it back up I have to
again loosen the valve to let out some air. What causes this? Is it
leakage at the valve? This occurs on more than one brand of butyl
inner tube.

Paul Hobson wrote:
Tubes lose air because they are under high pressure relative to the
atmosphere, which causes diffusion from the tube back to its
surroundings. If you push out all of the tube and seal it, now the
atmosphere is at a higher pressure than the tube. So it would make
sense that air would diffuse back into the tube, right?

Agree. I think the unspoken bit here is that some tubes are 'flatter'
or 'rounder' than others. If you cut out the valve, what cross section
does it display? Tubes which lie flat in that test would be less
likely to expand when left laying around I think than tubes which
'want' to be round in cross section.

Here's some idle speculation. What drives air molecules through the
rubber? Diffusion? Does the rate of gas diffusion through a material
depend on pressure?

In an inflated tube, constrained by the tire, the rubber of a properly
sized inner tube is in compression. That implies that any pores in
the rubber should be closed tighter than they would be if the tube
were not inflated. If the rate of diffusion is not dependent on the
pressure of the gas (independent of the compression), then the rate
should be slower as the pressure increases because the compression is
increasing, causing the pores to close. That would imply that the
rate is greatest with an uninflated tube.

Not sure I believe any of that...maybe someone who knows something
about this can enlighten me.

Joe Riel

Joe Riel wrote:
I notice the opposite effect, which I find curious. That is, a
completely deflated inner tube, with its valve (presta) tightly
closed, partially reinflates after several days if it is not tightly
compressed. That is, when I attempt to roll it back up I have to
again loosen the valve to let out some air. What causes this? Is it
leakage at the valve? This occurs on more than one brand of butyl
inner tube.

Paul Hobson wrote:
Tubes lose air because they are under high pressure relative to the
atmosphere, which causes diffusion from the tube back to its
surroundings. If you push out all of the tube and seal it, now the
atmosphere is at a higher pressure than the tube. So it would make
sense that air would diffuse back into the tube, right?

A Muzi writes:
Agree. I think the unspoken bit here is that some tubes are 'flatter'
or 'rounder' than others. If you cut out the valve, what cross section
does it display? Tubes which lie flat in that test would be less
likely to expand when left laying around I think than tubes which
'want' to be round in cross section.

Joe Riel wrote:
Here's some idle speculation. What drives air molecules through the
rubber? Diffusion? Does the rate of gas diffusion through a material
depend on pressure?

In an inflated tube, constrained by the tire, the rubber of a properly
sized inner tube is in compression. That implies that any pores in
the rubber should be closed tighter than they would be if the tube
were not inflated. If the rate of diffusion is not dependent on the
pressure of the gas (independent of the compression), then the rate
should be slower as the pressure increases because the compression is
increasing, causing the pores to close. That would imply that the
rate is greatest with an uninflated tube.

Not sure I believe any of that...maybe someone who knows something
about this can enlighten me.

Yes I believe tubes permeate air to a greater or lesser
extentwith afew variables liek material, wal thickness,
pressure.
To your comment about the tube being 'in compression', don't
you think in most cases a bigger factor is that the tube is
simply larger in use when it fills the space inside the casing?

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

Someone wrote:
Sandy wrote:

From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post

(she's referring to Le Cycle)

--
Paul M. Hobson
Georgia Institute of Technology
..:change the f to ph to reply:.

Dans le message de ,
Paul Hobson a réfléchi, et puis a déclaré :
Someone wrote:
Sandy wrote:

From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post

(she's referring to Le Cycle)

Please do not tamper with my gender !
--
Bonne route !

Sandy
Verneuil-sur-Seine FR
--
Les faits relatés ici ne sont que pure fiction, et ne sauraient être
utilisés ou rapprochés d'une situation réelle existant ou ayant
existée

Paul Hobson wrote:
Someone wrote:
Sandy wrote:

From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post

I also assumed that the source was Le Cycle, but the source should have
been stated in the text.

--
Tom Sherman - Fox River Valley

Someone wrote:
Paul Hobson wrote:

Someone wrote:

Sandy wrote:

From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post


I also assumed that the source was Le Cycle, but the source should have
been stated in the text.


Oh...maybe you missed in controversial thread involving this article.
apologies

--
Paul M. Hobson
Georgia Institute of Technology
..:change the f to ph to reply:.

Sandy wrote:
Dans le message de ,
Paul Hobson a réfléchi, et puis a déclaré :

Someone wrote:

Sandy wrote:


From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post

(she's referring to Le Cycle)


Please do not tamper with my gender !

Sorry. The only Sandy I know in real life was a female.

\\paul

Paul Hobson wrote:
Someone wrote:
Paul Hobson wrote:

Someone wrote:

Sandy wrote:

From its November 2005 issue :

Winner overall : Décathlon butyl (1,5€)
Loser overall : Challenge latex (10€)


INDEFINITE PRONOUN ALERT!

What is "its"?


Look at the the subject line of your own post


I also assumed that the source was Le Cycle, but the source should
have been stated in the text.


Oh...maybe you missed in controversial thread involving this article.
apologies

Regardless of that, what happened is that Sandy added "Le Cycle" to the
/Subject:/ line, and many people don't notice stuff like that when reading
threads. (I, for one, often don't even take note of WHO posts something
unless something calls attention to it.)

It was easy to miss, but also easy to figure out once the "indefinite
pronoun" was spotted.

Bill "slow morning, obviously" S.

"Paul Hobson" wrote: Sorry. The only Sandy I know in real life was a
female.
^^^^^^^^^^^^^^^^
My condolences to her family.