Mailing List Archive Mailing List Archive
Scott, I think his problem is in recognizing the difference between thermal conductivity and heat capacity or just what the hell causes us to get cold. So I'll take a stab at explaining it real simple like (not for you, but for those haven't taken thermo or heat transfer). Heat capacity is a measure of the amount of energy required to increase the temperature of a substance. To figure out how much body heat this substance (argon, air, etc.) is going to accept before reaching thermal equilibrium you will need to know how much of it you are dealing with (mass, moles, whatever). If that was the only property in play our bodies would heat the gas until a thermal equilibrium was achieved between our body and the gas; thereby ending the driving force (the thermal difference) for the energy transfer. So after a while the gas would be at body temp and because our bodies actually require cooling we would overheat. That doesn't happen because there's something on the other side of the gas that wants the heat too: the water. And it wants the energy more than the gas does. So the two main thermal bodies are your body and the water. The driving force is still the temperature gradient, this time between you an the water (for me that's usually around 30 Kelvin). But your suit creates a gap between these two bodies that the energy must traverse and this is where heeat transfer comes into play. There are three ways for heat to be transferred: radiation, convection and conduction. Radiation requires a large temperature difference, and while you think that that water is cold, 30 Kelvin isn't jack. Convection is heat transfer through a fluid medium (liquid or gas) and is what is actually happening in any airspace (or argonspace if you will) within your suit. Convection requires conduction initially at the interfaces and is the movement of the heated fluid between the interfaces. But we're going to idealize this system by saying that the airspace is not very thick and that your underwear also limits movement of the gas and therefore conduction is the primary means of of heat transfer between your body and the water. Convection will cause the heat transferred to the water to be dispersed throughout the ocean and heat capacity says that the little piddling of energy your losing ain't going to warm it up; thus maintaining the thermal driving force for energy transfer ! between your body and the water. So now that we've established that conduction is what we're worried about, we'll discuss that a bit. In general, as density increases so too does thermal conductivity. That's why we put a gas between us and the water, because we want to slow the flow of energy to the water (a crappy conductor is also known as an insulator). Here are the thermal conductivities for the four gases divers should be concerned about: Argon: 0.1772 Nitrogen: 0.2598 Oxygen: 0.2674 Helium: 1.52 All of these values are in mJ/(cm*K*s), where mJ are millijoules of energy transferred across the gas, cm is the thickness in centimetres of the layer of gas the energy is crossing, K is the temperature gradient in Kelvin, and s is each second. Therefore it should be very easy to see if all other variable are the same, the heat loss while using argon is less than that with air and substantially less than that with a trimix. Now you could just wear really thick underwear and jack alot of gas into your suit to increase the thickness of the gas layer and thereby decrease heat loss, but that would make you a tool. I still haven't gotten around to using argon, but that's the theory as best as I can explain it at 4AM. PS If there's actually anybody still reading at this point, you've managed to last longer than I ever did for a heat transfer lecture. Travis >From: "Scott Landon" <js_landon@ho*.co*> >To: gzambeck1@me*.ne* >Cc: trey@ne*.co*, techdiver@aquanaut.com >Subject: Re: argon for the cerebrally challenged >Date: Thu, 13 Jul 2000 00:08:28 GMT > >okay greg, watch closely. > >i take a fixed amount of air and add one molecule of helium - thermal >conductivity goes up. > >i add a gram-mole of helium - thermal conductivity goes up. > >i add a pound of helium - thermal conductivity goes up. > >it does not matter how much. the thermal conductivity goes up. i guess >everybody still appears 10 feet tall to you. let me know when you come back >down. the final absolute value of the thermal conductivity of the mix is >irrelavent. it went up no matter what. > >i am curious as to your education level because this very simple concept is >obviously falling on deaf ears. > > >>From: gzambeck1 <gzambeck1@me*.ne*> >>To: Scott Landon <js_landon@ho*.co*> >>CC: techdiver@aquanaut.com, trey@ne*.co* >>Subject: Re: argon for the cerebrally challenged >>Date: Tue, 11 Jul 2000 19:50:11 -0400 >> >>Yes you do need to solve for weight and volume Scott. You mix by volume >>Helium >>is 97.08 cf./lb. air is 13.45 cf./lb. Helium has roughly 5 times the heat >>capacity but is over 7 times the volume for the same weight. Joel is >>correct in >>his statement about trimix breathing mixtures. But an insulating gas has >>to be >>evaluated based on the moisture in the divers suit. >> >>Tell Trey I have talked to DuPont chemist also try a handbook on chemistry. >> >>Greg Z. >> ------------------------------------------------------------ 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'.
Navigate by Author:
[Previous]
[Next]
[Author Search Index]
Navigate by Subject:
[Previous]
[Next]
[Subject Search Index]
[Send Reply] [Send Message with New Topic]
[Search Selection] [Mailing List Home] [Home]