Dinghy Pressure due to temp?

[Cheechako]

Originally Posted by CAELESTIS

Ok, engineers, if I have a 10 RIB inflated to 3.5psi at 70*F, what is the psi at 90*? What about 115*F?

Thanks!

I dont know but I just made sure mine was good and hard at full sun in the day. It would be a bit slack every morning for a bit.

[C420sailor]

Originally Posted by jlcarreiro

Does the water temp factor in?

Depends. Where I’m at (US NE) a properly inflated dink on deck will shrivel once placed in the water.

As such, I prefer to inflate on deck, then top off a few minutes after splashing.

When it comes out, I let some air out before it heats and expands.

[Ded reckoner]

Originally Posted by Clivevon

How is that working in terms of protecting against UV damage? Is the paint flaking off? What kind of paint did you use exactly?

There is a product we've used for the last couple of years, 303 Aerospace Protectant, which claims to provide UV protection. We apply it periodically, more in the summer months.

[betwys1]

Originally Posted by CAELESTIS

Ok, engineers, if I have a 10 RIB inflated to 3.5psi at 70*F, what is the psi at 90*? What about 115*F?

Thanks!

Before Faraday, balloons were animal bladders.
After Faraday, it was finally noticed that adding volume to a latex balloon variously increased, decreased or held constant the pressure differential to the atmosphere, which was a smack in the face to people who sought insight from ‎Boyle, ‎Charles & ‎Gay-Lussac.
If pressure times volume is not constant at constant temp for a toy balloon, then I suggest a value for the Young's modulus of the hypalon/neoprene laminate is needed, or even better, an experimental determination of the Pressure v Temp relation for your hard bottom inflatable might be the most accurate way to go. As sea water is almost always cooler than air temperature in a cruising situation, over pressure on deck seems to be the principal danger, and something like a sheet of aluminized polyester film "space blanket" might be helpful to hold the surface temp down.

[StuM]

Originally Posted by betwys1

Before Faraday, balloons were animal bladders.
After Faraday, it was finally noticed that adding volume to a latex balloon variously increased, decreased or held constant the pressure differential to the atmosphere, which was a smack in the face to people who sought insight from ‎Boyle, ‎Charles & ‎Gay-Lussac.

Why a smack in the face? Anyone who understood what Boyle,Charles ad Gay-Lussac were saying knew that volume is one factor of those gas laws and than the changing volume of an expandable container was a fundamental aspect of the relationships. In fact a change in volume with pressure in an elastic container is at the heart of one of those laws.

If pressure times volume is not constant at constant temp for a toy balloon, then I suggest a value for the Young's modulus of the hypalon/neoprene laminate is needed

The expansion of the hypalon/neoprene walls of an typical inflatable or RIB is negligible over the pressure range being talked about (0 - 10PSI. above atmospheric)

or even better, an experimental determination of the Pressure v Temp relation for your hard bottom inflatable might be the most accurate way to go.

I'll bet that if you do that, it will be extremely close to theory.

[JimZeigler]

It is Gay-Lussac law (P1/T1 = P2/T2). Convert temperatures to Kelvin and convert to absolute pressure. The 3.5 psi is pound per square inch GAUGE. The true pressure is 18.2 pounds per square inch ( 3.5 push 14.7). So I end up with 4.2 psig at 90 F and 5.1 psig at 115 F.

[betwys1]

Originally Posted by StuM

Why a smack in the face? Anyone who understood what Boyle,Charles ad Gay-Lussac were saying knew that volume is one factor of those gas laws and than the changing volume of an expandable container was a fundamental aspect of the relationships. In fact a change in volume with pressure in an elastic container is at the heart of one of those laws.




The expansion of the hypalon/neoprene walls of an typical inflatable or RIB is negligible over the pressure range being talked about (0 - 10PSI. above atmospheric)

I'll bet that if you do that, it will be extremely close to theory.

Not being a betting man, I looked up the mechanical properties of hypalon in

Nicholas P. Cheremisinoff Ph.D., in Condensed Encyclopedia of Polymer Engineering Terms, 2001

CHLOROSULPHONATED POLYETHYLENE
More commonly known as Hypalon. This is a special purpose rubber formed by substituting chlorine and sulphonylchloride groups into polyethylene. General properties include moderate mechanical properties; excellent resistance to ozone, oxidation, weathering, and oxidizing chemicals. Its upper temperature limit is ca. + 150 °C. Hypalon® or Chlorosulfonated Polyethylene Rubber is made by DuPont-Dow. It is an elastomer and is a thermoset material. There are many grade variations of this specialty elastomer. A typical grade is 29 % chlorine content and 1.4 % sulfur content. Its neat product form is white, odorless chips. A distinguishing feature is that it is readily soluble in common solvents. The polymer has the following characteristics:

1.good low temperature flexibility,
2.very good abrasion resistance,
3.good chemical resistance,
4.fair compression set resistance,
5.fair flame resistance,
6.very good heat resistance,
7.excellent ozone resistance,
8.fair petroleum oil resistance,
9.excellent weather resistance,
10.good low temperature properties, and
11.fair tear strength.

In terms of processability, it is described as demonstrating fair extrusion, good molding, and fair calendering. Some general physical properties are reported in Table 1, however the reader should examine individual grades. The physical property data below (except viscosity and density) are for vulcanizate.

