in: Swampfox; Swampfox > 2007-09-21;
| # Posted 2007-09-22 16:31:51 | |
| StoraMoo: | Hi. Everyone says that there is less air at 8,000 feet than at sea level, but how much less?
Is there a quick formula to use to determine the percent of air or oxygen at different elevations? |
| # Posted 2007-09-22 21:04:27 | |
| Swampfox: | StoraMoo, I'm only a futile swampfox, so what do I know? Not much! So all of the following may be wrong (and it also depends a little on what you mean by "simple" and how you intend to use oxygen percentages by altitude.)
Anyway, a rough calculation for elevations up to 10,000' and ignoring effects of temperature and voodoo magic would be to take an initial value of 29.92, and then adjust it by the following to obtain a new value: Average decrease Altitude Range per 1,000 feet Feet per in/Hg Sea level to 5,000 ft 1.006 in/Hg 994 5,000 to 10,000 0.862 1,160 10,000 to 15,000 0.740 1,350 Sea Level to 10,000 ft 0.934 1,070 Divide the new value by the initial value, and that should give a per cent of sea level oxygen at the altitude you selected. When the US Army is calculating adjustments for 155mm (and larger) artillery pieces, the field artillery manual specifies an additional adjustment for G (the Gagarin). If PG are more than 3500 kms from the firing piece, no adjustment is required. Within 3500 kms, some further adjustment will be required due to G induced gravitational distortions. If PG are eating Krispy Kreme donuts and within 3500 kms, no artillery operations are permitted since it will no longer be possible to predict where artillery rounds will impact. (Footnote: some grammar above are Blind Tangerine adjusted.) |
| # Posted 2007-09-23 03:46:59 | |
| PG: | It's amazing, I've been reading this biography of Einstein, and he's coming up with all this stuff about gravitational forces and the spacetime continuum, and I read what SF says, and ..... It all makes sense! Except the Krispy Kreme part (that makes sense, but that's not something Einstein knew about). |
| # Posted 2007-09-24 02:32:28 | |
| StoraMoo: | I was thinking about something more in lines with this, "For every 1,000 of elevation the air is 1% less dense."
Throwing G and PG and Einstein is a bit too complex. And if we are tossing in Krispy Kreme and Einstein, where do bagels fit? |
| # Posted 2007-09-24 09:51:31 | |
| jjcote: | Air Density decreases at a rate of 2.9% - 3.0% for each 1000 ft. of elevation above Sea Level. |
| # Posted 2007-09-24 10:20:27 | |
| Swampfox: | I've never seen PG with a bagel, and offhand it would be a surprising thing. There aren't enough calories in a bagel to make it worthwhile.
Think of him as a sort of a human equivalent of the hummingbird, and you can see why he needs to stick to very calorie dense objects. |
| # Posted 2007-09-25 05:38:02 | |
| StoraMoo: | According to JJ's reference, turbos suffer at higher elevations. Is my little Subaru turbo in danger if it visits Laramie? |
| # Posted 2007-09-25 09:15:35 | |
| jjcote: | If you read it closely, you'll see that it says that turbos do take a loss at high altitude, but not as big a loss as non-turbo engines. Turbos are great at altitude, in fact, the only propeller-driven aircraft that can fly above some particular altitude are turbocharged ones. (That website, by the way, appears to be focused on high-performance snowmobiles and jet-skis.) |
| # Posted 2007-09-25 09:16:33 | |
| ebuckley: | High altitude is where turbos shine. At the Pike's Peak hill climb (and other high-altitude races), they have to separate out the turbocharged cars because any normally aspirated car of similar performance at sea level will be gasping for breath by the top while the turbo cars will suffer much smaller losses. The reason for this is that most turbos are capable of generating more boost pressure than the intake manifold can handle. At low elevations, a popoff valve regulates this. As you go up, the popoff valve stays shut and the boost pressure drops to what the turbo can actually produce - but that's still a whole lot more than what get's sucked into a conventional intake. |
| # Posted 2007-09-25 10:26:26 | |
| cedarcreek: | Altitude has thinner air, and that means the turbo spins faster than at sea level. If you do something like Pike's Peak in a stock turbocharged vehicle, you should avoid running it hard because it's very unlikely that the designers expected someone to do that. At any reasonable altitude and at moderate throttle settings, you're almost certainly fine, although I'd add that if you have a turbo and you don't use full synthetic oil, you're asking for bearing trouble. Also, don't run it really hard and then just shut it off. As you get close to where you'll be stopping, go easy so the turbo isn't spinning fast when you turn off the engine and the oil flow stops.
Eric is right about turbos shining at altitude, but Pike's Peak is sort of an extreme example. If you're racing there, you need to have your turbo selected and set up by someone who is familiar with compressible gas dynamics and related topics, or it's gonna break. I am no expert here, but I understand it's important to keep the compressor on its map. At altitude increases, the pressure ratio (plotted on the y-axis of the map) tends to go up, and mass flow (plotted on the x-axis) tends to decrease, and this corresponds with higher rotor speeds and the possiblity of hitting the surge line, which is sort of the west boundary of the map. It's more than a little complicated. |
| # Posted 2007-09-25 20:53:24 | |
| ebuckley: | Actually, I talked to a Subaru tech guy about that (not that I was going to race my WRX up Pike's Peak, but I was curious) and he said the stock WRX turbo should be able to take it. Granted, the WRX setup is a lot closer to a true race turbo than most. He did, of course, recommend letting it spin down after as you should always do when running the intake hot. The full synthetic oil is a good call, too. I do that simply because I like to go longer between oil changes, but it's good for the turbo, too. |
| # Posted 2007-09-26 18:14:52 | |
| cedarcreek: | I've been thinking about this for a few days now, and I think I made a mistake. Most of my interest in high altitude turbos is related to diesel engines. Gas engines run very limited amounts of boost compared to diesels. Gas engines are limited by knock to maybe 6psi or so at sea level, while diesels often run over 20psi. (Diesels knock in normal operation.) Just eyeballing the map, it seems like it's a lot easier to keep on the map with a gas engine, so it's certainly possible that Pike's Peak is within the design space of a stock turbo installation. |
| # Posted 2007-09-26 23:55:54 | |
| ebuckley: | I'm sure Swampfox is appalled that we've turned his log into a discussion of forced induction engines. Anyway, the WRX popoff is controlled by the computer. It will let the boost go all the way up to 13PSI. If it detects pre-detonation, it backs it off. |
| # Posted 2007-09-27 00:53:39 | |
| Swampfox: | I blame it all on StoraMoo. He's such a troublemaker. |
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