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The density of air is ca. 7.5x that of He. Thus 1 kg of He will contain more 'moles' of He. The issue here is what is to B compared. Since it is the gas being breathed, it is best to compare the amount of heat it takes to 'warm up' a volume of breath, say 1000 ml. Since the specific heat of He is < spec heat of air, it will take < energy to condition same volume of He. Safe bubbles Esat Atikkan --- Steve Hogan <Steve.Hogan@tr*.co*> wrote: > > > -----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'. > __________________________________________________ Do You Yahoo!? Talk to your friends online with Yahoo! Messenger. http://im.yahoo.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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