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But the important quantity isn't mass, it's volume. Air is in the neighbourhood of 7 times as dense as helium (7.232 at STP to be exact). Because the compressibility factor at 10 bar is close to 1, the gases can still be assumed to be ideal and the property of most interest will be molar heat capacities, not mass heat capacities. Air: Cp=29 kJ/kmol*K Helium: Cp=20 kJ/kmol*K To heat a like volume of air to that of helium in your lungs it requires almost 50% more energy. I'd give the thermal conductivities for suit inflation but I don't have that textbook here and I'm late for class, maybe tomorrow. Later, Travis >From: Steve Hogan <Steve.Hogan@tr*.co*> >To: techdiver@aquanaut.com >Subject: RE: Helium >Date: Thu, 16 Mar 2000 12:34:20 -0800 > > > >-----Original Message----- >From: Andreas Hagberg [mailto:andha513@st*.li*.se*] >Sent: Thursday, March 16, 2000 8:13 AM >To: Steve Hogan >Cc: techdiver@aquanaut.com >Subject: Re: Helium > > >> Helium is a small molecule. It does not take much energy to heat an amount of it (relatively speaking to lets say air). > > >err... So you're saying that it takes LESS energy to heat Helium than air?? >Well, not according to a nationally used collection of tables and formulas in Sweden. >The specific heat-capacity of air is 1 kiloJoule/kg*K. (Which means that is takes one kiloJoule to heat 1 kilo of gas 1 degree (Kelvin)) >And the specific heat-capacity of Helium is 5.1 kJ/kgK. Which in the same way as above means that it takes 5.1 kiloJoule to heat 1 kilo of Helium 1 degree. > >Since when is 5 kiloJoules LESS than 1 kJ??? > >This seems all pretty simple to me, but I might have misunderstood the whole thing. If so, please correct me. >/Andreas > > >Andreas, > >Look at it like this. While it does take more energy to heat a sample of helium vs a sample of >air, what happens in the lungs is different. When the heating is happening, the body is loosing >energy heating the HE, but the HE is transferring the energy back to the body (due to its high >thermal conductivity). The overall effect (the important one) is less energy is required by the body >due to the combination of the two properties. > >Does that make sense to you? >I should have done a more complete answer (like HE as a molecule, that is what I get >for not proof-reading) > >I was digging through the archives so you could have the math behind it. > >Helium also has a higher heat capacity than nitrogen. >Thats right (He: cP = 5.23 kJ/kg*K vs N2: cP = 1,038 kJ/kg*K), >but thats only half of the story. You have to take the mass of the >employed gases into the calculation, too. >Lets first look at the breathing cycle: > * Inhale breathing mix at ambient temperature/pressure > * Hydrate and warm up the gas > * Exhale breating mix with 100% relative humidity and body temperature >The body loses its heat by breathing off the warm gas (ie. convection). >The energy which is expended can be calulated with: >Q = cP * m * delta T >Q: Energy [kJ] >cP: heat capacity [kJ/kg*K] >m: mass [kg] >delta T: temperature difference [K] >The temperature difference delta T is the difference between body >temperature >and the ambient pressure, both do not change while breating another mix. >The mass of a gas is dependent on the number of molecules and their >specific >weight. The number of molecules in a fixed gas volume (loung) is only >dependent >on the temperature, therefore if the temperature is constant the number >of >molecules is constant, too. The specific weight is proportional to the >relative >molecular weight Mr (Mr_He = 4, Mr_N2 = 28). >Now you can see that Q ~ cP * Mr. >What does this mean? >You lose less energy when you breath high helium mixes!!! >> A diver will feel >> colder breathing heliox than when breathing mixes containing at least some >> nitrogen. >Right, but that has to do with the heat transfer characteristics of >helium. >Helium transfers heat much more rapidly than nitrogen. This is also the >reason >why you dont want helium as dry suit inflation gas. >Regards >Frank Riffel > >Regards > >Steve > > > >-- >Send mail for the `techdiver' mailing list to `techdiver@aquanaut.com'. >Send subscribe/unsubscribe requests to `techdiver-request@aquanaut.com'. ------------------------------------------------------------ Mr Hat's Hell Hole - We ARE South Park - http://www.thehellhole.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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