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From: Ray M Keller on 10/07/97 04:07 PM I wrote this some time ago for the rebreather list and it still holds true today. If you want to know the physics behind the thermal properties of Argon read on.... To: edb2otid@mh*.ns*.go*.au*, rebreather@nw*.co* cc: Subject: Re: Argon as suit inflation gas...? This is an effort to explain to Bruce etal. the physics of why Will's Willie stays warmer in Argon vs Air. This is going to be a bumpy road so please read through this a couple of times before you raise questions or provide comment. Argon has better insulating properties than air. This is a statement that we have all heard hundreds of times. But, what does it really mean to a man hanging on a deco line with a chilly willie? To answer this question we must first understand how the following two things relate: 1. The thermal conductivity of Argon Vs Air and 2. Heat transfer Part 1 Thermal conductivity of Argon Vs Air In one ATM at 270 Kelvin, Argon will conduct 0.1619 millwatt per centimeter Kelvin (mW cm-1 K-1) of energy whereas Air will conduct 0.2374 millwatt per centimeter Kelvin (mW cm-1 K-1). In one ATM at 40 degrees Fahrenheit or 4.4 degrees C, Argon will conduct 40.09 cal/(sec)(cm2)(C/cm) x 10-6 whereas air will conduct 58.31 cal/(sec)(cm2)(C/cm) x 10-6. In one ATM at 40 degrees Fahrenheit or 4.4 degrees C, Argon will conduct 9698.13 Btu/(hr)(ft2)(F/ft2) x 10-6 whereas air will conduct 140105.71Btu/(hr)(ft2)(F/ft2) x 10-6. Part 2 Heat Transfer Heat is transferred by conduction, convection and radiation. The principle that is applicable to this discussion is stagnant gas conduction* in a steady state of heat transfer. This involves two fluids (liquid or gas) at different temperatures, separated by a solid barrier, such as a tube or pipe, or in this case the wall of a dry suit. The rate of heat flow q from the hot fluid to the cold equals the product of a proportionality factor commonly referred to as the overall heat-transfer rate U, the amount of barrier surface (undergarments and dry suit) through which the heat flows A, and the temperature difference between the two fluids Dt (water and gas inside the dry suit). q = U A Dt The above equation applies for local conditions only, unless the three factors on the right side remain constant with respect to the flow paths of the fluids. With changing conditions this equation must be expressed in differential form. dq = U Dt dA These two equations are directly applicable to systems involving conductive and convective heat transfer and may be adapted easily for use in systems involving radiant heat transfer. The rate of heat flow q, called duty or heat exchanged, is the rate of transfer of energy with respect to time through the heat transfer surface. The overall heat transfer rate U, accounts for the multiple resistances r, to heat flow that are present in a system. Such resistances include the fluids themselves, fouling deposits that may be present on the hot and cold surfaces of the barrier separating the fluids, and the barrier wall resistance itself. The overall heat transfer rate is equal to the inverse sum of all of the resistances in a series as expressed in the following equation: U = 1/(rh + rhf + rw + rcf + rc) This equation applies directly to heat transfer through barriers with constant area normal to the direction of heat flow, e.g. flat plates or the wall of a dry suit (this means no cooling fins). The inverse of each of these resistances is referred to as conductance, h(h = 1/r). The higher the conductance (remember part one from above?) or lower the resistance, the greater is the ability of the fluid or material involved to transmit heat. In the case of hot and cold fluids, the conductances, hh and hc, are generally referred to as film coefficients. These coefficients depend on the mode of heat transfer, i.e. conduction, convection, radiation, or a combination thereof. Numerous other factors influence fluid-film coefficients, including, but not limited to, system geometry, fluid-flow velocities, fluid physical properties, difference between bulk fluid and wall temperature, etc... The steady-state rate of heat flow, q, is equal through all series resistances. *Conduction refers to the molecular transmission of heat through a body from point to point within the body or from one body to another in direct contact with it. Conduction is generally limited to the transfer of heat through a solid, such as a metal tube wall, or a series of solids in contact with each other. Heat transfer by conduction may occur in stagnant liquids or gases when no mixing occurs. The basic theory of heat transfer by conduction was established over a century ago by Fourier who expressed it as: q= dQ/d0 = -kA(dt/dx) It states that the rate of heat flow is equal to the thermal conductivity k multiplied by the area of heat transfer and the temperature gradient. A, is the surface area normal to the direction of heat flow, and dt/dx is the rate of change of temperature with distance in the direction of heat flow. My name is Raymond M. Keller, I am a chemist with a nuclear DOE facility responsible for the interpretation of organic and radiochemical environmental analytical data. ----- Original Message ----- From: Christopher Brown <hokiediver@ya*.co*> To: <techdiver@aquanaut.com> Sent: Wednesday, February 21, 2001 7:47 AM Subject: Re: Helium thermal properties > Brad is correct. There are two types of specific > heat, constant volume (Cv) and constant pressure(Cp). > I think the one we are concerned with in dry suits or > your lungs is the constant pressure, since the volume > of the dry suit (or lung) is not fixed. In your tank, > it would be a different story, since the volume is > fixed and the pressure is slowly varying, but who > cares how chilly your tank is? There is not much > difference the magnitude of Cv and Cp. > > Argon Cp=1.14 kJ/(kg*C) > Helium Cp=22.8 kJ/(kg*C) > Nitrogen Cp=2.06 kJ/(kg*C) > > As you can see, the Helium Cp differs by a bunch. > Roughly speaking, at the same mass (kg) and initial > temp (C), Helium would carry away 22 times the energy > (aka heat) (kJ) as Argon. The Nitrogen is included as > a reference, since that's about what air would be... > > To do Brad's second reccomendation (amount of body eat > "lost" when filling with Air vs. Argon), we would have > to use the first law of thermodynamics, but that is > another bullet in another foot. > > Chris Brown > Suffolk, Virginia > > --- Brad Beskin <bradb@ex*.ne*> wrote: > > Helium is lighter (less dense)than air, and > > therefore it conducts heat much more quickly. This > > is the reason you do not inflate your drysuit with > > backgas. Use an independent inflation system with > > argon or air (argon is very dense and therfore > > insulates well). > > > > To answer your physics question: > > c = the specific heat of a gas (the amount of heat > > necessary to change 1 kg of the substance 1 degree > > celcius) > > This would be helpful (in that shoot yourself in the > > foot kind of way) to calculate the joules of body > > heat you're gonna lose if you fill your drysuit with > > He instead of Ar or Air. > > -CB > > From: moorea(uol) > > To: techdiver@aquanaut.com > > Sent: Tuesday, February 20, 2001 8:05 AM > > Subject: Helium thermal properties > > > > > > Hello all ! > > I´ve been diving since 86, and nowadays i made my > > own mixes, but now, i decided to go certified. > > My instructor, talking about thermal loss in a > > trimix class, explain me that the heat exchange is a > > direct relationship with gas density. > > I´ve read and learn in the past (including in > > phisics class at regular school) that heat exchange > > depends of thermal conductivity of the media (Q = > > mc delta t) where "c" is the thermal conductivity. > > Can you help me with that ? > > Am i right ? > > > > thanks > > > > Marcelo Moorea > > > > > __________________________________________________ > Do You Yahoo!? > Yahoo! Auctions - Buy the things you want at great prices! http://auctions.yahoo.com/ > -- > Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. > Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. -- Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'.
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