Fitting a large K type thermocouple on the inside of my furnace bore has really made a difference to tuning the furnace fuel/air ratio for the highest temperatures. I bought a 10 inch long K type probe with the ceramic beads over it from Ali Express a while back and last week I drilled through the refractory from the outside and angled the thermocouple into the bore about halfway up the side of the crucible. The thermocouple junction is naked but sits inside a small ceramic cup which I packed with kaolin clay to give some protection from the combustion gases. So while the furnace is running you can set a given fuel flow and vary the airflow and see what the temperature does in realtime: this really does take all the guesswork out of running an oil fired furnace in a big way. I learnt that I'd been running too lean all this time and maximum heat occurs with large yellow rich looking flames spitting 12" out the furnace lid. Also it became apparent the furnace needs constant adjustment as it heats up and the fuel system also warms up and changes characteristics. I can highly recommend a thermocouple as a tuning aid for your furnace, this unit has fairly thick wires so it's reasonably rugged and still works after reaching 1305 degrees C/ 2381 deg F. Gettin up there!! Running the furnace at the hottest setting: Probe tip in the bore, hole sealed with kaolin clay. Some brass screw terminals to thermocouple extender wire.