Painted Heat Sinks

Is it true that emissivity + reflectivity = 1?

That seems intuitively to be the case. The brass was shiny enough in the IR that what I was measuring was the reflected image of the room, not the temp of the brass itself.

So you can measure emissivity at thermal wavelengths by heating up the thing in question and scoping it with a cheap IR thermometer. There must be an equation somewhere.

John

infrared

could

The still-air convection cooling of a surface is messy: depends on size, shape, orientation. Figure very roughly 150 k/w per square inch, or 1000 k/w per square cm (although it's nonlinear as hell). Assuming that, my 100 sq cm hunk has a convection coefficient of about 10 K/w, not much less than the 12.5 k/w for the radiation case cited. But if there was any air flow, or if the heatsink has fins, or if e < 1, the balance is further tipped in favor of convection.

If anybody has better numbers, I'd like to hear them.

John

I have a cheap ($70 roughly) Extech that seems pretty accurate, but Fluke has a new one (for $99 I think) probably better quality and it has a smaller field of view, which is good.

Fluke also has a full IR imager for a mere $9995, which is sorely tempting.

John

Heatsinks are not painted because they don't need to be. They are usually made from aluminium or copper and run hot, so it is not going to suffer from damp-related corrosion. Aluminium ones are often anodised and black.

High temperatures might make the paint flake off, you could see what they use for radiators.

I wonder if anyone has suggestions for infrared remote thermometers with laser sights.

Rod Elliot's (ESP website) contains some information on this subject.

That's interesting, do you have any model #s? I have a Extech multimeter and it is super nice (military surplus).

It depends on the composition of the paint.

is an anodized finish unsuitable?

Bye. Jasen

It depends on the heatsink compound. some formulations have copper or silver particles in them and the only ooze down to the diameter of the pparticles.

Bye. Jasen

Sure, but I meant the regular white thermal grease. Just hand pressure will squeeze it below 100 microinches, which was my measurement resolution.

One fun thing to do is to add a dash of Cataphote glass beads (they're used as retroflectors, usually) into thermal epoxy, to force a small insulating gap. I guess some fine sand would work, too.

Here in California, they stencil big STOP letters in the street at an intersection, in addition to the regular STOP sign. They put down a stencil, spray white paint, remove stencil, then toss handfulls of the Cataphote stuff onto the wet paint. The wind blows it around, and the sidewalks sparkle for some weeks until it washes away.

It also comes in "spacer grade", with uniform glob sizes, but that's much more expensive.

John

Fat! 34 - 22 - 34 will have better longterm reliability.

John

infrared

could

Yes, but the numbers are very rough and the assumptions - zero air flow, e=1, 50C rise, no heatsink fins, no reflections from enclosure walls - unreasonably favor radiation.

If we use the somewhere-cited convection equation

Theta = 50/(sqrt A) A in sq cm,

Then the convection theta here is 5 K/w, further diluting the share assignable to radiation.

But the only way to really predict a heatsink's behavior is to test it.

John

The Extech is a 42520. Seems pretty good.

John

Our sparky uses one when checking the power switchboards. Don't recall the make off-hand but I think it's a Fluke.

Ken

infrared

could

Are you saying (for this simplistic example) that the power dissipated through radiation is about 4 Watts and the power dissipated through convection is about 5 Watts? If so, then the radiation is a very significant portion of the total.

John, too.

to,

infrared

could

affecting

Then that would mean 10 Watts convected and 4 Watts radiated (about 28%).

In fact, I usually guess at about 30 to 35% radiation under conditions that would be ideal for radiation. Still, that is a rather significant amount and should not be ignored.

You are correct, of course, that the only way to know for sure is to test. However, I find it handy at times to estimate such things before having the object available for tests.

Returning to the subject of painting the heatsink, I remember a design manual I had about 30 years ago mentioning that the radiation of a heatsink could be improved by painting with oil-based paint. They said the color was relatively unimportant. The booklet was produced by Motorola or National, I think, and it concerned designing voltage regulator circuits. In fact, I think the manual's title was something like Voltage Regulator Design Handbook. But, don't hold me to that -- thirty years is a long time.

Cheers, John, too.

I think you can buy specially coated aluminium from Alanod in Germany, this metal is "shiny" in the long-wavelength infra-red, but "black" in the visible and near-infra-red where the sun emits most of its energy. The intention is that this metal should be laser welded to copper pipes for use in solar water heaters, hence they want it to absorb sunlight and then not re-emit the energy. It is the opposite of what you want on your PA equipment.

Chris

I've done the calcs for 0.15mm thick powder coat, and its a piss-poor thermal conductor - IIRC a few thousand times higher than aluminium. which is why we anodise that particular heatsink. And why we castigated the twit who decided powder coating was a good idea.

Cheers Terry