TeaTreeOil
Well-Known Member
We went over this. Light is radiant energy. Once again you confirm aluminum is an excellent reflector of radiance. Bravo! Encore!
Reflective material
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mindphuk
Well-Known Member
We are not talking about Al reflectors. We are discussing foil. The very reason for it's poor performance is exactly what you are saying and I have never denied, is that its specular reflectance is severely decreased as it crumples, folds, bends, etc.
I noticed you haven't commented on the background information on the patent re: insulating materials, explaining why foil is so good reflecting and containing thermal energy.
I'll quote mine a bit here: "The surface of aluminum has the ability not to absorb, but to reflect, 95% of the infrared rays which strike it. Since aluminum foil has such a low mass to air ratio, very little conduction can take place, particularly when only 5% of the rays are absorbed."
Notice how the thinness (low mass) contributes directly to it's inability to conduct heat well. This is because we are talking about foil, not aluminum in general.
Trying to parallel a 1 watt IR LED in your remote with the heat radiation coming from an HPS is ridiculous. Your wife would give you a failing grade.
mindphuk
Well-Known Member
pssst! (I've never denied it)
TeaTreeOil
Well-Known Member
I've always glued it to a flat surface, where it doesn't crinkle, fold, whatever. This is the same idea of any reflector.
Just because some people do it wrong says nothing about the material. It says, perhaps, a lot about the individual.
We agreed it's excellent at radiate reflection. You must be high.
Again, you restate facts ad nauseum. That's IR on the whole, but whatever. Thermal conductively is about surface area. Aluminum foil has greater surface area for its mass than whatever else you're comparing it with. This would make it better, not worse at thermal conductuively. I think you're thinking about thermal retention(like an insulator).
Conduction and insulation are two separate properties. A heated piece of glass(insulator) will stay hot much longer than metal(conductor) of similar size and mass.
So is your own correlation that near IR is somehow creating noticeable heat. You like my facetiousness, don't you!
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You're judging the material by the way people have used it incorrectly. (apples vs rotten oranges)
Try to separate correlation and causation.
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I might as well judge all Americans by what GWB has done. I guess you're all horrible war criminals.
mindphuk
Well-Known Member
Maybe because you haven't read the link in spite of it giving very good explanation of why aluminum foil reflects heat so well.
Sorry, I do know what I'm talking about. You have a few physics books in front of you. Why don't you open them up and read a bit.
Thermal conductivity is NOT about surface area, it is an inherent property of the material. It is designated by k. Aluminum happens to conduct heat pretty well. However, foil is a poor thermal conductor. Look up Fourier's law. The area you probably are referring to is designated by the letter A in the Fourier formula. It is the cross-sectional area, not surface area. Since the thinner the material, the lower its cross sectional area, the equation can demonstrate that very thin materials do not conduct heat well.
Hold a strip of foil in an open flame. The end in the flame will cook and you will barely be able to feel the heat.
Really, thanks for that information. Has nothing to do with the topic at hand however.
Are you saying an IR LED will not cause something to heat up? IR radiation doesn't 'feel' hot any more than visible light does. ALL EM radiation, including visible light will heat any surface that absorbs it, no matter what wavelength. If that were not the case, we wouldn't be able to cook anything in our microwave ovens.
Until you read more on thermodynamics and emissivity, or at least acknowledge there might be something you don't know, I don't think we can continue this discussion.
mindphuk
Well-Known Member
Since we have already established reflectivity is wavelength dependent, your comment means nothing. Reflecting heat well does not automatically make a good visible light reflector. Look at the host of exotic ceramics, made specifically to reflect heat. Can you claim they are just as good reflectors of blue light? X-rays? Microwaves?
I know physics well enough that even if I were high, I would still be trouncing you.
TeaTreeOil
Well-Known Member
LOL!
Why are heat sinks made from thin slices of metal attached to a *much* thicker base? Why not just a big block of metal? Because the thick part is able to absorb more heat and retain it, while passing it onto the fins. The fins are less able to retain heat, and are more inclined to radiate it.
You're talking about the capacity of absorption related to the rate of entropy.
