08-06-2008, 04:18 AM
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
[signature]
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
[signature]