Cheap thermometer calibration technique?

I know this subject of "cheap" and "calibrate" used in the same sentence may well be anathema to some of you but I need to verify that either my IR temp
gun is accurate or my DMM/thermocouple is, or neither. Accuracy to 2 or 3 degrees F is fine.
I'm looking for suggestions for a simple way to provide some kind of common temperature "standard" (I use the term loosely, here) I can compare these against.
Thanks,
--
DaveC
snipped-for-privacy@bogusdomain.net
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The freezing/melting point of ice is quite exact. Boiling water is good if you correct for your local air pressure.
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Freezing point and melting point are also pressure related, and so they too vary.
Also, water has a very low emissivity, and would be a very poor choice for a reading on the IR instrument.
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On Tue, 08 May 2007 17:16:56 -0700, The Great Attractor

0.98, about as high as anything else on the planet.
John
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["Followup-To:" header set to sci.electronics.design.] The Great Attractor wrote:

In the chart the link to which YOU keep posting here, nothing has a higher emissivity than water.
Which makes water an extremely good choice for a reading on the IR instrument.
robert
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Freezing and boiling water are obvious points.
Tapwater stirred with a lot of crushed ice will get within 10-15 mK of 0 deg C. Boiling water, corrected for barometric pressure, will get within your error budget.
You can buy a thinfilm platinum RTD, for a few dollars, that is accurate to a fraction of a deg C, if you measure it with a good ohmmeter. Some of the semiconductor temp sensors are better than 1 deg C.
Fever thermometers are very good close to body temp. That and ice point is a combination that's hard to beat.
John
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John Larkin wrote:

Don't forget that with your IR gun, the emissivity of the source is important in calibration. I suspect water and ice are not friendly calibration sources. Maybe a black anodized heat sink that has been heated to some calibrated temperature would be a good source. Does your gun have an emissivity adjustment?
Chuck
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Very good call, Chuck.

Spot on <sic>!

Not if the surface that got anodized was "shiny", as it were. A good black body source is aluminum (thick) as it conducts heat fairly evenly, but the ideal surface is a very matte finish, or even concentric rings cut into the face and then painted or anodized after a grit blast session.
Shiny is bad, which is why water is bad, despite it also being a very good, even conductor of heat. The shiny surface reflects the IR back into the medium, hence reduced emissivity.

You can find a fairly decent emissivity chart here, as well as a very good primer on the subject:
http://www.mikroninfrared.com/App_Notes/Principles_of_IR.pdf
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["Followup-To:" header set to sci.electronics.design.] The Great Attractor wrote:

[...]
Are you illiterate? Why don't you look up water in the data YOU provide and see for yourself how shiny it is? Hint 0.98 isn't shiny. 0.98 is about as black as it gets.
robert
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On Tue, 08 May 2007 08:15:21 -0700, John Larkin

For his thermocouple. Not good choices for the IR device, however.
The body temp thermometer is VERY accurate usually (mercury type), but you need a good black body source for the IR cal session.
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snipped-for-privacy@privacy.net wrote:

I don't think i have boiled any water or iced a thermometer lately. I try to check out an unknown by a known in the shop. Sometimes I may have to use a third or fourth device to be sure. That is a problem to contend with, its very difficult to place more than one probe in one spot. I try to test using a beaker of water being stirred very rapidly, otherwise temperature differential will kill you. Measuring IR can be trickey, as I found out at home measuring aluminum foil. The heat of the reflection is what you will measure. Since I have a few thermometers on the wall at home. viewing the IR probe vs the readout gives a feel for the accuracies involved.
greg
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On Tue, 08 May 2007 15:35:34 GMT, snipped-for-privacy@pitt.edu (GregS) wrote:

If you IR measure shiny copper or brass, what you're measuring is your own reflection.
I dab things with black whiteboard marker to improve the emissivity. Apparently Scotch Magic tape has high emissivity, too.
John
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On Tue, 08 May 2007 08:44:00 -0700, John Larkin

Not really. It depends on the focal length of the instrument. Shiny surfaces do reflect a lot of their IR emissions back down into the medium however.
A mirror finish of nearly anything yields results based on reflections, yet is still very dependent on the optical system utilized by the instrument.

