Let's Talk Temp and Pressure...

[SHigSpeed]

So, since we recently had a quickie refresher on buoyancy and displacement volume, I thought I'd muse a bit about pressure and temperature.

So, the common knowledge is that if you inflate your tube/'toon in the chill of the morning and then leave it out in the sun on the shore, you'll get more pressure.

But how MUCH more pressure?

We'll, the bogeyman has always warned of blown seams and catastrophic failure, but is there a way to prevent this?

Of course it's always been said to let some air out in the heat of the day or as you beach your craft or as you put it in the car/truck. Wise and simple precaution.

BUT, let's run the numbers shall we?

Say you inflate your tube at freezing, 32 degrees F, to a reasonable 4 psi. The day warms up to a crazy 100 degrees due to global warming. The glacial pool you were floating in goes from grayling to peacock bass!

So, your tube hasn't lost any air or gained any air, the volume remains the same since the bag hasn't stretched.

Any first year physics student will tell you that ANY gas (air, nitrogen, xenon, any mix of these) will gain pressure in a linear manner directly proportional to the change in ABSOLUTE temperature.

This absolute temperature is what's important.

Though 32 degrees F SEEMS cold, and will freeze water, the coldest you can get !evar! is -456 F, or 0 Kelvin. So the 32 degrees is actually 273 Kelvin (Kelvin being the absolute temperature scale). So when the temp rose to 100 F, it's actually 311 Kelvin. This change is 311/273-1 is 14% increase in temperature. SO, if the air in your tube/'toon went from 32 to 100, then the pressure went from 4 psi to 4*1.14 = 4.5 psi.

Certainly not a problem. In fact, for it to go to 5 psi, still probably within the tolerance of most if not all tubes (where it's happily rigid - no jokes here! - at 4 psi and starting to sound like a dodgeball to the face when plinked at 5) you'd have to take the air in the tube to 154 F. Probably not unrealistic inside a parked car.

So I guess if you start at a reasonably rigid 3.5 psi at freezing, you're only at 4.7 psi at 200 F!! Probably won't see much higher temps than that.

Anyway, just some interesting facts. If it weren't for liability issues, it'd be really cool if they would make a Boston valve with a 4.5 psi squeaker-leaker to make certain that you never creep over the safe pressure.

BTW, it doesn't matter what material your bladder is for these laws to hold true! <ducking and running!>

_SHig
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[macfly55]

since I am not a physics anything.. I will try to recap in my own words and then ask a question..

basically you are saying that a tube/toon inflated to 4psi will only "expand" to 4.5 psi under a range of conditions from 32 - 200 degrees (air and water temps) .. now with that said. and if my recap is correct.. I have a question.

what affect.. if any .. will altitude have on that same 4psi inflated tube or toon? .. the reason I ask this is that I have seen numerous posts/discussions about the craft failing when it is transported inflated from sea leve or near sea level to a higher altitude.. say less than a 1000 ft.. and in some case over 1000 ft..

MacFly [cool]


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[SHigSpeed]

[quote macfly55]since I am not a physics anything.. I will try to recap in my own words and then ask a question..

basically you are saying that a tube/toon inflated to 4psi will only "expand" to 4.5 psi under a range of conditions from 32 - 200 degrees (air and water temps) .. now with that said. and if my recap is correct.. I have a question.

what affect.. if any .. will altitude have on that same 4psi inflated tube or toon? .. the reason I ask this is that I have seen numerous posts/discussions about the craft failing when it is transported inflated from sea leve or near sea level to a higher altitude.. say less than a 1000 ft.. and in some case over 1000 ft..

MacFly [cool]

[/quote]

Good question!

Though if you read my previous comments again, the 32 - 100 swing takes the tube from 4 to 4.5. The 32 - 200 went from 3.5 to 4.7.

At any rate...

Let's say a 'toon at sea level has 4 psi relative to atmospheric pressure, which is normally around 14.7 psi on a normal clear day. Basically, the air outside the bag is pushing in at 14.7 psi, the air inside is pushing out at 18.7. The difference is the apparent, or "gauge" pressure. So if you took that same 'toon into space (no atmospheric pressure), a gauge would indicate 18.7 psi.

That said, the atmospheric pressure at 1000 ft is around is 14.1 psi (normal clear day again) so if it was at 4 psi at sea level, it'll show 4.6 psi. At 2000 ft it's 13.6 so the tube is now showing 5.1 psi. 3000 is 13.2. Now add to that temperature and you'll definitely be in trouble.

