I recall a Jacques Cousteau documentary years ago, where the divers breathed a helium-oxygen mixture. They came up to a decompression chamber, but I must ask: why the helium along with the oxygen?
So they would sound like Donald duck when talking on their underwater microphones, of course!
100% oxygen is too likely to cause seizures at depths greater than about 20ft. So you need to cut the oxygen percentage down by adding another gas.
Switching to air (about 78% N[sub]2[/sub] and 21% O[sub]2[/sub]) gets the oxygen level to an acceptable range for diving down to just past 200ft but nitrogen brings its own problem. Nitrogen starts to display some narcotic-like properties at depths greater than about 80ft so you need to cut the nitrogen percentage by adding or switching to some other gas. Or just dived “narced” - feels like being drunk - and accept the risks that come with poor decision making in a hostile environment.
Helium works nicely as that other gas but is costly. Helium does not seem to affect the nervous system until extreme depths (greater than 500ft).
Most dives much deeper than about 150ft are with a breathing gas mixture known as trimix (He + N[sub]2[/sub] + O[sub]2[/sub]), but in some cases heliox (He + O[sub]2[/sub]) may be used. The deeper you go the lower the oxygen and nitrogen content and the greater the helium.
Typical recreational scuba dives are shallower than 130ft. Most are shallower than 100ft. Recreational divers tend to tolerate the nitrogen narcosis between depths of 80 and 130ft. The drunken feeling vanishes instantly upon ascent.
You’ve gotta love the SDMB for answers. Thank you, Iggy!
Of course you can add those people that think honey is better for them than sugar. (Actually it’s closer to the “dreaded” HFCS than sucrose is.) Actually come to think of it a lot of people are surprised when I tell them that the acid in tonic(coke, pepsi, etc) actually break down sucrose into fructose/glucose which is what HFCS is. (I really should find a chemist and find out how long it should take for a bottle of coke made with sucrose to break it down and get to equilibrium.)
I’m not arguing what they ARE, I’m arguing what they’re CALLED.
Just last year I looked up in wikipedia and other resources (online) and the answer was very equivocal. Looking there now I see it’s changed, but I’m not convinced there is an authoratative answer. I’d like to see a reason for any of the claims that “Canadian goose” is incorrect. As I said, either is a reasonable translation from the Latin, which is the official name. Common names often vary from the official scientific name, and there is no absolute right and wrong.
Actually, air tanks for deep dives have lower O2 concentration, not higher (than atmospheric). I don’t know of any reason to increase O2 concentration, for diving.
At a partial pressure of 6 atmospheres, O2 becomes a poison. So, if it’s 20% of air (as a rough estimate), air becomes poison at 8 atmospheres. Every 33 feet down (again, roughly, to make the math easy) adds one atmosphere, so 33 foot depth = 2 atm, 66 = 3, … 233 = 8. So, much below 200 foot depth, air is poison. (I don’t remember the reason, just the fact, from normal divers’ traning. Wikipedia has an article with a number of different causes.)
So, for diving below 200 fee, one uses a mixture of less than 20%.
Regarding nitrogen, it causes two problems. One is narcosis, as mentioned above. The other is “the bends”, which happens because nitrogen gets dissolved in the blood, and when you rise (decompress), it “bubbles” out, and the bubbles can block blood vessels. Helium (for reasons I don’t remember) doesn’t have this problem, and there are other gas mixtures that avoid it. Those are used for diving below the “sport diving” limit of 100 feet.
OK, back to misconceptions:
A common misconception is that in “long lived”, “lived” should be pronounced with a short ‘i’, like the past tense verb “lived”. If we were describing cats and calling them “nine-lived”, how would we pronounce it? If we were discussing Henry VIII, how would we pronounce “the eight-wived king”? (OK, perhaps a bad example, but illustrates the point.)
English is democratic, so anything (no matter how wrong) becomes “correct” when it’s accepted and repeated by enough of the population (and not admonished by the experts, unlike “ain’t”, even though that was originally a perfectly good contraction for “am not”.)
So, the “wrong” pronunciation for “short lived” becomes accepted, leading to more confusion and contradictions in the English language we love so well.
Well, how many sources and cites would it take to convince you? You trust Audubon, Peterson’s field guides, and Cornell? How about Linneaus himself, who actually named the bird? You trust that guy to know his way around taxonmy, right?
My lil niece calls me unca. She’s not yet two; her verbal abilities appear normal for her stage of development.
Seriously. Learjeff, you can call them all *Susan *if it makes you happy, but the proper name for the bird is the Canada Goose. It’s in the book.
Do you have an authoritative cite for this assertion? If you’re just reasoning by logic and analogy, it can go either way. “Long” can be either an adjective (“a long life”) or an adverb (“he lived long”), and “long-lived” makes sense either the way you interpret it (“having a long life”) or as an adverb+participle ("having lived a long time), analogous to “well read” or “poorly dressed.”
I think you are confusing “misconception” with “different from the way we speak the language where I grew up.”
Gas blends with higher than atmospheric concentrations of oxygen are used quite commonly, though the benefit comes from lower concentrations of nitrogen.
Nitrox blends of 32% or 36% O[sub]2[/sub] are available at many dive shops and are routinely used in recreational scuba diving. These blends have less N[sub]2[/sub] than atmospheric air and thus the diver absorbs less nitrogen at a given depth. This allows the diver to extend his time underwater without undue risk of decompression sickness (“the bends”).
But there is a trade off. The higher oxygen percentage limits how deep the diver can go without risking oxygen toxicity. Indeed the accepted limit for dive planning is a partial pressure of oxygen of 1.4 with a contingency of 1.6. Oxygen toxicity can lead to a seizure which is a very bad thing to happen 100ft underwater.
Gas blends for dives deeper than the recreational limit of 130ft may start to cut the oxygen percentage below that of atmospheric air. Usually due to the cost of preparing such gas blends such changes are not made until the dive planning is exceeding 150ft or more. It is required below about 200ft. Far less than 1% of all scuba dives are to such depths.
Helium certainly does have this problem! It is particularly noted for increasing the risk of skin bends. Any inert gas contributes to the risk of the bends. However helium is more rapidly diffused than nitrogen. In gas blends with a substantial nitrogen fraction the diffusion rate of the nitrogen will be the controlling factor for decompression times.
Misconception: That the opinions of an at independent news publication’s editorial board has influence on the reporting done by the publication’s news staff.
Then why are wings shaped as they are?
Airfoils come in many shapes, but the general idea is to maximize lift and minimize drag. That said, even a flat plate can generate lift given a suitable angle of attack.
The common myth is that the air above the wing crosses in the same time as the air below the wing. Then it is said that the shape of the wing makes the air on top have to travel farther in that same amount of time, leading to reduced pressure via Bernoulli’s principle. But (1) This “equal transit time” hypothesis is false, and (2) in some counterfactual universe where it were true, the curvature of the upper surface of the wing would need to be a lot more dramatic than it typically is to generate sufficient lift in this way.
Most people think air friction causes heating, like when a space craft re-enters at high speeds, or like how the meteor over Russia burnt up. This is not what causes the intense heat.