You can heat up one end of foil while holding the other end because the heat is being rapidly conducted into the surrounding area, and that area has a massive surface to mass ratio, so the heat isn't going to be able to amount to much. It can't travel very far because the second law of thermodynamics(entropy). Instead of the outer shell of aluminum releasing heat to the inside(as it would with a thicker piece of aluminum), it's released to the other side, and very little energy is 'stored', it's pretty much instantly radiated. We've gone OVER THIS! Why do you not get it yet?
You seem to have a faint grasp of the concepts, but not how they actually work in reality.
The light will also apply momentum and pressure to any surface that absorbs it. So what? This is defined roughly as the light (energy) absorbed divided by light speed.
The thicker a material the more likely it will absorb a greater amount of EM energy.
Glad to see you got to emissivity. This is pretty important. For instance, snow is only around .9 reflective-coefficient. But the emissivity is .99!
Emissivity is not as important as reflective. As emissivity takes into account diffusion. Whereas the law of reflectivity states angle on incidence is equal to the angle of reflection. Emissivity doesn't. It's the sum total of all reflectance.
Reflectance is not equal to reflectivity. A mirror is reflective. White paint has reflectance.
EDIT: Trouncing me? You don't even make any sense. You seem to explain the concepts well enough but you fail at their application to the real world.
Bare aluminum averages 90% visible light reflectivity. And 95% IR reflectivity.
No, one does, of course, not mean it's good at *ALL* spectrums. But Aluminum is over 70% on average across the entire EM spectrum. The thickness of the material matters little to it's reflectiveness after a certain point.
Mylar is a very poor reflector when the aluminum is applied to PET at very thin coatings.
Why are heat sinks made from thin slices of metal attached to a *much* thicker base? Why not just a big block of metal? Because the thick part is able to absorb more heat and retain it, while passing it onto the fins. The fins are less able to retain heat, and are more inclined to radiate it.
You're talking about the capacity of absorption related to the rate of entropy.
You can heat up one end of foil while holding the other end because the heat is being rapidly conducted into the surrounding area, and that area has a massive surface to mass ratio, so the heat isn't going to be able to amount to much. It can't travel very far because the second law of thermodynamics(entropy). Instead of the outer shell of aluminum releasing heat to the inside(as it would with a thicker piece of aluminum), it's released to the other side, and very little energy is 'stored', it's pretty much instantly radiated. We've gone OVER THIS! Why do you not get it yet?
You seem to have a faint grasp of the concepts, but not how they actually work in reality.
The light will also apply momentum and pressure to any surface that absorbs it. So what? This is defined roughly as the light (energy) absorbed divided by light speed.
The thicker a material the more likely it will absorb a greater amount of EM energy.
Glad to see you got to emissivity. This is pretty important. For instance, snow is only around .9 reflective-coefficient. But the emissivity is .99!
Emissivity is not as important as reflective. As emissivity takes into account diffusion. Whereas the law of reflectivity states angle on incidence is equal to the angle of reflection. Emissivity doesn't. It's the sum total of all reflectance.
Reflectance is not equal to reflectivity. A mirror is reflective. White paint has reflectance.
EDIT: Trouncing me? You don't even make any sense. You seem to explain the concepts well enough but you fail at their application to the real world.
Bare aluminum averages 90% visible light reflectivity. And 95% IR reflectivity.
No, one does, of course, not mean it's good at *ALL* spectrums. But Aluminum is over 70% on average across the entire EM spectrum. The thickness of the material matters little to it's reflectiveness after a certain point.
Mylar is a very poor reflector when the aluminum is applied to PET at very thin coatings.
mindphuk
Well-Known Member
Pretty bold statement considering you are contradicting yourself in the same sentence. What surrounding area? The air? That would make it radiation, not conduction. You claim I don't get it, yet a good heat conductor BY DEFINITION, would absorb the heat and then conduct it up and burn your fingers. It seems you are the one that doesn't 'get it'.