Table 1. Average Physical Properties of a Hypalon® Grade

Properties Values Comments US/other units
Density, g/cc 1.12 1.12 g/cc
Hardness, Shore A 70 45-95. ASTM D2240-81 70
Viscosity 28 Mooney Viscosity ML 1 + 4 28 at 100 ° C. ASTM D1646-81 28
Tensile Strength, Ultimate, MPa 20.6 Upper limit with carbon black stocks. ASTM D412-80 2,987 psi
****************************************
If you think a plastic with an ultimate 3000 psi conforms to an equation like PV/T = constant, I urge you to look at the P vs V graph for a typical party balloon, which as you may know, operate at even lower excess pressures.

Here is a sample relation - fig 2 here:
https://en.wikipedia.org/wiki/Two-balloon_experiment

Tell me is this is extremely close to theory!

[StuM]

Originally Posted by JimZeigler

It is Gay-Lussac law (P1/T1 = P2/T2). Convert temperatures to Kelvin and convert to absolute pressure. The 3.5 psi is pound per square inch GAUGE. The true pressure is 18.2 pounds per square inch ( 3.5 push 14.7). So I end up with 4.2 psig at 90 F and 5.1 psig at 115 F.

See Posts #7. More like 5.0 @ 115°F
Pressure @ 115°F = (18.2 * 319.261 /294.261 - 14.7 = 5.046

[StuM]

Originally Posted by betwys1

If you think a plastic with an ultimate 3000 psi conforms to an equation like PV/T = constant, I urge you to look at the P vs V graph for a typical party balloon, which as you may know, operate at even lower excess pressures.

Tell me is this is extremely close to theory!

If you think that the 3000PSI tensile strength of hypalon or the expansion of a thin party ballon has any relevance to the behaviour under low pressure (up to 10PSI) of a thick tube made of hypalon covered multi component FABRIC , you need to think again.


"Hypalon fabric is used extensively to make high end products in the marine industry. It's outer layer of CSM provides resistance to UV rays, abrasion, hydrocarbons and extreme temperatures. The polyester weave gives the fabric high tenacity and tear resistance which in turn provides good mechanical resistance. The neoprene in the fabric provides fire/heat resistance and also seals the fabric to make sure it is air tight."


(Pease excuse the spelling of "tenecity" in the attachment, it's not my image

[Tusindtak]

Jim Ziegler's post #36 is correct, assuming the fabric stretches minimally.

[Redline452]

While I am an engineer, I try to temper that with a bit of a practical side ;-), so I'd say that if you are equipped to measure the fact that



"I have a 10 RIB inflated to 3.5psi at 70*F"


then take out your pump and pressure gauge the next time it's 90F and measure again.



The colour of your dinghy and whether it's in or out of the water will determine how much heat it absorbs (my Zodiac's instructions say to leave the back ends of the tubes in the water when beaching in hot temps). Everything can be calculated, and calculations can be confirmed through experimental measurement. If the two disagree, reality generally trumps theory.

[chrisr]

Originally Posted by Redline452

...reality generally trumps theory.

now i know you are an engineer and not a scientist !

cheers,

[Ded reckoner]

From betwys1: “…Tell me this is extremely close to theory!”

And from Redline452: “…reality generally trumps theory.”

The simple solutions given were based on the ideal gas law (PV=nRT), but, it was pointed out, this is an approximation. The solutions returned should have been given as integer values, because more significant digits are inappropriate.

Usually, the next most sophisticated model for gases is the van der Waals equation, [P+an^2/V^2][V-nb]=nRT. Obviously, this doesn’t reduce down to spreadsheet solutions as neatly as the ideal gas law. The term an^2/V^2 is an empirical correction for molecular attraction and nb is an empirical correction for the volume of molecules. Determining these corrections for non-ideality in a real gas consisting of O2, N2, etc. is non-trivial for our dinghy problem. So, it is left as an exercise for the OP. :-)

BTW, the van der Waals equation for non-ideal behavior is still inaccurate for real gases. State of the art thermodynamic representations of non-ideal gases are used by the chemical and petrochemical industries and solved on workstations. Those equations still have values that must be empirically determined for best accuracy. As the Nobel prize winning physicist Richard Feymann put it, “It doesn’t matter how beautiful your theory is, It doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”

[Dallasron]

dont be afraid to under inflate a bit and for gods sake never ask an engineer a question! The answer always starts with "well, when the earth was cooling...." Pick a sunny day during the season you are in. Set your psi when the sun is high and if it feels too baggy at night add a pound. I was enjoying drinks in BVI when somebodys tube blew. No big pop but just watched it go flat. check with those around you. You will be adusting it as the seasons change.

I think the real question is: How much psi can a hypalon dinghy stand given it is recomended at 3.5psi...If 8psi is safe,,,rock on

[SeanPatrick]

Originally Posted by chrisr

seeing as how we are talking about temperature, this seems like an opportune time to mention that degrees kelvin is named in honour of lord kelvin...who happens to be a distant relation of mine

That's cool. But Kelvin is not measured in degrees.



BTW, here's a tip for everyone: if you hold [ALT] and press [2][4][8] on a number pad, you get a degree symbol: °.