You need a type R thermocouple for iron. But I have never used one for aluminum or iron. I've just poured when it acted right on the stirrer (1/4" steel rod) . For aluminum, it should come out clean, not globbed up. For iron it should come out without slag attached, and like dallen says, maybe getting shorter, and pointed!
Oddly enough I did feel the need to use a temp sensor when pouring diecast metal lately doing thin section lost foam. It's a lot lower temp pour than even aluminum. The reason -- I wanted hot metal to melt out the thin sections, but not fuming zinc, so settled on 450C for pouring. The only way to tell that closely was use a thermocouple. I used a Harbor Freight multimeter that comes with a K-type thermocouple. I put the thermocouple in a couple feet of 1/4" stainless tubing and crimped the end. bent it to a curve for easy dipping. It worked well.
Oddly enough I did feel the need to use a temp sensor when pouring diecast metal lately doing thin section lost foam. It's a lot lower temp pour than even aluminum. The reason -- I wanted hot metal to melt out the thin sections, but not fuming zinc, so settled on 450C for pouring. The only way to tell that closely was use a thermocouple. I used a Harbor Freight multimeter that comes with a K-type thermocouple. I put the thermocouple in a couple feet of 1/4" stainless tubing and crimped the end. bent it to a curve for easy dipping. It worked well.