Why?

[The Chairman]

Hey DrDeco,

Another lucid response, I like that. I helped with research on free radicals in hydrogen for the army at the University of FLorida back in the 70s. The cool part of it (pardon pun) was working with liquid helium (as well as l-N2) and seeing how it reacted in all three stages. We had the largest Supercon magnet at the time (for almost a year, actually) and while the chemistry was a bit beyond this mere mortal lab technician, I was exposed to a lot of gas physics and basic gas chemistry. When I took my Dive master and was asked for the change in pressure when a tank cools, I was able to use STP and figure it precisely. I do not like the method of using the "finagle factor" that they give you in the book.

So can you tell us WHY neon is not being used? Was there a draw back that you omitted from your tretise? Again, I need to do advanced nitrox and trimix, and am not looking to short change those classes, but learning is fun and I like to do it. You can really never know too much about these things...

Pete from Orlando...

 

[Dr Deco]

Pete:

The rational behind the use of neon was its decompression characteristics between nitrogen and helium (that is good), its lower thermal conductivity than helium (so it keeps the divers feeling warmer, and its improved speech characteristic (allowing more intelligible speech).

The real driver in this was (if I remember correctly)the easy availability of neon worldwide. The US Navy was interested in this gas (and funded the program) because it could be obtained all over the world. As the growth of industrialization has occurred worldwide, there are now air liquefaction plants everywhere. The liquefied air is distilled to collect the oxygen (very valuable for industrial processes, steel making for example) and liquid nitrogen (good for cooling). A very low boiling point fraction distills off first and contains a mixture of helium and neon. This is generally not of any real value in most places of the world. The theory was to buy this anywhere from Africa to Asia.

While an interesting idea, it is based to a large degree on needed a worldwide supply against the possibility that your shipments from the United States could be slowed down or halted. This did not occur under the world conditions of the 1970s.

There were not any negative characteristics that would make this gas unacceptable. If for some reason countries without much helium so desired, they could use this neon/helium mix.

In deep sea diving, the helium is collected in a two hose system and not discarded as by open circuit SCUBA. Thus, the cost and lack of availability of helium is not such a big economic consideration.

Dr Deco

 

[gozumutti]

Dear Doc Deco,

I told a friend about your explanation of oxygen toxicity and he has asked me to translate it for him...you don't mind do you? Don't want to break any rules here......

gozu

 

[The Chairman]

Hey Dr Deco,

Is this a rebreather perchance? Also, what are the mechanisms that make divers lose more body heat with helium? I know (at least I think I know) that the nature of the helium gas at the same pressure of air is far lighter, hence less dense, which would affect sound travel (why sounds in the water tend to sound so much lower). Still a "thinner" gas should relate into a lower heat transfer, in the same that the denser water has a MUCH bigger heat transfer than the thinner air.

See, you answer a question, and I got three more. Sorry about that, but this is enlightening.

Pete from Orlando...

 

[Hocky]

Dr Deco and Net Doc

Thanks for replying to my original question, I appreciate the input, and I have passed this information to a number of my students.
Its interesting to see the questions that are now coming in with regads to gasses,Im learning all the time.I'm sure others are as well, and I want to thank you for the time and effort you have put into the answers. I appreciate it and I'm sure that there are others out there that appreciate it too. Well done!
Hocky

 

[Atomox]

The properties of thermal conductivity have not been fully explained by the kinetic theory of gases. But experimental evidence provides us with the following results.

At 20° C the thermal conductivity of water is 0.60 W/m × K, and the thermal conductivity of air at 20° C is only about 2.5^10-2 W/m × K. To a rough approximation, liquids conduct heat about 10 times better than do gases.

The properties of the thermal conductivity of dilute gases (versus the free-molecule region or the dense-fluid region) exhibit some unusual characterisitcs. The most striking is the lack of dependence on pressure or density. There is a dependence on molecular mass, with light gases usually better conductors than heavy gases. The thermal conductivity of a dilute gas increases with increasing temperature and there also seems to be some correlation between molar heat capacity and thermal conductivity.

The thermal conductivity of mixtures falls between the conductivities of its components, however, there is an odd regularity. The conductivity of a mixture is always less than an average based on the number of moles (mole fraction) of each component in the mixture. This appears to be related to the different effect that molecular weight has on thermal conductivity.

Brad

 

[Dr Deco]

Dear Pete:

REBREATHER?
The system for helium about which I wrote was for commercial divers who are diving from a bell. Here the diver has a two hose system and the expired gas is sent back to the bell and the surface ship where it can be stored and recovered. This is sort of an “industrial strength” rebreather!

HEAT LOSS?
The loss to which I was referring was for a diver in a decompression chamber. When commercial divers make deep dives that require decompressions lasting several days, the deco time is spent in a Deck decompression Chamber (DDC) which provides amenities such as a toilet, bed, and room to walk around (a little anyway). It is in the helium atmosphere of the DDC that the heat loss occurs. :cold:

Normally we are comfortable when the room temperature is about 72 Degrees Fahrenheit. For divers in the DDC, they like to have it about 84 degrees. :tree: The rapid motion of the very low molecular weight helium molecule allows it to conduct heat away from the diver very quickly (diffusion loss). Atomox has a more detailed explanation.

There is also a problem of heat loss through the very dense breathing gas but this is a situation at depth. Often the breathing gas must be heated, not an easy matter if proper control of temperature is a factor. Heat control in the water in commercial situations is often a considerable problem when the depths and durations are considered.

Dr Deco

 

[Dr Deco]

Dear Gotzumutti:

Yes, it is all right to translate my responses into different languages. (I assume that you meant that, and not my English into yet simpler English. I try to make the response “readable.”) =-x

Many individuals from non-English speaking countries write and tell me that the material is passed on to their friends. (Most likely, many of the replies are translated.)

Dr Deco :doctor:

 

[gozumutti]

Thanks Doc, you will now become immortalized in German

 

[Dr Deco]

Dear Gozumutti:

In actuality, I worked in Germany for several years (1977 to 1980) in the Underwater Medicine section of the Institute for Aviation Medicine (DFVLR) in Bonn.

For curiosity, the following were published during that period.

• MR Powell. Untersuchungen an Ratten über den Einflus von Stickstoff und Helium auf die erste Austauchstufe bie Tieftauchgängen. In: Tauchmedizin, [F. Gerstenband, K. Seeman, and M. Lazaronni, eds.], Schlütersche, Hanover, West Germany, 26-30, (1980).
• W Thoma, MR Powell, and K Klein. Erfahrungen mit Kontinuerlicher Transkutaner Sauerstoffmessung mit dem Oxymeter unter Hyperbaren Bedingungen. In: Proceedings, Congress on Hyperbaric Medicine, Fussen, West Germany, (1980).
• MR Powell, HD Fust. The Influence of Inert Gas Concentration on Pulmonary Oxygen Toxicity. In: Proceedings, VII Symposium on Underwater Physiology, Undersea Medical Society, Bethesda, MD, (1980).
• MR Powell, W Thoma, HD Fust, and P Cabarrou. Gas phase formation and Doppler monitoring during decompressions with elevated oxygen. Undersea Biomed. Res., 10 (3), 217-224 (1983).

Many thanks for your nice comments....

Vielen Dank fuer Ihre sehr nette Bemerkungen!

Mit freundlichen Gruessen,
___________
Dr Deco