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Old 11 June 2015, 10:28   #21
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Originally Posted by Max... View Post
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).
Its the temperature of the AIR in the tubes not the surface of the fabric that matters. Especially since the tops of the tubes usually become very hot but the sea water cooled bottom does not. If the issue is overpressure it COULD be (part of) the issue after a long time, but for cooling down, if the tubes were filled with "ambient" air it isn't "superheated" instantly.

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Originally Posted by Tim in Yorkshire View Post
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.
Absolutely (although I am not sure Henry is to blame - he was dissolving gasses in liquids) - this is a point I've made in the past (and more subtly further up this thread). Water (vapour or liquid) is a far bigger issues than temperature alone.
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Old 11 June 2015, 11:11   #22
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Yes, but....bear with me... if the 2mm or so of fabric is at silly high temperatures then this will indeed heat the air within via conduction which will quickly raise the air pressure regardless of the outside ambient air temperature. Of course as you say the water underneath is much cooler but a lot more tube is in the sun than in the water...goodness knows how you could calculate this lot but if you leave an inflatable boat in the sun the pressure goes up for sure as we all know.
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Old 11 June 2015, 11:54   #23
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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!
The math is straight from ISO compliance leak down of life rafts and inflatable boats. You will find that math utilized by life raft service stations to diagnose leak down compliance for warranty purposes.
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Old 11 June 2015, 20:24   #24
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The math is straight from ISO compliance leak down of life rafts and inflatable boats. You will find that math utilized by life raft service stations to diagnose leak down compliance for warranty purposes.
A bit of googling suggests you are referring to ISO 9650-1 (2005) s. 6.8.8.1 ? (and the same section of part 2)

In which case this line "The test is only valid if the temperature variation is <= 3 degC." Appears to be important - and underpins the assumptions of their approximation.
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