Skin Oil on Power Components

A month ago I bought an IR sensor thermometer from Harborfreight. $20, with laser dot. Measured a metal plate at room temp with a thermoelectric probe, 70.2F. Then I held that thermometer at it from a few inches away, 70.2F. Amazing.

It's easy to measure the temp of anything when everything in the room is 70F. In fact, there's no need to measure anything when everything is 70F.

The problems are:

1. The field-of-view of cheap ir thermometers is very large and fuzzy. The laser dot is deceiving, when the sensing cone may be 30 degrees or more.

1. The emissivity of the target matters a lot. If you heat up a sheet of copper, or copperclad, and IR it, what you're measuring is the reflected background (which may be your own body, or the walls of the room) and not the copper, because copper is nearly a perfect mirror at thermal IR wavelengths.

Painting a metal surface with black whiteboard marker makes it almost a perfect ir blackbody. It's easy to test for reflections; aim the ir gun at the surface at an angle, and wave your hand in the "reflection" zone. Ice, incidentally, is a very good blackbody, so check your ir gun by aiming it deep into a cup of crushed, melting ice.

1. A cheap ir thermometer will make big errors for objects that are not close to room temp.

John

I just did a google on 'human +"skin oil" +"smoke point"' and the first four hits were this post. ;-)

I got over 1,000,000 hits on "sebum", however. :-)

Good Luck! Rich

They don't work on aluminum. :-)

Cheers! Rich

In Kirby "sales" school, they have an attachment called a "dirt meter", which is essentially nothing but a piece of filter paper that demonstrates how much dirt the Kirby picks up. (I had a job demonstrating Kirbys once.)

The trainer said, vacuum the mattress, using the dirt meter, and light the dirt pad, to show how much body crud gets dropped on the mattress. Presumably (I never tried it), it smells like your cannibal BBQ. ;-)

Once, about 40 years ago, I fell asleep on the beach at the Gulf of Mexico. When I finally woke up, I could hardly move, for the sunburn. After a few days, I started peeling in sheets.

As an experiment, I took about a 3" by 7" piece of my shed skin, and lit it.

It went up like flash paper, but melted into a blob like burning plastic. And, of course, stunk up the whole room.

Cheers! Rich

Ours was $90k in 1987, and it was LN cooled CCD and was only 4 fps. It was only 16 color, and about 640x400, and had no NTSC out.

Now, you can get a pretty damned good one for less than $3k.

That is not very expensive at all by comparison, and it far outperforms its ancestor.

Not really. Some are fresnel. Many have no optics at all.

The dot is supposed to represent the center of the FOV.

Only if it is polished. If it is rough, any "reflection will be diffuse.

Bullshit. "Black body paint" achieves this. Nearly all else achieves less... far less even, and it doesn't really have much to do with our perceived "color" of the media one is "painting" with.

Won't do much if the wavelength of the "gun" as you call it, is outside the emission range of a human's body/skin.

Yeah.. keyword CHEAP. If it makes such errors, then it cannot really make the claim of being such an instrument at all.

Talk about snake oil!

It depends on the surface finish, and the spectral bandwidth of the "instrument".

And most of them have a huge amount of parallax, so the dot aligns with the ir pattern only for far-away objects.

A fuzzy reflection is still a reflection, which means you're measuring the temp of the background, not the object.

Well, I tried it, and it works very well. So you are saying that I'm wrong. Refresh my memory: what's our respective history on being wrong?

Objects at body temp emit blackbody radiation in roughly the 7-14 micron range, so that's where an ir thermometer will work if it wants to measure same. Since the detectors are mostly bolometers, that's where they *have* to work.

But what does the skin thing have to do with checking an ir thermometer with ice?

The cheap ones can be fine if you understand them. My $70 Extech works great for heatsinks and enclosures and stuff. It's useless for checking most ic's on pc boards. Our FLIR will graph the temp profile across an 0603 resistor.

John

Well... it's only a matter of time till somebody puts a video of human sunburnt shed skin sheets on fire on Youtube. :)

D from BC British Columbia Canada.

Paint it with black marker-board stuff, which wipes off easily. Black electrical tape isn't bad, either. Gotta get the emissivity up.

Copper and brass are really bad. They'll typically read room temp even if they're hot enough to take your skin off.

John

Thank you for sharing that with us.

John

No, you are not. It depends on the instrument, and how the instrument is set up.

You don't know much about windows, do ya boy?

Do you know what a pyrex window crops IR at?

Do you know what a Germanium windows crops at?

They don't *have* to do anything that their designer doesn't set them up to do.

You were talking about waving your hand in front of a Copper plate, idiot. I see it doesn't take much for your Alzheimer's to kick in.

Vinyl tape? That is pretty retarded. The perceived COLOR does NOT have ANYTHING to do with it, dipshit. The blue masking tape from Home Depot is better as it is a matte surface and far thinner than the vinyl electrical tape. Surface quality IS a BIG factor.

I can only hope you found out the hard way. Try looking at a Teflon coated skillet. First outside on the Al surface where the heat was introduced, then, inside.

No instrument required. Hold a hot skillet up near your face. The Smooth Al side barely casts a heat flux, but the Teflon side feels like you could warm the entire room with the radiation coming off of it.

Even with the right word, it's still a Google challenge. The smoke point of human sebum is very obscure. (My sebum smoke point is about 150C from my adjustable temp soldering pen experiment.)

Sebum on a resistor can be a cheapo overheating indicator. I suspect as temperature rises, sebum will smoke first, then the resistor smokes next.

Thanks for the word.

D from BC British Columbia Canada.

I didn't say it did. I said that black whiteboard marker is very good, and black electrical tape is pretty good. Which are true.

OK, I'll try that some time. But a visually shiny surface doesn't mean it's not black in the thermal IR. Water and ice are both shiny, but have very high thermal emissivity.

John

Mmm, smells like overheating electronics ;-)

Tim

-- Deep Fryer: A very philosophical monk. Website @

It was aluminum!