Use a non-contact infrared thermomator as a pyrometer?

[wa8dof]

Hi Guys

As the title says, I am wondering if a cheap infrared thermometer could be used to measure the temperature of a pot of molten aluminum. I have been casting aluminum for several years and have never had a pyrometer, but judged pouring temperature by color and fluidity of the melt. It usually works fine, but I would like to be more precise.

I was in Harbor Freight the other day and saw an infrared thermometer that was rated to 2200 degrees F (I think).

Have any of you experienced casters used a device such as this?

Thanks for reading and stay well.

Dave

 

[retailer]

I can't see why not if the device is rated at 2200F, I have one that is rated at 550C I used it a few times to check my kiln when tempering spring wire, works ok but that 550C limit is an issue, one day I'll fit a pyrometer.

 

[kiwi2]

Hi Dave,
I'd be a bit careful about trusting the absolute temperature indicated by one of these as the readings are affected by the nature of the surface you're looking at (the emissivity). A matt black surface will give a different reading to a polished metal surface even if the two are at the same temperature. You might be better off using a K type thermocouple meter. If you get one with fibreglass rather than plastic sheathing you should be able to dunk it directly into the molten metal without harming it and it will tell you exactly what the melt temperatue is.
Regards,
Alan

 

[stragenmitsuko]

Yes the bright surface of the molten ali throws off the reading .

I use a cheap Ktype lcd meter off ebay , battery powered .

Combine it with a less cheap long stainless ktype thermocouple with stainless braided cable .
Use a drilled carbon rod to cover and protect the stainless from contact with the alu .

total cost abt 30$ and works great .

pat

 

[Shopgeezer]

Why the carbon cover? Does stainless react with aluminum? Would the carbon affect temp readings?

 

[Richard Hed]

I do not know if anyone makes a temperature detector of the nature I am about to describe, but it should be a matter of easy construction: since all physical objects (except black holes) radiate heat in the form of light and that that light's wavelength is directly proportional to it's temperature (we can tell the temperature of the stars by the wavelength of the most light given off by that star), then a meter to detect the wavelength (thus the energy and heat) should be a relatively simple matter. I know Gayle's temp detector in Breaking Bad must have workt like this but am not sure.

Actually, all those of you who use your eye to see the red, orange and white different degrees of a pour are doing this already--it just takes experience to know what color and patience and careful discernment to see the various shades of red and orange. But to be EXACT, it takes some technology.

 

[stragenmitsuko]

Molten aluminium sticks to stainless , so after a few uses you end up with a blob kind of thing .
It does not stick to the carbon rod so the measuring probe stays clean .

 

[wa8dof]

Thanks for the input and ideas, guys. I like the idea of using a "non-contact" device such as this to avoid sticking something into the molten aluminum.

I guess if the readout is "repeatable" it would be OK, even if not totally accurate. By that I mean that once I found a pour temperature that I liked, I could get a reading and then duplicate it in future batches, even though the absolute temperature might be inaccurate.

Anyway, I shall continue to think this out and maybe I will take a chance and just try it. I can probably find a use for the infrared thermometer if it does not work as a pyrometer. The weather is improving and I should be able to pour in the next few weeks.

Thanks again for taking the time to advised me.

Dave

 

[Cogsy]

I do not know if anyone makes a temperature detector of the nature I am about to describe, but it should be a matter of easy construction: since all physical objects (except black holes) radiate heat in the form of light and that that light's wavelength is directly proportional to it's temperature (we can tell the temperature of the stars by the wavelength of the most light given off by that star), then a meter to detect the wavelength (thus the energy and heat) should be a relatively simple matter. I know Gayle's temp detector in Breaking Bad must have workt like this but am not sure.

Actually, all those of you who use your eye to see the red, orange and white different degrees of a pour are doing this already--it just takes experience to know what color and patience and careful discernment to see the various shades of red and orange. But to be EXACT, it takes some technology.

I believe that similar to emissivity differences in IR, you would need to account for reflectance in the visible spectrum as well as ambient effects. Molten aluminium is a good example - it appears silver to the naked eye (until it gets really hot) yet is actually emitting in the visible red range at melting temps.

