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Insulating Properties of Propane

12/12/2011 9:56 AM

Morning Everyone.

I am following a high altitude long duration balloon flight over the night and it survived the extreme cold of last night.

But the payload did freeze overnight after the sun went down. And stopped transmitting it's location. but after about 30 minutes of sunshine this morning it has re satarted up again posting it's position reports. you can actually see this on the page the tracker is being displayed on,

http://aprs.fi/?call=ng0x-2&mt=roadmap&z=7&timerange=86400&_s=ss_call

you see the large gap of no data from overnight.

OK, this payload the builder tried something he thought would help buffer it from the cold, he encased it in of all things "Propane" He said that propane has better thermal properties for insulating than plain "AIR" does.

Is this true?

in addition, how much difference does it really make when it is at 60,000 feet, and the densities of both the "AIR" and the "Propane" have dropped to roughly 0.0941% of that at sea level.

Anyone care to try to calculate the ummm "R" value of these two gasses at that altitude?

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#1

Re: Insulating Properties of Propane

12/12/2011 10:00 AM
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#2

Re: Insulating Properties of Propane

12/12/2011 10:13 AM
  • 1Determine the mean velocity of the gas, <v>. This is calculated using the kinetic equation for gases, <v>² = (3(R)(T))/M where "R" is the universal gas constant, 8.3145 J/mol K, "T" is the temperature of the gas in Kelvin, and M is the molar mass of the gas. Solve for <v>. For example, the mean velocity of oxygen, with a molar mass of 0.016 kilograms per mole, at 273 degrees Kelvin is [3(8.3145)(273)/.016]^1/2 = 461 m/s.
  • 2Look up the molar heat capacity of the gas, c_v. This is found on the standard periodic table of the elements. This property is the amount of energy required to raise the temperature of one mole of this particular element 1 degree Celsius. The molar heat capacity of oxygen gas is 919 J/(kg)(K).
  • 3Calculate the mean free path of the gas. According to Widener University, the equation to estimate mean free path, lambda, is lambda = (RT)/[(2^(1/2))(pi)(d)(L)(P)] where "R" is the universal gas constant, "T" is the temperature in Kelvin, "d" is the collisional cross section, "L" is Avogadro's number 6.022 x 10^23 mol^-1, and "P" is the pressure of the gas in Pascal. The mean free path of oxygen at 273 degrees Kelvin and 105 kPa is therefore (8.3145)(273)/(2^1/2)(3.14159)(3.39 x 10^-10)(6.022 x 10^23)(105) = 0.7 x 10^-7 meters.
  • 4Solve the thermal conductivity equation, k=[n<v>(lambda)c_v]/3L where "n" is the number of particles per unit volume, <v> is the mean velocity of the gas, lambda is the mean free path, c_v is the molar heat capacity, and "L" is Avogadro's number. The units of thermal conductivity will be Watts per meter Kelvin.The thermal conductivity of oxygen gas is therefore 0.026 Watts per meter Kelvin.

    Read more: How to Measure Thermal Conductivity in Gas | eHow.com http://www.ehow.com/how_7815446_measure-thermal-conductivity-gas.html#ixzz1gKk1x5Mr

http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=53

AIR

Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 23.94 mW/(m.K) = Air

Thermal conductivity (1.013 bar and 0 °C (32 °F)) : 15.198 mW/(m.K) = Propane

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#5
In reply to #2

Re: Insulating Properties of Propane

12/12/2011 10:38 AM

The temperature at 60,000 ft is nearly -70F...I don't think anything would work for long...Maybe a heat source with the payload all insulated in aerogel mat...

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#3

Re: Insulating Properties of Propane

12/12/2011 10:18 AM

Morning, though it's nearly evening here.

Insulation doesn't stop heat transfer. It only slows it down. So the temperature of the payload is a function of size, shape and internal heat dissipation as well as insulation, and a whole raft of other things. Calculating the R-value doesn't actually achieve anything; what would one do with the data (rhetorical question)?