Not very much though... if at all. Surface quality is the most determinant factor, not "color".
A sprinkling of copier toner would work better, but be much "messier".
Is that a "messier function"? :-]

Yes, but the underlying heat has to push through the tape medium, and there are losses.
Any matte finish brings one closer to ideal. Flat black paint (very thin coat) is best.
Take a look here for some really good facts:
http://www.mikroninfrared.com/App_Notes/Principles_of_IR.pdf
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On Tue, 08 May 2007 17:29:04 -0700, The Great Attractor

How can the focal length change the emmisivity or the effective temperature? If the surface is a mirror in the thermal IR, all you ever image is a reflection of *something*, but you don't measure the temperature of the mirror itself. Shiny copper has an emissivity of about 0.05, so 95% of whatever the IR meter is seeing is the reflection of something else.

Having tested a number of ways to increase the emissivity of shiny metallic surfaces, I know that black whiteboard marker brings the emissivity almost all the way up to 1.0. And it's easy to wipe off after you're done. Is 1.0 within your definition of "if at all"?

Do the math. Delta-t across scotch tape, working against air, is usually tiny.

Gosh, didn't somebody recently say that
'Surface quality is the most determinant factor, not "color"'?

Funny, they keep mentioning "black" too.
John
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On Tue, 08 May 2007 19:36:52 -0700, John Larkin

Can you really be that dumb?
If the instrument has a one inch focus, your reflection is hardly going to affect what it reads from the face of a "shiny piece of copper". It will read the copper, and be off be the emissivity factor.
Focus is extremely important in the energy gathered by an IR instrument. EXTREMELY. Do you think the 1000' focus device looking at the space shuttle launch pad would get enough energy to give a calibrated reading if it were focused for 20 feet, despite being 1000' away from the target being measured?
Use some common sense, man.
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On Tue, 08 May 2007 21:36:47 -0700, The Great Attractor

Hhat can I say but WrongAgain?
Look into a mirror that's reflecting the sky into your eyes. You see the color of the sky. Now take off your glasses or equivalent to defocus the scene. You still see the color of the sky.

If defocussing means you are averaging unwanted objects into the image with the desired target, sure you need to focus. But no amount of focussing, from any distance, will make the emissivity of shiny copper any better, or improve the temperature measuring accurscy.
If you poke the germanium lens one inch away from the copper, and focus perfectly, you'll be measuring mostly the reflection, namely the temperature of the lens itself, not the temp of the copper. Try it.

OK, common sense: look in a mirror. You see your face, not the mirror. Get closer; you still see your face, not the mirror.
Try it.
John
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On Tue, 08 May 2007 19:36:52 -0700, John Larkin

You're an idiot, and there are NO 1.0 emissivity sources.

I think you need to re-learn what is in the realm of possibility. Try the PDF I posted a link to for starters.
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["Followup-To:" header set to sci.electronics.design.] The Great Attractor wrote:

I agree that illiteracy combined with extreme stupidity is a tough burden to carry.
robert
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On Tue, 08 May 2007 21:38:24 -0700, The Great Attractor

Did you miss the word "almost"? 3rd line, 2nd word in my paragraph above.
With an imager, it takes a few seconds to test a surface to get a pretty good idea of its emissivity. The whiteboard marker, and the water, and the ice cube, all passed my tests for "so black that it doesn't matter", or, equivalently, "almost 1.0"

I read it; you didn't.
John
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On Tue, 08 May 2007 19:36:52 -0700, John Larkin

It isn't the gradient across it, it is what it inhibits THROUGH it. It does have good surface quality for high emissivity, however.I merely stated that there would be losses.
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