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Old 09 June 2015, 22:32   #1
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air temperature in correlation to air pressure

does ambient air temperature have an affect on how much air pressure is in the tubes?

I pulled the boat out of storage today, I pumped my tubes full of air it was around 100 degrees then the temp dropped down to the 80s and the tubes were deflated (these are wing tubes)

the tubes are not damaged and I dont hear/feel any leaks coming out of the valves
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Old 09 June 2015, 22:43   #2
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yes it does.
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Old 10 June 2015, 02:59   #3
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Boyles law iirc.

Higher temperature = higher pressure.
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Old 10 June 2015, 03:18   #4
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Quote:
Originally Posted by Searider View Post
Boyles law iirc.

Higher temperature = higher pressure.
Charles law actually, but it doesn't explain what people see with tubes. The change in absolute pressure from Charles law alone in going 80 to 100F is tiny. Other issues are at play... Moisture (which is not an ideal gas) and atmospheric pressure changes. The really big issue is that on very hot days the glue can soften which combined with a pressure increase will cause a pop!
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Old 10 June 2015, 03:25   #5
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Yes significantly, which is why its best not to inflate your tubes to a very high level in the morning if you know the weather will warm up considerably in the afternoon. especially if your not with the boat all day.

Ive seen lots of posts about pressure and the use of pressure guages, I dont use a pressure guage, I just feel if the tubes seem OK when I go out in the morning, if they feel overly hard after the air has warmed up (which doesn't happen that much in the UK) then I may let some air out if sitting in the sun.

Other than that I dont generally worry about it, my tubes are never rock hard and never overly soft so I perhaps have the right balance for the weather here. I would expect in the med for example people have to be a bit more careful.
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Old 10 June 2015, 07:54   #6
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Moisture is a big part, especially if you have accidently got some free water in the tubes...
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Old 10 June 2015, 08:12   #7
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Yes significantly, which is why its best not to inflate your tubes to a very high level in the morning if you know the weather will warm up considerably in the afternoon. especially if your not with the boat all day.

Ive seen lots of posts about pressure and the use of pressure guages, I dont use a pressure guage, I just feel if the tubes seem OK when I go out in the morning, if they feel overly hard after the air has warmed up (which doesn't happen that much in the UK) then I may let some air out if sitting in the sun.

Other than that I dont generally worry about it, my tubes are never rock hard and never overly soft so I perhaps have the right balance for the weather here. I would expect in the med for example people have to be a bit more careful.
You really *do* need to use a gauge IMHO - certainly with an airfloor SIB (not so much with RIB's) where it is critical for performance and the last 1/2 psi in the floor really makes all the difference.

As an example - we went out Saturday and anticipating the effect of the still very cold sea I went with just under 1/2 psi extra in the floor and tubes to compensate (so around 3.9 psi in the tubes). All fine and dandy and the boat performed great.

After a hour stop for lunch 3 hrs after launch and with the boat sitting in the sun I stuck the gauge on the tubes and they were approaching 5.

Sure you can ping the tubes or bend the cones etc but it's very hard to judge properly and it really is worth using a proper accurate gauge to be sure.

Quote:
Originally Posted by Poly View Post
Charles law actually...
Amontons Law?

http://en.wikipedia.org/wiki/Gay-Lus...emperature_law
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Old 10 June 2015, 10:25   #8
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I've tried to run the calculations on Guy-Lussac's law and it says that a 40 or 50 degree F temperature swing is good for a small pressure change. Maybe half a PSI ? I don't recall exactly. Anybody better at physics?

Jason
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Old 10 June 2015, 11:30   #9
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-1 degree Celsius = -4 millibar pressure change.

100F to 80F = 6.666 Celsius. Thus, your boat should have lost 26.664 millibar of pressure (or 0.39 psi).

Same goes for barometric pressure. -1 millibar of barometric pressure = +1 millibar of tube pressure.
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Old 10 June 2015, 12:10   #10
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-1 degree Celsius = -4 millibar pressure change.

100F to 80F = 6.666 Celsius. Thus, your boat should have lost 26.664 millibar of pressure (or 0.39 psi).

Same goes for barometric pressure. -1 millibar of barometric pressure = +1 millibar of tube pressure.
That's pretty significant, here in the UK it can easily (well, alright once in a blue moon) be say 12 deg C first thing when you check tubes in the morning and double to 25 deg C early afternoon. That's just the measured air temperature though, surely the direct sun acting directly on the tubes will raise it more??
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