Arthur Peabody

I calculated the permeability of my tubes to air, and the numbers seem high. Do you see something wrong with my calculation? I measure my 700x28 tube to have a radius of 1 cm, a thickness of 1 mm, a toroidal radius 33 cm, thus has an area of about 1200 cm^2, a volume of about 600 cm^3. Air is permeable to butyl rubber at 2×10^-9 in units of cm, atmospheres, and seconds. (I rounded all these numbers.) I inflate my tires to 7 atmospheres. From this I calculate that it should lose about 2.5 psi/day, 18/week, which seems like a lot - though it is my experience.

Kontact 

Yup, about that or less. But your tube is inside a tire and a rim, which means that the permeability isn't going to be the same as a tube in open air.

FBinNY 

So, you have a theoretical calculation and empirical experience in agreement, apparently answering your question.

What is the problem?

FWIW it doesn't matter whether it SEEMS like a lot, or a little, or what it should be. It is what it is.

Crankycrank

Agree the above posters and IMO not worth spending the time to find an answer. Also different brands use different grades of rubber which will bleed air at different rates even given all tube measurements are equal and other parameters can also come into play. If your tube goes flat in 3 days, look for a leak. And as FBinNY said "it is what it is" Keep it pumped up as needed and just enjoy the ride.

bblair

Must have been a rainy day and you already read your copy of Bicycling Magazine twice.

A few strokes on the floor pump is part of my preride ceremony, like slathering on sunblock and loading my route.

wheelreason

Sounds about right, but it varies from tube to tube, conditions, and pressure. I've had "identical" tubes, neither with a hole where one held for months, and the other barely a week.

Iride01 

You could do some practical scientific testing with real inner tubes and tires to find out yourself if your calculations match the real world.

My one question on your permeability is if this is for a piece of butyl constrained by a tire or is this just through a butyl membrane with pressurized air on one side and normal atmosphere on the other.

Arthur Peabody

If my calculation is correct, a 'perfect' tube would lose 18 pounds/week, be flat in 5.

I have my own data. Unfortunately I get punctures often, have no spare wheel to mount a tube and tire on to test. I was hoping that someone else had looked into it.

? It's a tube inside tire. A tube can hold about 2 psi if not constrained by a tire. I don't think the tire affects permeation.

I didn't want anyone to spend any time on it. I asked in case someone had.

FBinNY 

The tire most definitely plays into this. Tire walls though permeable, are not completely permeable, and so would slow bleed, How much would depend on the details, but it really doesn't matter because, as I said earlier, "it is what it is". A small daily loss of pressure is normal, flat in a week isn't. Everything in between is a toss up.

I don't understand your last comment, since you apparently did the math, and are also tracking your real world experience, so obviously put some time into it. Likewise, I wonder what you expect to learn here that would trump what you've already learned/observed.

Kontact 

Why would you think that? If I painted the outside of the tube, you would probably agree that would affect how porous the tube is. Why wouldn't pressing the tube firmly against something that is also airtight not also affect air loss?

KerryIrons

As others have noted, the tube is not "seeing" atmospheric pressure, as the tire itself is also a "resistance" to permeation. That said, tubes seem to vary widely in permeability, so there are probably formulation issues that influence this. For example, I use Michelin A1 Airstop tubes with my 25 mm Continental GP 5000 tires, and they lose around 7 psi per week. I marvel at those who say they pump their tires before every ride, as I only need three pump strokes per week to bring my tires back to 6 bar from 5.5 bar. Other sources of slow loss than permeability are leaky valve seals and pinholes.

Iride01 

So you figured your air loss from your actual experience with your tires and built a calculation around that. Now you saying your actual numbers disagree with your calculated numbers? <grin> <funning with you while I try and wrap my head around the entire question and methods.>

My Continental Tire tube in my front wheel loses air faster than my rear wheel with a Bell inner tube. The Bell is a little bit heavier. They don't lose air as fast as the PSI gets lower.

wheelreason

Wow, I'm gonna need a safety stop after all this misconception about pressure...

Bill Kapaun

I question the consistency of the tube wall thickness. Whenever i've chased small leaks by pumping the tube up, it always bulges somewhere first rather than evenly inflating. Something is "inconsistent".
I have identical tires/rims/tubes on my hybrid, but the front loses more air then the rear even though I pump the rear 10 psi higher
It's the first time I've noticed that "reverse" phenomena.

Sy Reene

Isn't there also variability in tube wall thickness? They sell standard thru extra-light versions of tubes that have the same nominal size.

Now, how come nobody does this? Or maybe use something like flex seal.

