air temperature in correlation to air pressure

[Ribdude]

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

[zip]

yes it does.

[Searider]

Boyles law iirc.

Higher temperature = higher pressure.

[Poly]

Originally Posted by Searider

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!

[boristhebold]

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.

[Tim in Yorkshire]

Moisture is a big part, especially if you have accidently got some free water in the tubes...

[Max...]

Originally Posted by boristhebold

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.

Originally Posted by Poly

Charles law actually...

Amontons Law?

http://en.wikipedia.org/wiki/Gay-Lus...emperature_law

[Bigtalljv]

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

[office888]

-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.

[Max...]

Originally Posted by office888

-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??

[office888]

Originally Posted by Max...

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??

You are completely correct, sun will raise the material temperature.

Today in Charleston : 87F (30.6C), scattered clouds at 12:30

I used an infrared temperature gauge on both of my inflatable boats.

My FC470 (black hypalon) registered 157F peak (69.4C)
My other boat (light gray hypalon) registered 127F peak (52.8C)

My FC470, has overpressure valves all over the place, so I don't really worry about it. Plus, it has enough seam leaks at pressure (the downside of a beat up, former military hypalon boat that probably got dropped out of an airplane a few times!) where it loses 10% of its pressure in a day until it gets down to about 1.5 psi tube pressure.

My other boat does not have overpressure valves, so I air it down to about 2 psi before I store it.

However, once the boat is in the water, the material will begin to cool, and the pressure will drop dramatically. Same goes vice versa...if you pull your boat up on the beach in the midday sun, the temperature/pressure will increase.

This is why I tell people a pressure gauge & an electric pump is probably their best investment for inflatable boat ownership.

[jyasaki]

Originally Posted by Max...

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.

I just made a run from Albion, CA (near Mendocino) to Oakland; temps went from upper 50's (F) at Albion to about 110F in Booneville, and back to about 80F in Oakland (with a drop to about 70 in Richmond.) Started with a tube pressure of (at a guess) about 2 psi, dropped it again in Booneville (they were really tight); got home with fairly soft saggy tubes. Beats repairing blown seams though.

jky

[Bigtalljv]

That's California. I do the long beach @ 50F and foggy to the valley @110f and back to Ventura @50. Makes for a couple stops to deflate and inflate. I'd be interested to see how that works now that I have a pressure gauge.

Jason

[zip]

What's a good gauge, and how do you put air in your tubes?

[Ribdude]

thanks guys for all the responses, interesting info

[Bigtalljv]

I got this one. It's a low pressure gauge which is what I wanted.

http://rivergear.com/product.php?productid=54

I have an electric air pump now. The fancy two stage 12v one wired in the boat. Can't recall the brand right this second. The popular one on here.

Jason

[Poly]

Originally Posted by office888

-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.

Nah,

P2 = P1 T2 / T1

T has to be in Kelvin.

(for 100F falling to 80F...)

P2 = P1 * 300/311

So the pressure change is 3.6%... probably less than the accuracy of most cheap gauges!

[Max...]

Yes, but looking at the whole picture, that's just the air temp Poly - as ofice888 posted the actual temp the tubes heat up to when exposed to the sun is much more (significant).

[Tim in Yorkshire]

Also have a look at how the partial presure of water changes with temp, apply Henry's law and work out what that does to the overall pressure if you have some liquid water in your tubes.

[jyasaki]

Originally Posted by zip

What's a good gauge, and how do you put air in your tubes?

I have a Bravo 12HP wired into the boat, with enough garden hose tacked on to reach all the valves.

jky