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Old 28 September 2012, 12:54   #1
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Tube Pressure Calculation

I know this is a subject that's been flogged to death but if I can find an answer to this then I'm sure it could be useful to lots of people. Hearing about a few seams going this year due to the hot weather got me worried, I tend to deflate the tubes when I leave it but always let far too much air out.

The question is quite simple, if I inflate my 6m RIB to a 2psi when the air temperature is 20C then what will the tube pressure be at 30, I'm sure the answer depends on a fair few factors, diameter of tubes, number of chambers and so on but a rough guide could save a heap of time putting air back in I needn't have let out in the first place.
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Old 28 September 2012, 13:12   #2
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If the air in the tubes went up to 30C due to heat conduction from the outside air then the pressure would rise by 3.4% according to the ideal gas laws (which are a good approximation at these pressures and temperatures).

In a fixed volume, the pressure is proportional to the absolute temperature.

Absolute temperature in Kelvin = temperature in C + 273.15

hence new pressure = 2 * (30 + 273.15) / (20 +273.15)

The problem is that the air inside the tubes is being heated beyond the outside temperature by radiation in a similar manner to a greenhouse. The maximum inside temperature is much more difficult to predict and is influenced by many factors such as the amount of radiation that is reflected - ie. the colour of the tubes. Darker tubes reflect less/absorb more and will be more likely to become over pressured. My tubes are dark blue and I have relief valves and these often blow even at this latitude (20 miles north of Trump's new golf course!).

Richard
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Old 28 September 2012, 13:13   #3
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I kept mine at 2 psi in Italy thus summer and it got to 35+ at times and they didn't increase that much and that was using a pressure gauge
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Old 28 September 2012, 13:15   #4
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P1*V1/T1 = P2*V2/T2

So if you assume volume is constant (not quite, fabric does stretch a little bit).

2/20 = X/30
X= 3psi

The problem is the air temp inside the tubes is not the same as the 30C outside temp. Its more like the temp inside your closed up auto on a hot day. (50C?)
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Old 28 September 2012, 14:32   #5
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So surely the pressure outside the tube ( ie the normal atmoshpheric pressure ) plays a part ?. It been a long time since I did the sums...but isn' that why weather ballons get very big as they go up ?
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Old 28 September 2012, 14:39   #6
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Seas tend to be near sea level, and weather does not change atmospheric pressure that much, if it does your rib pressure is the least of your worries.....

Is the Rib on water or on beach/trailer when you leave it? On water is not as bad due to water cooling some of the tubes, as we all know.
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Old 28 September 2012, 14:54   #7
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My boat sits on the trailer but it has a heavy dark blue cover, on a hot sunny day the tubes are baking when I first take the cover off, even this time of year, in the morning the side that's had the sun on it is pretty firm and the other can feel almost empty and loose.

I was wondering if the pressure could rise by 1psi, or even 2 as that what some release valves release at (4psi), I guess the thing to do is to get out my thermometer and pressure gauge and take some measurements.
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Old 28 September 2012, 15:27   #8
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Quote:
Originally Posted by simsy View Post
Seas tend to be near sea level, and weather does not change atmospheric pressure that much, if it does your rib pressure is the least of your worries.....
Driving to altitude has a HUGE effect however.
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Old 05 October 2012, 11:42   #9
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[QUOTE=captnjack;490620]P1*V1/T1 = P2*V2/T2

So if you assume volume is constant (not quite, fabric does stretch a little bit).

2/20 = X/30
X= 3psi



Formula correct, units wrong. Temperature must be absolute - meaning relative to absolute zero 0K or -273.15C or -459.67F.

Add 273.15 to each denominator and see me after class.

Gay-Lussac's law. P proportional to T at constant V, T absolute.

Richard
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Old 05 October 2012, 13:10   #10
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As Richard says the direct temp effect on pressure is fairly small even if the average temp rose to 50ppm deg c (more likely to be just the surface temp of the tube).

However there are two effects that doesn't consider. 1. The gas is likely moist and therefore not ideal, the difference between minute condemning out and arising could be huge. 2. As tubes heat up blue softens which weakens it. Failure is more likely and a small amount of extra pressure might help it on its way.
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