Author Topic: How to use an Infrared thermometer (emissivity)  (Read 702 times)

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Offline gimpo

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How to use an Infrared thermometer (emissivity)
« on: January 05, 2021, 01:46:34 pm »
Hello,
I hope this is the right place to post my question.

I just bought a chinese cheap IR thermometer, and it is working quite well. I'm just a little bit confused about the right setting for the emissivity value when checking the surface of a NEMA11 stepper motor.

As I can see (see attached image), the front/back plate pf the motor is made of some alloy (Aluminium? Not 100% sure on this), while the black zone in the middle should be coated copper.
Since the emissivity is related to the material on the surface, and I don't know the composition of the coating, how I can set the right value for my IR thermometer???  :-//

I would to like to roughly measure the surface temperature of the motor to check if it reaches critical values (100 °C or above). An error of 5 or 6 °C is more than acceptable for me.
 

Online Kleinstein

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Re: How to use an Infrared thermometer (emissivity)
« Reply #1 on: January 05, 2021, 02:28:41 pm »
Clean metal surfaces have rather low emission factor, but more like a good reflector. So even if you know the factor it still does not work as the rest would come from somewhere else.  To get a good reading on essentially need high emissivity. The cheap thermometers usually has a fixed value set, like 95%.

The good thing is that most organic coatings like paint or plastics have such a high essentially. So the easy way is to get some suitable sticker or paint to mark metal surfaces of interest.
In theory one may check how bad aa surface is by moving a hot plate (e.g. some 60 C surface)  in the background / in reflection. If this shows up in an IR picture of IR thermometer reading, the emissivity is not good.
 

Offline gimpo

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Re: How to use an Infrared thermometer (emissivity)
« Reply #2 on: January 05, 2021, 03:39:40 pm »
The good thing is that most organic coatings like paint or plastics have such a high essentially. So the easy way is to get some suitable sticker or paint to mark metal surfaces of interest.
That is a nice trick! Thanks.  :-+

I could also carefully attach a metal thin stripe of some metal (copper or steel) with well-know emissivity value. Unfortunately all metals have a good reflection value
 

Online Kleinstein

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Re: How to use an Infrared thermometer (emissivity)
« Reply #3 on: January 05, 2021, 05:10:24 pm »
The IR thermometers are not very focused - so the sticker has to be relatively large. It works better with an IR camera, even low resolution (but more than 1 pixel).
 

Offline Gyro

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Re: How to use an Infrared thermometer (emissivity)
« Reply #4 on: January 05, 2021, 05:48:33 pm »
The good thing is that most organic coatings like paint or plastics have such a high essentially. So the easy way is to get some suitable sticker or paint to mark metal surfaces of interest.
That is a nice trick! Thanks.  :-+

I could also carefully attach a metal thin stripe of some metal (copper or steel) with well-know emissivity value. Unfortunately all metals have a good reflection value

If you google 'emissivity chart', you will get a lot of hits for charts covering most common (and uncommon) materials, including specific paints and tapes.

Check out the Thermal Imaging forum section too. I'm sure there is a lot of emissivity data in there too _ I remember @Fraser discussing different paints for calibration sources, although you probably aren't going to need resolution to that degree.
Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline gimpo

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Re: How to use an Infrared thermometer (emissivity)
« Reply #5 on: January 05, 2021, 08:38:50 pm »
I've looked at different emissivity tables, unfortunately all of the metals have a low value, as I understand this increase the margin of error. Materials with high value should be preferred.

The problem is that I know nothing about the black painting visible on almost all of the stepper motors. I suppose that it is the insulation layer used for the winding wires.
Without a knowledge of the material even the most detailed emissivity table is useless...  :'(

I will take a look at the imaging forum.
 

Offline gimpo

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Re: How to use an Infrared thermometer (emissivity)
« Reply #6 on: January 05, 2021, 09:55:05 pm »
Probably you was referring to this thread started by Fraser.

In the thread the author suggest how to build a rudimentary black-box "for the rest of us" to calibrate an IR sensor.
As I understand the key is the paint. A matt black paint. Because "all" matt black paint have a good emissivity value (0.90 or higher).
Even if not explicitly written, I suppose that after calibration the next step is to use the same paint on your target object to get a "good enough" value.

