How to Measure Temperature of Molten Metal in Induction Furnace?

I do not think you want to use the vinger test method!

http://www.micro-epsilon.com/applications/areas/Temperatur/index.html

I have not done on the metal but did on glass. I was melting two phase glass and wanted to measure temperature of second phase which was below first phase and end up doing

Microwave temperature measurement.

Infrared measurements should work. The slag temperature should be very close to the melt temperature. It is also common to use a disappearing filament pyrometer for these types of measurements.

Surface slag temperature I have seen as much as 50 oC off based on slag composition which is generally glass and is oxide based melt . If we do n ot measure metal temperature correctly we do have high loss of metal.

During my days as metallurgiest this was a tough issue and there is not an easy solution for this. In small batch one may suscefully use IR if melt is in rocking furnace base and on and off you end up with metal surface.

Other alternate is the caliberation of slag to metal temperature and the yes IR will work other tools are ultrasonic and microwave base measurements but I am out of touch

You are correct, the slag surface temperature will be somewhat lower than the melt, but not much lower. The emissivity of the slag is higher than that of the melt. This means that a thermometer calibrated for molten metal emissivity will read lower when measuring slag temperature. If one calibrates the IR thermometer for slag emissivity, or uses an automatic emissivity compensating thermometer, one will obtain a close reading. Also, you can correct for any residual temperature offset.

You can use IR with greater accuracy if you skim back the slag before taking a reading. Depending of the charge content the amount of slag and its emissivity can vary considerably. The emissivity is the basis of the IR reading and your calibration. Taking a reading of the actual metal bath is better. There are quartz tip pyrometers either portable or wall mount which are good for that temperature range (cast iron about 3000F) but I had broken a few of those trying to penetrate the slag so even there you're best to trim back your power level on the furnace, skim and read whichever method you use.

i have come across "Cyclops 055 Meltmaster" as a IR portable thermometre. this of land brand. how is the product? can i use it for the purpose i m looking for?

I'm not familiar with that one. What is critical, and hasn't been discussed, is that if you decide to use an IR thermometer, it can't be used to read through an optical window in the furnace. If you must read through a window, then a disappearing filament pyrometer or a thermocouple are your best bets.

sorry for the delay but can you please explain what exactly do you mean by optical window?

An optical window is a small window, usually made of quartz, that allows the viewer to view the molten metal. You can not obtain an accurate reading of the melt temperature through one of these windows with an IR thermometer because the instrument will measure the surface temperature of the window and not the temperature of the molten metal inside the window.

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