So if I go from my home at sea level to my favorite alpine lake at 7000 ft (11.3 psi), I'll have to keep the inflation at home to only an amazing 0.6 psi at home or I'll exceed 4 psi. This is somewhat misleading though as at 0.6 psi you probably won't achieve full inflation (rigid skin) so realistically much of the extra 3.4 psi will go into fully filling out the tube, and not increasing the pressure (hope that makes sense...).

_SHig
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[flygoddess]

The biggest noticiable change I have experienced is when I head to the hills so to speak.
Not using a gauge on my tube, It is slightly inflated in the back of a 6' truck bed with a 9' tube to the point that the tips of the tune are folded in (a shell on the truck.
Being 4,226 FT here, and the destination is 10,000 ft. by the time we are there, the tubes are pressed hard against the window of the shell.
Now we take them out and set them in the water (which temp depends on the heat of the week or so). Generally we have to give them 6 to 10 more pumps to fill.
I have done this with a trailer which is tuffer because you do have to have the TOON somewhat full to strap down. We have to stop half way up and let a little air out or the straps could cause damage.
Again the OUTER cover is the biggest factor and how it is sewn or welded.

I am not an engineer, it is all trial and error, and common sense.
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[macfly55]

yes.. that made a lot of sense.. so as altitude increases the "gauge" reading on the tube will increase (to a point anyway)...

... and yes I found it amazing that at 7000 ft you would start your tube at 0.6 psi to ensure that it does not go over the 4 psi and that altitude..

.. so .. wouldnt it be easier, and maybe safer, to not inflate your toon at all until you get to your higher altitude destination.. and then fill it??

MacFly [cool]
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[SHigSpeed]

Yup, I'd say not to PRESSUREIZE it until you get there, but you can still FILL it before you leave.

Easy thing would be to fill the 'toon/tube 'till just before it goes fully rigid. At that point, you basically have zero gauge psi inside, and under 14.7 psi absolute inside as the outside pressure is pushing the bag in until the pressures reach equilibrium.

This way, if you go from sea level at 14.7 psi to 10,000 feet at 10.1 psi, you'll gain a MAXIMUM of 4.6 psi, but much of that reduced atmospheric pressure will simply go towards filling out the max volume of the bag until rigid. Only then will the gauge pressure actually rise.

All of the warnings and calculations for temperature/elevation change only fly if you start off with a fully inflated bag at max volume.

_SHig
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[macfly55]

okay ya got me going on this.. so I have to ask to clarify...

what is the difference between pressurising it and filling it?

MacFly [cool]
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[SHigSpeed]

Filling is making it CLOSE to final shape, but there's really no pressure in it. The skin isn't taut. The air inside is the same gauge pressure as the air outside. Basically, if there wasn't gravity to cause the thing to flatten, if you pulled the plug, it wouldn't "exhale".

Pressurizing it starts when the skin is taut (in an ideal world, there'd be no stretch to the bag and once you started pressurizing, the volume would remain the same) and at this point the thing WOULD exhale if you pulled the plug (no gravity again).

When you do just a fill, the absolute change in atmospheric air pressure is the MAX increase in gauge pressure you'll see. Real world it will be much less as much of the initial expansion of the air inside will go towards tightening the skin and only then will the gauge pressure start to rise pound for pound with the reduction of atmospheric pressure outside of the skin.

_SHig
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[macfly55]

so what I got out this one is.. FILL is the point where if you open the valve it will not hiss at you and start to deflate (well maybe slowly deflate).. and once you got to your destination (higher altitude in this case).. you would finish pressurizing it (add more air until it is taut and ready to do into the water..

MacFly [cool]
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[SHigSpeed]

Kinda... You'll never be able to crack a valve without it hissing because the weight of the bag pressurizes is somewhat.

BUT, as long as there are some waves in the fabric, you should be fine.

_SHig
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[macfly55]

I for one want to thank you for all the great info and insight.. I learned a lot from your explanations..

MacFly [cool]
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[SHigSpeed]

No problem man! I want to feel like I didn't go to school for nothing!

Besides, my wife just doesn't get me when it comes to fishin'.

_SHig
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[macfly55]

lol.. my wife keeps asking when Im going to go fishing next.. and then of course she is handing me that infamous " honey do list" while she asks that question.. [][]

MacFly [cool]
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