First you claim heat is conducted in the foil, then it isn't? I agree that an aluminum bar will conduct heat very well. However, to conduct, it must absorb. My point was that foil cannot absorb enough heat to conduct it up to your fingers. It reflects or radiates. It also seems like later on you agree with this by claiming 95% reflectance (see below)
Please show me which thermal energy equation uses surface area in it's computation. I have already pointed out that in Fourier's law, the area used is the cross sectional area perpendicular to the direction of heat flow. Take the best heat conductor you can find and then make it very thin, is it still a good conductor? (answer- no)
Now you're getting it. Very thin material cannot absorb the EMR, therefore it cannot conduct efficiently. Also very dense materials are better conductors too, which is why an increase in mass increases the conductivity coefficient. If the molecule doesn't absorb the EMR, then it cannot pass it off to it's neighbor. Simple, basic.
Emissivity certainly does depend on angle. The nice thing about emissivity is that it is easier to disregard wavelength. If emissivity isn't that important, why is it used in engineering when working on heat transfer problems?
Reflectance is the coefficient of reflectivity. Are you trying to claim a mirror doesn't have reflectance? Reflectance is the coefficient of reflectivity. It is the ratio of reflected light energy to the incident. Things that don't absorb, reflect, plain and simple. Both white paint and mirrored silver have reflectance.
I'll let others be the judge of that.
This seems to be something we agree on.
I think you are arguing the wrong topic again. I have agreed that aluminum reflects light. This discussion is about the so-called 'hot-spots' with foil, which occur because it is too good of a reflector for heat.
Link to my post where I claim that the thickness of the material has to do with reflectance? I said it has to do with conduction.
You claim I'm not making sense, but at least I'm consistent. Either foil is a good conductor and (1) absorbs heat energy and passes it along to it's neighboring aluminum molecule, or is more like a black body and (2) re-radiates, or it (3) reflects the photon and does not conduct the energy.
Most materials do all three to some extent, which is why these values are expressed in percentages. If a material is 95% reflective, which by your own admission it is, then only 5% is left to conduct or re-radiate.
Just so we are clear. I am not claiming Al does not reflect well. I am not saying white paint reflects better than Al. I am saying that foil is a poor choice for a DIY reflector because it reflects heat extremely well and will burn your plants.
You seem to be arguing this point yet many growers have experienced this. It is not some urban legend, and can be explained scientifically by looking at the properties of foil.
TeaTreeOil
Well-Known Member
The aluminum and the air. The aluminum is conducting upon itself, as it's a solid object. Radiation escapes rapidly, and also convection currents draw heat away(wow, I just repeated myself). Heat isn't being conducted to the aluminum from the bulb. The bulb's heat is transferred only by convection and radiation unless it's actually touching the foil. We went over this!
Heat is conducted internally by the foil. The absorption in is basically less rapid than the radiation & convection lost. You can heat up the side of an aluminum can, holding the other side vertically on top of it. Then take a lighter and heat up the bottom part of the can. It'll quickly oxidize and turn black. In about 10 seconds your hand should be getting quite warm. Remove the flame. You can now instantly touch the black oxidized spot you were burning and it'll only be slightly warm. The nearly burning sensation in your hand is now gone, and the can feels equally warm throughout. This takes less than 10 seconds. This is predicted by the second law of thermodynamics.
LUX! For one.
Read above. It is simple, and basic.
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A mirror has reflectance and reflectivity. Reflectivity requires reflectance. But I'm unaware of any materials that are reflective and do not have reflectance.
Aluminum is a good conductor, not great, but good. It also re-radiates and it also reflects a significant amount of photons.
For foil to burn a plant the distance and absorption would have to result in photons hitting the plant more directly than the light itself. Or is it a combination of reflected light added to directed light that burns plants? Or is it just the fact that your bulb has a 1100C arc, and emits really intense radiant(IR) heat?
I've used fluoros for years. Never had hot spots with them and foil. I acknowledge that aluminum used too close with HID could cook plants. The HID itself will, afterall. The foil can basically double, triple, maybe even quadruple this.... Not good if your light source creates terrible amounts of radiant 'herbicide'.
