Originally Posted by Polwart
Boyle's law won't explain the problem fully though. (Actually its not Boyles law that applies anyway - Boyle's law is the relationship between P and V at constant temp).
Arrrgghh - as a diver I should have got this right
Please don't tell the BSAC
Originally Posted by Polwart
Even with an extreme temperature rise achievable with sunlight (especially in the UK) you can't get the sort of pressure increase that it would take to burst tubes just from air/heat. A temperature fluctuation of maybe 70 K is about the most that's likely - that is going to cause a pressure change of 25% or so - not enough to blow tubes unless you were already overinflated (and a big stuff would have done for you).
So pressure rises (and tube failures) associated with sunlight must be caused, at least in part, by some other factor, possibly:
if tubes are filled at high humidity and then allowed to cool the moisture will condense reducing the pressure. If they are then topped up then when they warm up the moisture evaporated causing a pressure increase. Likewise with any sea/rainwater that finds its way inside the tube during filling/repair.
(b) Heat softening the glue on seams.
If tubes get seriously hot then potentially you weaken the glue - then with (or even without) a slight pressure rise it goes pop.
(c) Atmospheric pressure.
still not going to burst a tube - but its part of the fluctuation.
the point of this post is that (a) and (c) affect all colours of tube much the same...
...oh and in Durham - you don't get enough sun to worry about it!
I really do not want to get into a drawn out public debate on this but .......nowhere in my original post on this subject did I mention tubes getting hot enough to burst like an over inflated childs balloon. I only stated that "pressure relief valves will be more advantageous in dark coloured tubes." because darker tubes get hotter and suffer a greater pressure increase. It is my belief that this will stress elements of the tubes more, seams, materials etc. which over an extended period may have a negative effect on the tubes.
To be fair, you point out yourself that softening of the glue is one possiblity. With temperature and pressure cycling this softening may take on a permanent aspect? I dare say there are a number of other effects that could occur, but not being an expert in these matters, I would not know what they are. Maybe one of our trade members may be able to advise?
What I am reasonably certain of is that raised temperatures are bad; bad for the adhesives, and bad for the tube materials. Further, temperature cycling does seem to be even worse than constant elevated temperatures for many materials. I am assuming that you do not contest the point that darker tubes will get hotter? This being the case if there is any way that I can minimise these effects on my boat, even in a small way, I will take it. Hopefully then if my tubes are 'on the cusp' of catastrophic failure, I may just get away with it, but more importantly, my tubes should not be degraded quite so badly as others over a period of time, with the benefits that this will bring.
My original post was meant to help a fellow rib owner with tube colour choice; the other factor that I mentioned was the effects of exposure to the elements of different tube colours. With these factors in mind I stand by my original post, to the effect that IN MY OPIINION, grey is one of the better tube colours, if not the best.
Oh, and yes, my tubes are predominantly grey, and fitted with over pressure valves.
but I will not hold it against you if your tubes are another colour