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Old 20 November 2013, 04:10   #21
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Mountain , Queensbury ,Nr Bradford . Tuther side of big hill to you lol
Small world, I grew up 5 mins away from there.
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Old 21 November 2013, 11:40   #22
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if you do the physics you'll see that temperature change on air will make not a huge amount of difference to the pressure. However, any moisture in the tube has the potential to make much larger fluctuations.

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Re: nitrogen fills:
This will help you keep your rib inflated over the course of the year compared with air (wet or dry) if you didn’t otherwise fill or drain it, but won’t help keep daily fluctuations in pressure down as your rib heats up in the sun and cools down in the water.

Re: nitrogen vs dry air vs wet air:
The ideal gas law PV=nRT is king: given a chance to equalise in temperature with the surroundings (the rib tube fabric) , tubes that started at the same pressure and temperature, and filled with any gas will reach the same end pressure eventually .

The key here is ‘given a chance to equalise in temperature’. Typically if a rib is taken out of the water and left in the sun, the type of gas in the tubes will affect the RATE at which the pressure builds, but not the end pressure. Radiant heat from direct sunlight onto dark tubes will be a significant source of heat in addition to the heat from the ambient air. Coupled with the low conductivity of these gases , the equilibrium temperature may be quite high and may not be reached for several hours; the pressure would keep on rising. Reducing the RATE of pressure rise therefore is a good thing so we can get our boat into the shade/get into the water/let the tubes down/wait for the days to cool down before they pop.

Water vapour in the tubes will actually help minimise pressure fluctuations over a short period of exposure to heat by reducing the rate the pressure increases.. So for example if you take your boat out of the cold water, drive the 10 minutes home on a sunny day , and then immediately measure the pressure before putting it in the shade, a boat filled with damp air will not have risen in pressure as much as one filled with nitrogen or air.

Interestingly there would be virtually no difference in pressure in this scenario between a rib filled with dry air and a rib filled with pure nitrogen, and also the tubes filled with water vapour would be lighter to begin with!


Some sample numbers to give an indication of the scales involved (ideal conditions assumed), a typical 5m rib inflated to 2 psi at 15 degrees C, is heated with a 1kw heater for a little under 2 minutes. A rib filled with EITHER nitrogen OR dry air would increase in pressure to ~3.45psi. A rib filled with water vapour would increase to ~3.14 psi - around 20% lower pressure

I attach the calculations for both arguments for scrutiny, and further discussions around nitrogen filling for long term pressure stability.
Attached Files
File Type: pdf nitrogen fill.pdf (563.7 KB, 36 views)
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Old 21 November 2013, 12:51   #23
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'ere Ribgeek dont go spoiling a typical thread on here with " facts " like that !

Yours is a rather odd first post ! Please introduce yourself properly ( ie tell us what your usual aliarse is )
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Old 21 November 2013, 13:23   #24
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'ere Ribgeek dont go spoiling a typical thread on here with " facts " like that !

Yours is a rather odd first post ! Please introduce yourself properly ( ie tell us what your usual aliarse is )
I absolutely agree - it is outrageous for someone who clearly has a good all round physics degree and Phd to to suddenly post such good sense - totally not in keeping with tradition - a first posts should command / attract hoots of derision etc.

Reveal yourself son of Einstein!
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Old 21 November 2013, 16:01   #25
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My money is on Poly! He is stuffed with information and thinks "drawing breath" is when you exhale on cold glass to doodle in the condensate.
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Old 21 November 2013, 17:01   #26
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I absolutely agree - it is outrageous for someone who clearly has a good all round physics degree and Phd to to suddenly post such good sense - totally not in keeping with tradition - a first posts should command / attract hoots of derision etc.

Reveal yourself son of Einstein!
I thought it would attract hoots of derision! it's all just A-level physics this stuff, with wild assumptions thrown in left right and centre (especially the bit about pure water vapour), and using numbers to disprove someone's opinionated post is just cowardice.

Ive been pondering the nitrogen in tyres question for a while, and this post was just the excuse I needed to read a few old text books. The moisture question had me thinking too - as my tubes are old and knackered and full of water and if I could be bothered I could dry them out and just use dry air (scuba tank), and I always worry that they will pop when it is sunny... but I'm not going to bother now - just leave them damp and fill them from the Aldi workshop compressor with no filter on it like I've always done..

I'll still get free nitrogen fills from the local Costco when I get the tyres done as usual - as this has been proven experimentally, but I wouldn't pay for it or worry about topping up with air because I check my tyre pressures regularly and have an airline handy.


Urm, an introduction?
my name's Pete [Hi Pete], and I'm a rib-a-holic. I've been off the two-stroke for 3 years, 2 months and 10 days. Lord of the seas, help me to change the sparkplugs I can change, accept the seized bolts I cannot change, and the wisdom to know the difference between port and starboard.
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Old 21 November 2013, 17:06   #27
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Hi Pete - welcome to RIBnet. You're going to fit in here.


I can tell.
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Old 21 November 2013, 19:42   #28
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My money is on Poly! He is stuffed with information and thinks "drawing breath" is when you exhale on cold glass to doodle in the condensate.
It wisnae me - the comany shareholders decided I had to go to Germany to look after customers and so I am sitting in a crap hotel having just paid a bar bill that would buy a sr4 on eBay (or keep Bigmuz in malt for a week) and been neglecting my duties
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Old 21 November 2013, 20:16   #29
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I have been filling my tubes up for years with 78% Nitrogen ; )
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Old 22 November 2013, 01:54   #30
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...and have an airline handy.
Are you sure you're not Richard Branson under a pseudonym?
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