Use a non-contact infrared thermomator as a pyrometer?

[TimTaylor]

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]

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]

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]

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.

 

[dazz]

I think Tim Taylor above touched on the method to avoid differences in reflectivity.
If you have a hollow sphere, and heat that to any temperature, the light emitted within the sphere is the same, regardless of material and surface finish.
It is called a black body cavity.

For foundry work, a practical application is to place a light tight lid on the crucible with a small hole it is. Measuring the light emitted from the hole will give a reasonably accurate temperature reading.

 

[GreenTwin]

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

You can still pick these up online if you shop around.
I bought one.
I need to get it out and try it again.
At the time I bought it, I did not really know what I was doing with foundry stuff.

 

[Steamchick]

There is one on e&@y for £40....

 

[dazz]

Hi
This e-bay vintage optical pyrometer did not sell for $USD80. I think one of these would be quite adequate for the home foundry.

If you really want to leap forward to 1980's tech, then there is the Mikron type infrared pyrometer. Basically it is a volt meter connected to an infra-red sensor. The magic happens in the sensor. Mikron are still in business.

Dazz