 

[kiwi2]

Hi Richard,
There is a gizmo which has been around for yonks and which does pretty much as you describe. I used to work in a factory which had a stream of molten glass which needed to be kept at a specific temperature. The device consisted of a low power telescope with a filament in the optical path to the glass stream. This was heated by a torch battery through a variable resister which had a dial calibrated in degrees. At low power, the filament glowed dull red, and as the power was increased the wavelength decreased until it would glow a brilliant white. In use, the device was focused on the glass stream and the power to the filament was adjusted until the colour matched that of the glass. At that point, the filament disappeared and it's temperature could be read off the dial. It worked a treat but was only useful for temperatures above say 700 degrees C when a pronounced visible colour was produced. From memory, I think it was called an optical pyrometer.
Regards,
Alan

 

[ddmckee54]

I can guarantee that the Harbor Freight IR thermometer won't work. They don't like reflective surfaces.
Mine won't even give me a reading off my aluminum hot block or the aluminum print bed on my 3D printer. If I put a piece of blue painters tape on the print bed or the hot block, I can take the temperature from the tape, but not from the nice shiny metal.

Don

 

[awake]

No problem! I'll just put blue painters tape on top of the molten aluminum ... hmm, wait, something doesn't seem quite right about that ...

(Sounds like one of those things that should be preceded by, "Here, hold my beer and watch this!" Not that I have any experience with such things, of course. Ahem.)

 

[RM-MN]

Does pure carbon float on molten aluminum? Maybe a carbon rod would work?

 

[wa8dof]

Thanks for the info Don. I was wondering if the reflective surface would effect the reading and now I know.

I was hoping the be able to read the melt temp without sticking a probe into the melt, but am now wondering if the thermometer will read the temperature of the melting pot, which should be about the same temperature as the melt. I use a steel pot and it glows red in use, but has a dull finish.

I think I will give it a try. Will report back if successful.

Thanks for all the input. Stay safe.

Dave

 

[ddmckee54]

If a video shows up on YouTube of somebody putting blue painter's tape on top of molten aluminum to get a Harbor Freight IR thermometer to register the temperature, remember it wasn't MY idea, it was Andy's. I'm not taking the rap for this one!

You guys will back me up on that right?

Don

 

[Richard Hed]

I believe that similar to emissivity differences in IR, you would need to account for reflectance in the visible spectrum as well as ambient effects. Molten aluminium is a good example - it appears silver to the naked eye (until it gets really hot) yet is actually emitting in the visible red range at melting temps.

Also, the wave-length is not a single one like a laser, it is a bell shaped curve of the closest ones to the one in the middle

 

[Richard Hed]

Hi Richard,
There is a gizmo which has been around for yonks and which does pretty much as you describe. I used to work in a factory which had a stream of molten glass which needed to be kept at a specific temperature. The device consisted of a low power telescope with a filament in the optical path to the glass stream. This was heated by a torch battery through a variable resister which had a dial calibrated in degrees. At low power, the filament glowed dull red, and as the power was increased the wavelength decreased until it would glow a brilliant white. In use, the device was focused on the glass stream and the power to the filament was adjusted until the colour matched that of the glass. At that point, the filament disappeared and it's temperature could be read off the dial. It worked a treat but was only useful for temperatures above say 700 degrees C when a pronounced visible colour was produced. From memory, I think it was called an optical pyrometer.
Regards,
Alan

That is BRILLIANT! No pun intended.

 

[TimTaylor]

Also, the wave-length is not a single one like a laser, it is a bell shaped curve of the closest ones to the one in the middle

A hand held IR thermometer is essentially a single pixel IR camera. It is sensitive only to energy in the infrared spectrum, not visible. As others have stated, the emissivity of the surface at the wavelength being measured is going to be the primary factor affecting measurement accuracy. The maximum theoretical emissivity of a surface is 1.0. The emissivity of a surface can also change with temperature, sometimes quite dramatically. The typical emissivity of molten aluminum is 0.11 - this means that only 11% of the IR energy you see is being actually being emitted, and the other 89% is reflected from the background.

Another factor a lot of folks neglect to take into account is spot size. A 12:1 IR thermometer measures the average temperature of a 1" diameter spot at a distance of 12" - at 24", the spot is 2"diameter, at 36" it's 3".

Like most things, when it comes to IR, you get what you pay for. A cheap under $50 instrument is not going to anywhere near as accurate, or just as important, repeatable, as a higher quality unit. There is a reason a high end IR camera costs many thousands of dollars.

In summary, unless you can accurately compensate for the emissivity, a low end IR thermometer is not going to to give you the kind of accuracy you need to measure pour temperature of your molten metal.

 

[Richard Hed]

Thanx for that. If one were to have a really good thermometer, it would be necessary for it to measure IR AND visible. As when a molten metal is white hot, you can SEE the white, and that is not IR--there is plenty of IR there, but it will not be measuring the best form of light to use which will be in this case, visible. IR would be best when the metal is in a dull red to cherry red.

 

[awake]

Ah! I just had an epiphany. If the problem for the HF detector is the reflectivity of the surface, then the answer is simple: Just submerge the IR detector in the molten aluminum before taking the reading. What could go wrong?