The Metal Veg
Active Member
ohhh snnnaappp... lmao
Brick Top
New Member
This is just a question asked only for purposes of clarification and not at all intended to be a comment or statement but isnt what was written above an admission that hot spots due to using aluminum foil for a reflective material, which has previously been claimed to not exist and to have not been proven, actually in fact do exist and can be caused by using aluminum foil rather than some other form of reflective material and that the real reason you have not experienced them, or at least not to a degree that you noticed them or suffered any ill affects from them is not due to aluminum foil not creating hot spots but instead only because you use a form of lighting that operates at a much lower temperature than HID lighting?
The only reason I asked is that to me what you wrote above seems rather contradictory to much of what you have previously written.
Do you care to explain how something that you claimed does not exist and that cannot happen and has never been proven is now acknowledged by you as being a distinct possibility?
TeaTreeOil
Well-Known Member
I claim hot spots aren't created by foil.
They're created by the light. The foil is merely redirecting it as it's placed. The individual creates them by poor design. There is nothing inherent to aluminum, foil or otherwise that causes hot spots(as compared to say, a lens or magnifying glass). I asked for some thought into cause vs correlation. I ask too much.
If you used HID at distances greenhouses use them at there'd be no 'hot spot' issue. If you put the bulb and reflector too close it causes hot spots. This is the correlation. This is multiple causes creating an [undesired] effect. IR Light source + foil + not enough distance + poor design ... maybe means a hot spot. That's a lot at play besides foil to create these 'hot spots'.
And no, different reasoning when dealing with a different context/situation/environment/FACTS is not contradictory. Just as your remote control IR is the wrong wavelength and really minor intensity. Not all IR is harmful. The harmful radiation is in the micrometers.
While HID creates IR and shifts it to the visible with pressure and gases. Fluorescents emit UV, and phosphor coatings shift the light to the visible spectrum and some short-wave IR. Very little radiance is emitted in the micrometer wavelengths compared to HID.
They're created by the light. The foil is merely redirecting it as it's placed. The individual creates them by poor design. There is nothing inherent to aluminum, foil or otherwise that causes hot spots(as compared to say, a lens or magnifying glass). I asked for some thought into cause vs correlation. I ask too much.
If you used HID at distances greenhouses use them at there'd be no 'hot spot' issue. If you put the bulb and reflector too close it causes hot spots. This is the correlation. This is multiple causes creating an [undesired] effect. IR Light source + foil + not enough distance + poor design ... maybe means a hot spot. That's a lot at play besides foil to create these 'hot spots'.
And no, different reasoning when dealing with a different context/situation/environment/FACTS is not contradictory. Just as your remote control IR is the wrong wavelength and really minor intensity. Not all IR is harmful. The harmful radiation is in the micrometers.
While HID creates IR and shifts it to the visible with pressure and gases. Fluorescents emit UV, and phosphor coatings shift the light to the visible spectrum and some short-wave IR. Very little radiance is emitted in the micrometer wavelengths compared to HID.
Man, Al foil is covered by a dozens of myths, despite its very documented reflective properties. It's fun to notice that people just ignores that.
I see the mylar and foil this way. Mylar is more convenient to use, requires less effort to apply. Al foil requires more attention to apply (not because hot spots, of course), because it is very thin.
I use a soft bond to apply Al foil and it's great. I would say better than mylar because I can clear it very easily without worrying about damaging it.
Mylar: easy to use, maybe better, costs a lot.
Al foil: requires some experience, slightly less reflective, cost almost nothing.
I see the mylar and foil this way. Mylar is more convenient to use, requires less effort to apply. Al foil requires more attention to apply (not because hot spots, of course), because it is very thin.
I use a soft bond to apply Al foil and it's great. I would say better than mylar because I can clear it very easily without worrying about damaging it.
Mylar: easy to use, maybe better, costs a lot.
Al foil: requires some experience, slightly less reflective, cost almost nothing.
CopsPop
Well-Known Member
wow, there are like 5 pages of bickering between a few folks that hijacked this thread that has made it useless. isn't there some point where you just have to agree to disagree? Arrogant, stubborn, i know it all and you can't tell me anything new and I am always right.....
italianstallion24
Member
haha its also a four year old thread lol but no worries!