There is an International Standard Atmosphere that gives details of temperatures and altitudes.

If the propane has dropped to less than 1% of atmospheric pressure, and it's supposed to be insulating the payload, then there is a leak.

Why wasn't the payload tested in an environmental chamber on the ground for function at simulated altitude before it was sent up? Had it been, and the faults put right before launch, the payload would have maintained function instead of failing to transmit data overnight!

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#4
In reply to #3

Re: Insulating Properties of Propane

12/12/2011 10:22 AM

Yes we know what the temps were and could be. These flights for this group is nothing new.

It was the use of propane as an insulator / buffer concept that i was wondering about. if it truly would be better than just plain air.

Joe

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#6
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Re: Insulating Properties of Propane

12/12/2011 10:58 AM

It's still too late.

Why wasn't it tested before launch?

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#7
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Re: Insulating Properties of Propane

12/12/2011 11:10 AM

Not a clue. As to why not tested b 4 launch.

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#8
In reply to #7

Re: Insulating Properties of Propane

12/12/2011 11:21 AM

Everything else is academic.

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#13
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Re: Insulating Properties of Propane

12/13/2011 5:54 AM

yes, that would be the reason for the post on CR4. If you were to perform this experiment, and report back to us, it would not be academic.

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#14
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Re: Insulating Properties of Propane

12/13/2011 9:04 AM

What - with these feet?

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#9

Re: Insulating Properties of Propane

12/12/2011 12:35 PM

Propane is a (significant) component of Hychill refrigerant HCF12, so I would of thought it was a thermal conductor if anything.

I would of gone a bit old school and used a couple of aluminium clad resistors as heaters with a simple temperature switch to turn them on (and heat the package)once the temperature dropped below a certain value. You would have to have provided a separate power supply so they wouldn't drain the package supply.

Oh and encase the package in good ol styrofoam(or the ghg safe equivalent).

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#10

Re: Insulating Properties of Propane

12/12/2011 1:03 PM

What they (or you) should have done was used a light-weight solid insulation like Styrofoam and heaters around critical electronic and mechanical elements to keep the equipment operating within its temperature rating.

The insulation medium could be many things, but its main purpose in an application like this is to keep the heat in, not the cold out.

I don't see any benefit in using a fuel such as Propane as an insulation, it will cause more problems than it solves.

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#11

Re: Insulating Properties of Propane

12/12/2011 2:59 PM

I believe your batteries is the part that suffers most from low temperature. (specially most Li-type batts do) And don't get too optimistic about insulation. Even a "perfect" insulator will keep your payload temp about halfway between temp peaks, which is still very low. So I'd start by getting the right battery, bake-dry the rest of the device and air-tight seal it, to avoid any humidity penetration. Anyway think propane was suggested not because of insulation properties, (not much insulation to expect from any freely moving gas) but because of it's greenhouse properties like all air-phase hydrocarbons. However these properties do NOT show much at this scale, so nothing to expect using it. S.M.

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#12

Re: Insulating Properties of Propane

12/12/2011 10:39 PM

Most people who make these packages place them in a hollowed out low density urethane foam shell. The weight added is minimal and if it is well sealed, with only a tiny gap for pressure relief, it will help. You can also place a tiny thermal load in series with the battery inside the foam shell. This will provide some heat but this needs to be balanced against the duration of the trip. High performance lithium single use cells will allow for a greater amount of heating = greater stability and higher cost

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#15

Re: Insulating Properties of Propane

12/15/2011 12:06 AM

Have you considered heating the payload using your gas supply, rather than with less abundant electrical power?

A microscopic bleed and a catalytic media would do this 'flame free', and obviously it can be thermostatically controlled with fairly 'off the shelf' gear.

Another solution, depending on how things can be arranged, is to put the cold sensitive things in the balloon envelope.

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