Since almost all stepper motor are "painted" with a not-glossy, no flat, layer of black material in the section between the front/back plates. I think that I should get a good value even with my not-calibrated chinese IR sensor when setting the emissivity value around 0.92.
Yeah, I will not get an error of +/-5 °C, maybe I will be around +10/-10 °C zone but in any case it will be enough to discover if the stepper is over-heating or not.

I found very interesting the article below published directly from the FLIR company. They are an authority in the field of thermal scanning.
"Use Low-Cost Materials to Increase Target Emissivity"
https://www.flir.com/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/

Among other things, they recommend the use of the Scotch™ Brand 88 black vinyl electrical tape as a good way to increase the target emissivity to 0.96.  :-+
Also a stripe of Kopton tape is listed as a good method to read temperature with an IR sensor, but the emissivity value is not reported.

I've found the emissivity of the Kompton tape (0.95) in the article here below:
"Engineers warm up to IR vision"
https://www.edn.com/engineers-warm-up-to-ir-vision/

I suppose that the Kompton tape has to be preferred when your dealing with very small IC components. Since it is very thin and does not affect the total mass of the target object in a significative way.
What surprise me is that the Kompton tape is yellow, not black. And it is semi-transparent too!


Excuse me now, I have to buy some tape!  ^-^
« Last Edit: January 05, 2021, 10:01:12 pm by gimpo »
 

Offline Gyro

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Re: How to use an Infrared thermometer (emissivity)
« Reply #7 on: January 05, 2021, 09:56:50 pm »
I've looked at different emissivity tables, unfortunately all of the metals have a low value, as I understand this increase the margin of error. Materials with high value should be preferred.

The problem is that I know nothing about the black painting visible on almost all of the stepper motors. I suppose that it is the insulation layer used for the winding wires.
Without a knowledge of the material even the most detailed emissivity table is useless...  :'(

I will take a look at the imaging forum.

Yes, you want something with high emissivity for accuracy. That pretty much rules out metals, too sensitive to surface finish too.

I was initially thinking tape. Even if not that thermally conductive, it will conform well to the surface and should attain the temperature of that surface. White paper looks fairly consistent at around 0.9 - 0.94 and is thin, so self-adhesive paper labels might be a good option. They tend to be available in larger widths than tape and will show the laser spot well.

Paint is a messier option and probably won't reach its final emissivity until dry.


EDIT: Yes, thermal emissivity is a funny thing, it bears little relationship to an object's visible appearance (eg, Kapton).
« Last Edit: January 05, 2021, 10:01:20 pm by Gyro »
Chris

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Offline gimpo

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Re: How to use an Infrared thermometer (emissivity)
« Reply #8 on: January 05, 2021, 09:59:22 pm »
Yeah, tape is the way to go.
Read my post above.  :-+
 

Offline Gregg

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Re: How to use an Infrared thermometer (emissivity)
« Reply #9 on: January 06, 2021, 12:56:53 am »
The field being measured can introduce major errors in IR readings and most of the cheap devices don’t really tell the user how wide the measured field may be at any given distance and most of the cheap units have quite wide angles of measurement.  The better units have multiple laser dots to indicate the approximate area being measured. 
In the attached picture the Raytek device shows the spot actually being measured which is very helpful.  The blue paint is to provide better emissivity; I was out of flat black spray paint.  Most flat paints work better than shiny surface paint.  To check the effectiveness of a paint, just spray a little on the outer surface of an aluminum cooking pot and fill it with water near the temperature you expect to be measuring; the offset from the water temperature and your reading can then be used to make adjustment to your result.
I personally like a thin coat of quick drying flat black spray paint because it makes a tight bond with the substrate with minimal insulation added.
 

Offline gimpo

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Re: How to use an Infrared thermometer (emissivity)
« Reply #10 on: January 06, 2021, 10:33:18 am »
The cheap chinese IR I bought (see the first post) has a circle of little red dots to indicate the area measured.

Using a pot filled with water at know temperature is a nice idea. It should work with tape too.  :-+
 


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