Kontact 

Probably because it would cause the tube to degrade faster for little benefit.

One thing that may decrease air loss is a teaspoon of water. On hydronic heating systems, leak checks at 100psi are less effective if there is any moisture in the system.

wheelreason

Yeah, I always have a brass P/S adapter on my rear wheel, and hit the wheel with a little gas station air once or twice a year, they seldom bleed their compresssor tanks...

Chuck M 

The bikes I ride regularly seem to lose about 20 to 40 psi over a couple of weeks. The one I ride once in a blue moon never goes completely flat. Rather than overthink it and try to calculate what the pressure is 3.5 days from the time I hung the bike up on the garage wall, I just connect a pump and inflate the tires to the pressure I ride them at. If I plan to ride the bike I rode yesterday and only a short distance, I'll squeeze it and decide if I'm good to go. If I do find one flat when I want to go out and ride, I leave it and take another bike, because there is probably something wrong.

Mathematics is the last thing I will give thought to when going for a ride.

Arthur Peabody

But they don't seal.

That I put some time into it doesn't mean I expect anyone else to. I asked in case they had.

I don't know everything. I make mistakes.

There may be some substance I could paint on a tube that would decrease its permeability; I suspect something on the inside would be more effective, perhaps sealant. Tires aren't airtight.


Mine aren't.


I used in my example butyl rubber. I hope there are better substances.

I calculated theoretical air loss from first principles plus figures for the permeability of butyl rubber to nitrogen and oxygen from the NBS.

Yes. From the model of permeation the thinner the tube the more the permeation. There are thicker tubes too, usually sold as thorn-resistant.

I bet it doesn't work. You'd have to paint it on before you inflated it; the expansion would crack. I doubt FlexSeal would hold up to high pressures.

Kontact 

We aren't talking about the tire as a whole, but whether you can change the leak rate by putting the permeable tube up against surfaces that allow less passage of air than the atmosphere.

If your tube as X surface area, and half of that surface area is in close contact with something of low permeability, how could the tube leak at the same rate it would if it had nothing touching it? It's like a damn - you don't have to block all the water to decrease the flow markedly.

choddo

I think you’ve probably already spent more time writing this post, let alone finding out something you already knew - that tubes and tyres are not 100% airtight - than I have topping up air in the last 5 years.

But this statement

Quote:

Air is permeable to butyl rubber at 2×10^-9 in units of cm, atmospheres, and seconds. (I rounded all these numbers.

seems wrong - I would have expected it to be in cm^3 for the air as well as the cm/mm for the rubber thickness

You can try adding some graphene platelets
https:/onlinelibrary.wiley.com/doi/abs/10.1002/pat.4958

or some plasma oxidation of the butyl rubber surface followed by vapour deposition of SiCl4
https:/patents.google.com/patent/WO2014094123A1/en

KerryIrons

You are revealing that you don't understand how gas permeation works. Each layer (tube, tire) represents resistance to permeation. A non-tubeless tire still is a barrier to gas otherwise you could breathe right through it. Different butyl rubbers are formulated differently (filler, additives, etc.) so just the NBS permeability is just a single point on a spectrum. There are other tube materials than butyl, but it is pretty much the standard.

Arthur Peabody

Air moves between the tube and the tire, can escape anywhere the tire lets it. Every time I get a puncture in a tube there's a puncture in the tire: I patch the tube, not the tire.

But negligibly when talking about the flow from permeation. All the air that gets out of the tube gets through or around the tire more quickly than it escapes from the tube.

1 mm = .1 cm


It is, but negligibly so, compared to the loss from permeation, which is much less than breathing. The bead doesn't seal to the rim, and my tire has holes: every time a tube gets a puncture so does the tire.

choddo

Strangely I am aware of this. It was the lack of cm^3 volumetric units I was querying.

Your comment about the tube and tyre seems to be completely ignoring the fact the tube is pressed up against the interior surface of the tyre at 40-120psi.

Air permeates rubber by dissolving into it at a molecular level and migrating through the material. It will encounter the wall of the tyre and have a similar process, it can’t just wander off to the valve hole or a nearby spoke hole when the tube is fully inflated. (Which also means your calculation for time to deflate needs a bit of calculus)

Arthur Peabody

I gave the units used, not how much of each. The volume doesn't enter in to the rate of permeation, just the area and thickness, though the pressure loss that represents depends on volume.

If that were true I could just not-patch a punctured tube, rotate the tire around a bit, then inflate it. Air presses up against the tube pretty tightly but not so tightly that air molecules can't move between them.