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Use a non-contact infrared thermomator as a pyrometer?

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TimTaylor

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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.
Actually, you only need IR. I have been a certified thermographer for many years and done literally thousands of measurements in hundreds of applications. You can make very accurate and sophisticated measurements in complex situations using IR if you know what you are doing. It's more than just pointing at the surface and taking a reading.

Most of the industrial IR pyrometers and cameras use a mid-wave sensitive detector (7.5 to 13 micron). This is adequate for most industrial and commercial applications. Special applications may need a different wavelength. For example, if you need to measure through glass, such as a filament in a light bulb, mid-wave won't work as glass is an IR reflector at those wavelengths - you'd need to use a detector that measures in the 2-5 micron range.

Calibration is also important - a properly calibrated mid-wave IR camera, for example, can make accurate measurements up to 1200C. This type of calibration is done using a series of black body sources - not something the manufacturer of a typical low end handheld pyrometer is going to do.

Back to the OP's original question: no, a cheap IR pyrometer isn't going to give you an accurate reading. Could it be done with IR or other non-contact method? - Maybe, but it would be expensive and complex.

I do some metal casting, but the melt volumes are small - I use an electro melt furnace with a PID controller, so I know exactly what the temperature is.
 

Steamchick

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Perhaps sprinkle some graphite powder, or Molybdenum dioxide dust on the surface when measuring temp. To get an emissivity close to 1. G the carbon will burn to CO and then that will burn to CO when above 350deg. C. MOT will need to be swept off with the slag. This is just a guess, so an expert can tell me I am wrong and I won't be offended. (I Have not confirmed this idea).
 

Willyb

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Hi Guys

The biggest reason why sticking a bare thermocouple into molten aluminum won't work, is because the insulation cannot handle the heat and the liquid metal will short out the thermocouple. It also sticks badly to the tip of the thermocouple. That is why you need to have the thermocouple inside a carbon or graphite rod. Thermocouples produce a very small voltage depending on their type (example “K” or “N” type) and the temperature they are reading. The thermocouple connects to a meter that converts the voltage into the temperature and displays it. As far as the graphite rod affecting the temperature reading, I think if you wait a few more seconds everything will be the same temperature in the end of that rod. Have a look at myfordboys video below to see the pyrometer he built and uses. His uses a “K” type thermocouple inside a graphite rod. I think this is the most common thermocouple for molten aluminum.


Cheers
Willy
 

johnwm

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I just use a "vanishing wire" optical pyrometer that I found on EBay for a tenner. Very easy to use, and certainly accurate to within 10 degrees.
 

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