e.
Yeah, that's mostly correct. If it was out in space it would radiate to th e ~3K background. But here on Earth there is a 300K background. And that could be thought of as a heat load on the box...or another way to think abo ut it is that the 300K is less cold than 0K and so the heat flow from the h ot box is less. Maybe this will help,
George H.
e.
There's no net heat flow from a colder object to a hotter object, but Joerg 's "hotter object" while hotter than the immediate environment, is colder t han the sun.
It collects quite a lot of radiative transfer from the sun, and has to bala nce it's heat flows by re-radiating that heat (as well as losing heat by co nduction and convection). The immediate environment (local dirt) is also ra diating heat and that's another thermal flux warming the object.
Radiated power is proportional to the fourth power of temperature, so if Jo erg's box is at 100C - 373K and the local ambient is at - say 30C - 303K it 's radiating 2.3 times as much power into the environment as the immediate environment is radiating back at it. Treating the immediate environment (an d deep space) as a at 0K isn't consequently a particularly crude approximat ion.
ise) and setting that to 0K is actually good enough for all but the most pr ecise work.
Nice! OK playing with the numbers a bit...
172F ~ 77C ~350K, so the hot asphalt is radiating about 400-500W/m^2 or clo se to 1/2 the solar influx. And the rest goes either into the earth or get s taken away by convection. Still, ~1/2 of the loss is by radiation.hes
Seems about right I got something similar with "looser" numbers.
George H. (who is still getting his head around how big the radiation effects are.)
Why was a "past" point reached? I believe we can get there with the aluminum caps that Martin suggested. Polymer would require a bidirectional converter. Not rocket science but would add cost and real estate needs.
Got to be careful with searches. However, Digikey has the best search engine for electronic parts there is. Which is why they are my default source for prototype parts. Sometimes clients even want me to list Digikey P/Ns instead of mfg P/Ns in the components section of my module spec.
Unwind from it once in a while. It's good to keep up the old school design skills. Sunday after church a client presented me with a new tough challenge. I could have gone on the web as usual and start looking. But instead I took my street bike and zipped along some very quiet streets. Just horses, and maybe one slow car every 15 minutes. I eased off on the gears a bit for a lower power ride and started thinking, notepad in the handlebar bag. By the time I got home I had the basic concept and some ideas for parts, and then fired up the barbecue.
Many still still have most of the original panel gear:
Engines are a wear and tear item on aircraft and there is a prescribed (and usually non-negotiable) number of hours before an overhaul. So you may still have parts of the old 1940's engine propelling you but it's going to contain lots of new stuff to replace the worn out stuff. This doesn't go for everything though.
Example here: I have a very heavily used electric drill that is almost
100 years old now. Love it, because it's striong and has no plastic parts. Plastic wasn't invented yet back then, other than bakelite.
There is a business about 20mi from here that regularly services 100+ year old elevators that are still in regular service. In my lab that's not all that different. A drill that's almost 100 years old, and some
50's era instruments like this:There are situations where this dipmeter is the ticket because my transistor dipmeter can get fried if a DUT decides to burst into oscillation. Tubes don't usually get fried.
The Measurements 59 also produces nice deep dips with pretty weak inductive coupling. (I got one last year on your recommendation. Sure beats the Heath ones.)
Cheers
Phil Hobbs
The point of these is they're not going to dry out on you years down the road, which is something not on the charts for other caps with rubber seals. Trying to extrapolate 2000 hour load lifes into years and decades may not always be accurate due to failure modes from just sitting around.
Perhaps a few in museums. Certainly none in commercial operation.
*Very* few of the original engines exist anymore. They've been re-engined several times, to the point of one company doing a turbo-prop refit for them. The original engines that are left are *very* expensive.
Very heavy, sure. No thanks. You can keep it with the rest of your ludite stuff. ;-)
So default-y that it's possible certain *other* search engines may now recognize Digikey part numbers.
--sp
Visit some places outside the US, south of here. You'll be amazed what's still flying out there.
Nah, you can get one overhauled at many places. For example, in Texas:
This guy bought a whole DC-3 ( _with_ two genuine Pratt and Whitney R1830-94) for $5k:
Of course, one of the engines was pretty much shot but that's still a good deal.
That is just about the only machine able to drive a major auger into wood. I have yet to see such an extremely geared electric drill of modern age. But you have to watch your wrist, that can get hurt if the auger seizes up.
It sure does. But the real title with which it wants to be addressed is "megacycle meter" :-)
Good point. I'll have to investigate that.
Not by much. Radiation flux depends on T^4 and as such takes off rapidly with increasing temperature. It is only really a problem for radio astronomers with sidelobes of antennas seeing the warm Earth.
Not heat it up as such but slow down its rate of heat loss.
*Net* heat flux is always from the hotter to the colder body but if there is a choice of two colder bodies with otherwise equal properties you will lose heat faster to the coldest one.Every object is emitting radiation characteristic of its temperature and its emissivity at that particular wavelength. White paint reflects visible light but is pretty close to black in the thermal IR band.
Shiny metallic surfaces being the obvious exception they mostly reflect whatever light is incident on them and are poor radiators. There was a recent cryogenics thread that detailed this stuff here fairly recently.
But is q really 1k W/m^2?
I thought it is much more closer to 850W/m^2 due to Atmospheric dust.
Cheers
We're talking past each other.
Me (from above):
"Don't understand the question. Ripple currents into the amps aren't uncommon. I have one application where aluminums won't cut it so we're going to multiple polymers."
With some elaboration:
"85C. You have to watch the ripple frequency with aluminums, too. We're running 500-800kHz. According the usual suppliers, hybrid polymers will take it. Aluminums won't.
I was referring to *my* application.
es
lose to 1/2 the solar influx. And the rest goes either into the earth or g ets taken away by convection.
aches 69C.
Yes, me too. Thanks George. It's neat how topics wander...this ties in nicely with a current problem of mine.
Cheers, James Arthur
q = 1.0kW/m^2 x 5 hours is a decent rule-of-thumb *daily* average for the U.S. sunbelt.
Peak insolation runs up to 2x. Arizona, New Mexico, and Texas are likely higher; the north end is a bit lower.
That actually should factor into my earlier calculations--I was in one of those 2kW/m^2 locations.
Cheers, James Arthur
Crap. I misinterpreted that graph--it's not peak, it's some composite mess.
So, 2kW/m^2 is suspect, if not bogus. (It seemed too good to be true.)
Searching for peak insolation just gets a load of photovoltaic "peak daily output" nonsense, whatever the heck that is.
Cheers, James Arthur
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